Twin Trailer Trucks Special Report 211 (1986) / Chapter Skim
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From page 12... ...
Executive Summary INTRODUCTION Truck-tractors pulling twin 28-ft trailers are using the nation's highways increasingly as a result of the Surface Transportation Assistance Act of 1982, which required states to permit these vehicles on Interstate highways and on other principal roads. The act also legalized other larger trucks for nationwide use on these routes, and directed the National Academy of Sciences to monitor the effect of twin trailer trucks on highways and highway safety.
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From page 13... ...
Twmi TRAILER TRUCKS important effects of the new rules on highway transportation. The effects were estimated in these areas: Trucking industry use of twins, Safety consequences of using twins, Pavement wear and other highway features affected by twins, and Safety and pavement wear affected by 48-ft long semitrailers and 102-in, wide trucks.
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From page 14... ...
Executive Summary of their freight hauling that is switched from tractor-semitrailers to twins. Cost savings from twins are not expected to stimulate any appreciable new combination-truck travel.
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From page 15... ...
4 TWIN TRAILER TRUCKS differences in handling are corroborated by professional truck drivers surveyed, who report experiencing these characteristics of twins and generally prefer to drive tractor-semitrailers rather than twins. Taken together, these special handling characteristics are mechanisms that could lead to a higher accident rate for twins operating at highway speeds.
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From page 16... ...
Executive Summary Collectively, the recent data are stifi too fragmentary to confirm or revise the results of pre-1983 analyses that estimated the accident rates of twins relative to those of tractor-semitrailers, but they show no surprising changes in truck accident experience that could be attributed to twins. These safety findings are based on twins carrying general freight, their predominant use, and do not assess the potential risks involved should twins be used to transport hazardous materials.
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From page 17... ...
6 TwIN TRAILER TRUCKS and 10 percent of the expected 1990 annual productivity gains in trucking. More reliable estimates of the pavement costs attributable to twins and other large truck types will not be possible until pavement wear and rehabilitation costs can be related more explicitly to vehicle loads and environmental factors that vary from region to region and highway to highway.
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From page 18... ...
Executive Summary 7 trailer sales, the 48-ft length accounts for almost 60 percent and is becoming the industiy semitrailer standard, replacing the 45-ft semitrailer. The safety effects of 48-ft semitrailers have not yet been established.
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From page 19... ...
TWIN TRAILER TRUCKS The U.S. Department of Transportation should work with state agencies to improve the quality and consistency of state-collected data through oversight and technical assistance.
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From page 20... ...
Executive Summary 9 with the 102-in, body width should be required to be equipped with axles of the matching width to take advantage of this opportunity for improved stability. When considering any change to truck size and weight laws, government regulators should seek opportunities to link the change with vehicle requirements that assure that highway safety is maintained or improved in the process.
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From page 21... ...
1 Introduction Twin trailer trucks -- truck-tractors pulling two trailing units (Figure 11) -- are appearing more frequently on roads in many parts of the United States as a result of the Surface Transportation Assistance Act of 1982 (enacted January 1983)
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From page 22... ...
Introduction 11 roads nationwide, and they account for one combination truck in 30 on principal roads in the 14 eastern states that barred twins before 1983 (3)
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From page 23... ...
12 TwIN TRAILER TRUCKS trailers offer the advantages of increased volume capacity, and twins allow greater flexibility in handling and routing cargo in some types of trucking operations. The new federal limits sought to improve productivity by allowing larger trucks than had been legal in many states and by providing more uniform size and weight regulations on a network of major national roads in place of the varying indivithial state regulations in effect previously.
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From page 24... ...
Introduction 13 eral and state government agencies. The states and the federal government will therefore continue to confront the problem of balancing the benefits of freight cost savings against the potential safety, maintenance, and road construction costs of larger trucks.
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From page 25... ...
14 TWIN TRAILER TRUCKS vehicles are heavier on average than the trucks they replace and because of the way in which the load of a twin trailer truck is distributed across its axles, pavement wear will be moderately accelerated. Increasing the length, width, and weight of trucks can impede the flow of traffic, especially on congested roads.
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From page 26... ...
Introduction 15 METHOD OF THE STUDY The full impact of the 1983 liberalization of truck size and weight rules cannot be observed during the law's first few years. Adoption of new vehicle sizes and configurations will be gradual, occurring over a period of years, for several reasons: a substantial stock exists of older trucking equipment with remaining useful life, uncertainty and controversy continue over the extent of the routes where the larger trucks may operate, investment funds in trucking have been scarce because of economic shocks to the industry from deregulation and the severe recession in the early 1980s, and time is required for experimentation with various operating practices using twins.
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From page 27... ...
16 TwIN TRAILER TRUCKS A series of interviews with trucking companies using twins and with trailer manufacturers to learn the extent of expansion of twins use and the benefits and drawbacks of twins. ORGANIZATION OF THE REPORT In Chapter 2 the history and rationale of truck size and weight regulation, the size and weight provisions of the Surface Transportation Assistance Act of 1982, and the policy debate that preceded and followed passage of those provisions are outlined.
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From page 28... ...
2 Historical and Regulatory Background The new federal rules governing the use of twin trailer trucks and other larger combination trucks are only one recent development in a 70-year history of state and federal regulation of motor vehicle size and weight. Throughout this period, as automotive and highway technologies have developed, vehicles have become progressively larger and legal size limits have been revised to accommodate them.
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From page 29... ...
18 Twir. TRAILER TRUCKS In the first section of this chapter the history of state and federal truck size and weight regulations is reviewed from the enactment in 1913 of the first state limits through 1982.
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From page 30... ...
Historical and Regulatory Background 19 on construction costs. Similarly, vehicle weights also influence the structural design of new roads.
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From page 31... ...
20 TWIN TRAILER TRUCKS and solid rubber wheels of heavy trucks. State involvement in road construction and maintenance was expanding at this time, in part because under the terms of the federal-aid highway program created in 1916, state governments were responsible for administering federal road funds.
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From page 32... ...
Historical and Regulatory Background 21 increased to 55 ft by 1964 and 65 ft by 1982. Width limits have been the most stable and unifonn regulation, with most states agreeing on 96 in.
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From page 33... ...
10 20 30 1927 1949 1964 1982 Less than 96 MAXIMUM 96 cO 1 cD46 5D0 WIDTH (in.) 102 More than 102 Not restricted 40 or less 45[ soil MAXIMUM 55r I COMBINATION LENGTH (ft)
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From page 34... ...
40,000 or less 40,001-50,000 J I1 50,00160,000 MAXIMUM 60,001 -70,000 PRACTICAL 70,001-75,000 COMBINATIONb WEIGHT (Ib) 75,001-79,999 80,000 Over 80,000 Not restricted Legal I sc]
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From page 35... ...
24 TABLE 2-1 Dates of State Legalization of 65-ft Twin Trailer Trucks Before 1983 (6) State Date Route Resthctions Arizona NP Nevada NP Idaho 1937 New Mexico 1955 Hawaii Pre-1956 California 1959 Montana 1959 Oregon 1959 Utah 1959 Colorado 1961 Washington 1961 Wyoming 1961 Illinois 1963 Designated highways only South Dakota 1963 Indiana 1965 Kansas 1965 Missouri 1965 Designated highways only Nebraska 1965 North Dakota 1965 Designated highways only Oklahoma 1965 Texas 1965 Alaska 1966 Kentucky 1966 Designated highways only Michigan 1966 Designated highways only Arkansas 1967 Delaware 1967 Maryland 1967 Four-lane highways and direct routes to terminals Ohio 1967 Louisiana 1972 Four-lane highways plus 10 mi access Minnesota 1973 Designated highways only Wisconsin 1978 Four-lane highways only Florida 1979 Four-lane highways only Iowa 1980 Designated highways only New York 1981 Four-lane highways plus 1 mi access Mississippi 1982 Four-lane highways plus 12 mi access Tennessee 1982 Interstates and designated access roads Nora: NP never prohibited.
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From page 36... ...
TABLE 2-2 AASHTO Recommended Policies on Motor Vehicle Dimensions and Weights, 1932-1980 (7-10) Size or Weight Provision 1932 1946 1964 1974 1980 Maximum width (in.)
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From page 37... ...
26 TWIN TRAILER TRUCKS FIGURE 2-2 States allowing 65-ft twin trailer trucks on December 31, 1982 (2,5)
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From page 38... ...
Historical and Regulatory Background 27 FIGURE 2-3 States allowing 48-ft long sernitrailers on December 31, 1982 (2,5)
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From page 39... ...
28 TwIN TRAILER TRUCKS PROHIBITED ALLOWED FIGURE 2-4 States allowing 102-in, wide trucks on December 31, 1982 (2,5)
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From page 40... ...
Historical and Regulatory Background 29 Interstates) , but these revisions left intact the features of the federal rules that permitted the states to retain wide latitude in setting standards: they applied to Interstates only, more liberal preexisting state limits were allowed to stand, and the states could choose lower limits than the federal ones (Table 2-3)
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From page 41... ...
30 Twm4 TRAILER TRUCKS Summary Points: Truck Size and Weight Laws Before 1983 Legal limits on truck size and weight are necessary to control road construction and maintenance costs, to control traffic congestion, and to keep unsafe vehicle types off the roads. State governments historically have taken principal responsibility for size and weight regulation.
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From page 42... ...
Historical and Regulatory Background 31 along at a speed of 20 miles an hour. The result?
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From page 43... ...
32 Twm TRAILER TRUCKS 100 90 Cl)
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From page 44... ...
60 50 40 10 0 36 40 45 50 55 60 65 70 75 80 83 YEAR 9 0 (nw co<7 w -i LU - 02 5 z o 01- 4 I -- u- <0 2 LU CL 0!
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From page 45... ...
34 Twm TRAILER TRUCKS 60 50 0 0 0 - 40 x .0 I- Ui 30 U) 0 Ui o 20 w > 10 0 25 30 35 40 45 50 55 60 65 70 75 8083 YEAR FIGURE 2-7 Gross weight of loaded combination trucks on main rural roads, 1925-1983 (11,15,17-21)
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From page 46... ...
Historical and Regulatory Background 35 50 . 45 z w -J 140 035 0 0 I- Cn 30 25 46 50 54 58 62 66 70 74 78 82 84 YEAR FIGURE 2-8 Length of new van trailers sold, 1946-1984 (23)
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From page 47... ...
36 TWIN TRAILER TRUCKS 100 I'> Double trailers and other cornbaons 90 80 70 w Cr 60 Ft I tr U) 50 40 Three-axle 30 Iractor-semitrailers 20 10 Five-axle tractor-semilrailers OL.
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From page 48... ...
100 i :: 40 20 2,000 1,800 0 0 0L 1,400 x 1,000 w -- J 600 200 100 - Percent of state highway 90 system miles paved 70 -- J b 50 I -- U- o 30 I -- z Lu 70 cc Percent of state highway system miles w with lanes at least 11 ft wide 50 0' 25 30 35 40 45 50 55 60 65 70 75 80 83 YEAR FIGURE 2-10 Highway development trends, 1925-1983 (11,15)
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From page 49... ...
38 TwIN TRAILER TRUCKS maintained roads (generally the major rural routes in each state) less than one-third were paved in 1925; today nearly all are.
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From page 50... ...
Historical and Regulatory Background 39 Fatal Accident Reporting System (29) , although the National Safety Council has published rough estimates of combination-truck accidents, based on reports from a few states, since 1968 (30)
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From page 51... ...
8 5,000 4,000 Cr U) W I- CO 3,000 z _J _J <0 > 2,000 ZZ 2 1,000 0 75 76 77 78 79 80 81 82 83 YEAR 5L_ I I 1 I 75 76 77 78 79 80 81 82 83 YEAR FIGURE 2-11 Fatal accident involvements of combination trucks, 1975-1983: (a)
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From page 52... ...
Historical and Regulatory Background 41 Involvement Rates per Mile Traveled Virtually all data sources available indicate that heavy trucks and combination trucks (which account for nearly 90 percent of heavy-truck travel) have higher fatal involvement rates than other vehicles.
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From page 53... ...
400 Cl, I- z Wo 300 LCJ.J >W 0< > CC 200 ED 42 TwIN TRAILER TRUCKS 7 75 76 77 78 79 80 81 82 83 YEAR FIGURE 2-12 (a) Fatal accident involvement rate and (b)
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From page 54... ...
1.7 1.6 w i -- 1.5 LU 1.0 cc 9 0 : cc -- z .7 0 I -- < .6 ED 0 o U- 0 .3 I -- <2 Historical and Regulatory Background 43 75 76 77 78 79 80 81 82 83 YEAR FIGURE 2-13 Ratio of combination-truck involvement rate to all-vehicle involvement rate, 19751983 (29,31,32)
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From page 55... ...
44 TWIN TRAILER TRUCKS 25 \\\ /i____ Intercity for-hire (1) 20 Lii Intercity -J w -J 2 I 15 z 2 Intercity common -J 2 , Automobile transporler cc 81 10 U)
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From page 56... ...
Historical and Regulatory Background 45 eluding empty truck miles) increased 15 percent, from 8.7 to 10 tons (26,27,24)
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From page 57... ...
46 Twm TRAILER TRUCKS in which the combination-truck rate appears to have been increasing rapidly (1975-1981) , and recent experience (1981-1983)
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From page 58... ...
Historical and Regulatory Background 47 The long-term nationwide trend (over the past 50 years) in heavy-truck accident involvement rates certainly has been downward -- accident involvement rates for intercity freight trucks reported in the 1930s were as much as five times higher than rates typical today .
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From page 59... ...
48 TWIN TRAILER TRUCKS tate highway development shows the following trends: 1937 1983 Miles paved (%) 49 94 Miles with lane width at least 11 ft (%)
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From page 60... ...
Historical and Regulatory Background 49 prohibited from enacting new laws setting limits higher than these, but the grandfather exemptions of the 1956 and 1974 federal weight standards were retained. The states were prohibited from denying reasonable access from the Interstates to terminals and to food, fuel, repair, and rest facilities.
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From page 61... ...
50 Twm TRAILER TRUCKS This provision together with the one concerning trailer lengths in effect required all states to permit twin trailer trucks with each trailer at least 28 ft long. Fourteen states had barred twins with the previous industry standard of 27-ft trailers and 65-ft overall length (Figure 2-2)
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From page 62... ...
Historical and Regulatory Background 51 to be raised incrementally over 4 years so that eventually the fee on an 80,000-lb truck would be increased from $240 annually under the old law to $1,900 annually. The new federal rules created a nationwide network of major routes where trucks of the specified dimensions could operate, and these dimensions included the longest vehicles then in widespread use, with the exception of multitrailer combinations over 80,000 lb.
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From page 63... ...
52 TWIN TRAILER TRUCKS effects of various alternative nationwide uniform weight limits (at 80,000 lb gross weight in most cases analyzed) and length limits (requiring the states to allow at least 65 ft)
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From page 64... ...
Historical and Regulatory Background 53 With the framework of the 1983 act thus established, the specifics of the size and weight rules were worked out in a series of compromises among the affected parties. The background of each of the major provisions is described in the following paragraphs.
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From page 65... ...
54 TWIN TRAILER TRUCKS industry in order to obtain the greatest possible semitrailer length in states that regulated the overall length of tractor-semitrailer combinations but not the length of the semitrailer itself. Use of a cab-over-engine tractor rather than the conventional design allows up to about 7 ft more of semitrailer length within a given overall vehicle length limit.
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From page 66... ...
Historical and Regulatory Background 55 where the new standards would cause unsafe conditions, and I feel we can do this with a minimum of regulation and bureaucracy. Controversy over Implementation On April 5, 1983, FHWA published in the Federal Register a list of the federal-aid primary routes it had selected as an interim designated network, and the truck size and weight provisions of the Surface Transportation Assistance Act of 1982 became effective.
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From page 67... ...
56 Twir.
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From page 68... ...
Historical and Regulatory Background 57 factor of 4 the net trucking cost savings after taxes (49)
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From page 69... ...
58 Twm TRAILER TRUCKS on the grounds that FHWA had not adequately evaluated the safety of operating longer and wider trucks on the routes and had ignored the law's provision that the roads designated for 102-in, wide vehicles be highways "with traffic lanes designed to be a width of twelve feet or more" (35, Sec.416)
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From page 70... ...
Historical and Regulatory Background 59 Congress also was sympathetic to the tax complaints of the trucking industry, which had been hard hit by the 1980-1983 recessions and was experiencing dislocations as a result of economic deregulation.
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From page 71... ...
60 Twmr TRAILER TRUCKS REFERENCES U.S. Congress.
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From page 72... ...
Historical and Regulatory Background 61 Report of a Study of Highway Traffic and the Highway System of Cook County, Illinois.
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From page 73... ...
62 TwIN TRAILER TRUCKS U.S. Congress.
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From page 74... ...
3 Use Characteristics and Cost Impacts The twin trailers and the wider and longer trailers permitted in the Surface Transportation Assistance Act of 1982 (enacted in 1983) offer motor carriers opportunities for major productivity gains.
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From page 75... ...
64 TWIN TRAILER TRUCKS of deregulation. The industry had slipped into a major economic slump from which it had not emerged when the act was passed.
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From page 76... ...
Use Characteristics and Cost 65 riers) , individual owner-operators, and companies whose primary business is not trucking but who have their own fleets to move the goods they produce or sell (private carriers)
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From page 77... ...
- ICC REGULATED -- INTERSTATE HIRE FOR ARROUTE___ Gen;aI Freight -- COMMON I I INTRASTATE Special Commodities -- MOTOR • Machinery INDUSTRY F Petroleum CARRIER - Refrigerated Products IRREGULAR - Motor Vehicles ROUTE - Building Materials -- LOCAL - Household Goods - Other -- CONTRACT PRIVATE FIGURE 3-1 The structure of the motor carrier industry, 1984.
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From page 78... ...
Use Characteristics and Cost 67 For-Hire Carriers Unregulated Independent owner-operators make up a relatively small but important share of the industry. Various estimates suggest that there may be as many as 100,000 independent operators and recent statistics indicate that owner-operators account for 11 percent of combination vehicle miles (3,15)
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From page 79... ...
68 TwIN TRAILER TRUCKS Common carriers also transport special commodities and are the principal transporters of general freight. General freight is an ICC classification that includes most goods moved by truck except truckload-sized shipments of special commodities.
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From page 80... ...
Use Characteristics and Cost 69 one-quarter of the total revenue of the entire regulated motor carrier industry. The share of revenue increases to 40 percent for the top 50 revenue earners and to just under 50 percent for the top 100 revenue earners (16)
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From page 81... ...
70 Twmt TRAILER TRUCKS TABLE 3-2 ICC Disposition of Permanent Operating Rights Applications (19) Fiscal year Disposition 1979 1980 1981 1982 1983 1984 All applicants Full/partial 12,333 22,125 27,475 14,786 7,294 13,577 Denied/dismissed 711 610 939 767 10 44 New applicants Full/partial 689 1,423 3,702 5,728 2,240 6,537 Denied/dismissed 59 38 80 438 3 30 New applicants by type Common carrier _a _a 3,381 " 922 2,977 Contract carrier - - 1,546 - 487 1,705 Broker -° " 801 -- o 319 1,255 "Data not available.
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From page 82... ...
Use Characteristics and Cost 71 TABLE 3-3 Number of ICC-Regulated Motor Carriers by Class Size (21) Year Class I Class II Class III Total 1984 1,088 1,554 27,370 30,481 1983 1,139 1,631 24,411 27,517 1982 1,144 2,139 22,059 25,342 1981 1,031 2,293 -o 18,000 1980 947 2,164 -o 17,000 1979 992 2,754 13,337 17,083 1978 1,045 2,929 12,900 16,874 1977 1,052 3,101 12,453 16,606 NorE: Class definitions last changed in 1979.
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From page 83... ...
72 TwIN TRAILER TRUCKS In addition, LTL carriers may also reduce handling costs by taking advantage of the flexibility of twin trailer trucks. To illustrate, LTL shipments transported in a standard semitrailer must be handled numerous times between the points of origin and destination (Figure 3-2)
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From page 84... ...
Handling with standard Avoiding intermediate (40-48 It) trailer handling with twin trailers Avoiding terminal and intermediate handling with (win trailers Pick up shipments and load into truck OUTBOUND TERMINAL - - Unload freight [ Reload in to standard trailer Unload freight I Connect loaded 28-h trailer to another headed in same direction Reload into twin trailer and connect to another headed in same direction Pick up shipments Pickup shipments and load into and load into truck 28-h single trailer T INTERMEDIATE TERMINAL I r+ -- -- -- -- -- - _ Unload freight Separate trailers and rs reconnect to trailer to in t headed in same direction I Reload into trailer I _Eara headed to destination I L4 INBOUND TERMINAL - -- -- -- Unload freight Unload freight Separate trailers Reload into delivery Reload into delivery I truck truck I Delivery from same Delivery Delivery trailer FIGURE 3-2 Potential handling eliminated by twin trailer trucks.
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From page 85... ...
74 Twm TRAILER TRUCKS HISTORICAL USE CHARACTERISTICS Before 1983 most twin trailer truck travel occurred in the West, the bulk of it within California. Until the early 1970s, twin trailer trucks had a considerable gross weight advantage over tractor-semitrailer combinations because of the axle weight and length limits specified in California law.
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From page 86... ...
Use Characteristics and Cost 75 Even though twins were legal in 37 states before 1983, they made up a small share of combination-truck travel except in California. Only 9 other states (Arizona, Idaho, Montana, Nebraska, Nevada, Oregon, Utah, Washington, and Wyoming)
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From page 87... ...
TABLE 3-4 Estimated Total Vehicle Miles for All Combination Vehicles and Twins, 1982 Rural Urban Region and Rural and Trailer Type Interstate Primary Other Total Interstate Primary Other Total Urban Total California All 966 879 260 2,105 939 885 678 2,502 4,607 Twins 331 295 56 682 214 241 129 584 1,266 Other Western and Mountain All 2,124 1,298 588 4,010 419 256 434 1,509 5,519 Twins 214 104 78 391 33 48 23 104 495 Central and Plains All 8,822 7,841 1,716 18,379 4,511 1,579 2,033 8,123 26,502 Twins 212 117 12 341 59 27 63 149 490 East No twins All 3,989 3,412 1,444 8,845 3,043 1,838 707 5,588 14,433 Twins 0 0 0 0 0 0 0 0 0 East With twins All 3,292 2,516 958 6,766 1,290 1,490 1,103 3,883 10,649 Twins 5 5 1 11 1 0 0 1 12 U.S. total All 19,193 15,946 4,966 40,105 11,202 6,448 4,955 21,605 61,710 Twins 762 521 142 1,425 307 316 215 838 2,263 NoTE: Mileage is given in millions of vehicle miles.
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From page 88... ...
Use Characteristics and Cost 77 0-1 percent 1-4 percent 4-11 percent 30 percent FIGURE 3-4 Twins as a percentage of rural interstate combination-truck traffic before 1983 (2)
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From page 89... ...
TABLE 3-5 Traffic in Twins and Tractor-Semitrailers Before 1983 by Carrier Class (2) Percentage of Traffic by Region California Other Western and Mountain East, Central and Plains Tractor- Tractor- Tractor- Carrier Semitrailer Twins Semitrailer Twins Semitrailer Twins Class (N = 2,423)
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From page 90... ...
Use Characteristics and Cost 79 use in the Other Western and Mountain states but only 25 percent in California. These statistics do not distinguish between contract and common carriers, but other sources indicate that virtually all mileage in twins was produced by common carriers (3)
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From page 91... ...
TABLE 3-6 Traffic in Twins and Tractor-Semitrailers Before 1983 by Commodity (2) Percentage of Traffic by Region California Tractor- Semitrailer Twins Commodity (N = 1,897)
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From page 92... ...
TABLE 3-7 Traffic in Twins and Tractor-Semitrailers Before 1983 by Trailer Body Type (2) Percentage of Traffic by Region California Other Western and Mountain East, Central and Plains Tractor- Tractor- - Tractor- Trailer Body Semitrailer Twins Semitrailer Twins Semitrailer.
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From page 93... ...
82 Twmi TRAILER TRUCKS TABLE 3-8 Gross Vehicle Weight Distributions on Rural Interstates and Other Primary Highways Before 1983 (2) Weight Class (lb x 1,000)
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From page 94... ...
Use Characteristics and Cost 83 California 40 30 20 10 Th$n ,nd RAnG flthifl 30 20 10 Central and Plains 10 East (states with twins before 1983) 10 5 1970 19711972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 YEAR FIGURE 3-5 Trends in twins traffic volume on rural Interstates, 19701983 (2)
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From page 95... ...
84 TwIN TRAILER TRUCKS 1984 vehicle classification counts to FHWA, 18 indicated increases in the number of twins on rural Interstates (Figure 3-6)
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From page 96... ...
Use Characteristics and Cost 85 3.0 percent by the summer of 1984. In nine reporting Central and Plains states, it increased from 3.1 percent in 1982 to 3.8 percent in 1984, and in five Other Western and Mountain states (excluding California)
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From page 97... ...
.055 West Virginia Line .050 j A I I ' .045 !
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From page 98... ...
Use Characteristics and Cost 87 Total travel in twin trailer trucks in 1982 made up a small share of total combination travel (3.7 percent) , or about 2.3 billion vehicle-mi.
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From page 99... ...
88 TWIN TRAILER TRUCKS The previous industry standard, the 45-ft semitrailer, dropped from 75 percent of total sales in 1982 to 15 percent in 1984. Sales of 48-ft trailers (practically negligible in 1982)
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From page 100... ...
Use Characteristics and Cost 89 cubic capacity and operational flexibility of twins. In the West, sales have not greatly increased because twins have been permitted in most western states for some time.
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From page 101... ...
90 TwIN TRAILER TRUCKS Carrier Interviews To determine the investment plans of carriers adopting twins, 16 general freight common carriers were selected for in-depth interviews. These firms were chosen on the basis of size, location, and extent of experience with operating twin trailer trucks.
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From page 102... ...
Use Characteristics and Cost 91 large amounts of truckload business also expected the bulk of their LTL freight to move in twins. The exception to these conversion plans occurs among regional firms headquartered in the Northeast.
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From page 103... ...
92 Twm TRAILER TRUCKS also varied but centered on a roughly 9 percent reduction in breakbulk handling costs. Carrier executives noted that increasing the use of twins will increase some other costs.
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From page 104... ...
Use Characteristics and Cost 93 trucks as a result of the 1983 act (30)
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From page 105... ...
94 Twm TRAILER TRUCKS The twin trailer trucks already in use before 1983 by the carriers in the group have an empty (tare) weight 3,000 to 5,000 lb greater than that of the tractor-semitrailers in their fleets.
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From page 106... ...
Use Characteristics and Cost 95 trucking productivity by offering more volume without increasing the gross vehicle weight. These studies assumed that the cubic capacity of twins was the primary incentive for shifting to this configuration, but some noted that twins also offer considerable operating efficiencies to LTL carriers by reducing the number of times that freight has to be handled.
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From page 107... ...
96 TWIN TRAILER TRUCKS in the field study done as part of the Truck Size and Weight Study (33) potential problems for these carriers were noted that may temper the attractiveness of twins.
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From page 108... ...
Use Characteristics and Cost 97 In a report by the American Trucking Associations (25) , it was estimated that cubed-out freight (freight in which the truck is loaded to its volume capacity but weighs less than the legal weight limit)
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From page 109... ...
98 TWIN TRAILER TRUCKS LTL general freight common carriers will be the primary users of twins. The estimates of total vehicle miles in twins, aside from the overly restrictive limitation to Interstates in one estimate, range from 12 to 14 percent of total combination vehicle miles.
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From page 110... ...
Use Characteristics and Cost 99 The 60 percent projection is also applied to all Other Western and Mountain states and to all Central and Plains states. Use in the East will lag behind that of other regions for some time.
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From page 111... ...
TABLE 3-10 Projected Vehicle Miles for All Combination Vehicles in 1990 Region Rural Interstate Primary Other Total Urban Interstate Primary Other Total Rural and Urban Total California Base 1,132 1,030 305 2,467 1,101 1,037 795 2,933 5,400 Change -2 -1 0 -3 -1 -1 0 -2 -5 Other Western and Mountain Base 2,501 1,529 692 4,722 493 773 511 1,777 6,500 Change -22 -11 -8 -41 -3 -5 -3 -11 -52 Central and Plains Base 10,452 9,290 2,033 21,776 5,345 1,871 2,409 9,624 31,400 Change -79 -70 -15 -164 -40 -18 -18 -72 -236 East No twins before 1983 Base 4,726 4,042 1,711 10,479 3,605 2,178 838 6,621 17,100 Change -30 -25 -11 -66 -30 -18 -8 -56 -122 With twins before 1983 Base 3,895 2,977 1,133 8,006 1,526 1,763 1,305 4,594 12,600 Change -32 -25 -9 -66 -9 -10 -8 -27 -93 U.S. total Base 22,704 18,863 5,874 47,442 13,251 7,628 5,861 25,558 73,000 Change -165 -132 -43 -340 -83 -52 -37 -168 -508 Norn: Mileage is given in millions of vehicle miles.
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From page 112... ...
TABLE 3-11 Projected Vehicle Miles for Twins in 1990 Region Rural Interstate Primary Other Total Urban Interstate Primary Other Total Rural and Urban Total California Base 392 349 66 807 253 285 153 691 1,498 Change +12 +11 +2 +25 +8 +9 +5 +22 +47 Other Western and Mountain Base 253 123 86 463 39 57 27 123 586 Change +227 +110 +78 +415 +35 +51 +25 +111 +526 Central and Plains Base 251 138 14 404 70 32 75 176 580 Change +799 +710 +155 +1,664 +404 +141 +182 +727 +2,391 East No twins before 1983 Base 0 0 0 0 0 0 0 0 0 Change +301 +258 +109 +668 +310 +187 +73 +570 + 1,238 With twins before 1983 Base 6 6 1 13 1 0 0 0 14 Change +325 +249 +95 +669 +92 +106 +79 +277 +946 U.S. total Base 901 616 167 1,686 363 374 253 991 2,677 Change + 1,664 + 1,338 +439 +3,441 +849 +494 +364 + 1,707 +5,148 NoTE: Mileage is given in millions of vehicle miles.
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From page 113... ...
102 TwIN TRAILER TRUCKS If there were no change in vehicle miles of twins, it is projected that total combination vehicle miles would increase to 73 billion by 1990 (Table 3-10)
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From page 114... ...
Use Characteristics and Cost 103 stitutes are available, but none is closely comparable with LTL in terms of both price and service attributes. The alternatives are discussed in the following paragraphs.
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From page 115... ...
104 TWIN TRAILER TRUCKS service qualities in return for a relatively small cost saving, because handling, the largest LTL cost item, is unaffected. Piggyback often cannot even offer a lower freight rate than LTL service on short- and mediumhaul shipments.
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From page 116... ...
Use Characteristics and Cost 105 proportion if the growth in LTL traffic is unaffected by the cost savings due to twins. If twins were to directly stimulate LTL growth by encouraging shifts from truckload carriers, the freight productivity benefit would be more than proportionately larger.
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From page 117... ...
106 TwIN TRAILER TRUCKS price elasticity of demand for LTL service is probably in the low range of these estimates, -- 0.5 to 0.0. The cost and travel projections of this chapter predict a 2 percent average cost reduction in LTL service and anticipate a 6 percent reduction in total miles of LTL line-haul travel as a result of use of twins (assuming that modal diversion and new traffic generation are negligible)
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From page 118... ...
Use Characteristics and Cost 107 at the approximate industry line-haul cost of $1 per mile, represents a gross line-haul cost reduction of $493 million. The savings will not be quite this large because twin trailer truck miles cost about 2 cents more per truck mile than the tractor-semitrailer miles they will replace.
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From page 119... ...
108 TWIN TRAILER TRUCKS total LTL general freight in twins in 1982. Thus handling costs in the general freight common carrier segment of the industry should decline on the order of 6 percent (0.665 x 0.09 = 0.059)
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From page 120... ...
Use Characteristics and Cost 109 Thus, line-haul and handling costs for the LTL general freight common carriers will decline by roughly $700 million (1985 dollars) by 1990, offset by an added annual capital cost of $200 million for equipment and terminals.
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From page 121... ...
110 Twmr TRAILER TRUCKS L Arbruzzese.
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From page 122... ...
Use Characteristics and Cost 111 the Bridge Formula (B) to Limit Gross Weight.
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From page 123... ...
4 Safety Impacts Nationwide use of twin trailer trucks has been controversial primarily because of fears that twins will cause more highway traffic accidents than the vehicles they replace. Concern for safety was the major justification used by states for laws banning twins and the principal argument supporting the legal challenges to implementing the 1983 federal law allowing nationwide use of twins.
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From page 124... ...
Safety 113 3. Any net changes in the operating environment -- how truck travel is distributed across the various types of roads, kinds of drivers and firms that operate trucks, and so forth -- that will result from increased use of twins.
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From page 125... ...
114 TwIN TRAILER TRUCKS those relating to the characteristics of the driver, the vehicle, and the environment (Table 4-1)
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From page 126... ...
Safety 115 TABLE 4-1 Some Factors Contributing to the Accident Pattern Category Factor I Category Factor Driver Education and training Experience Age Maturity and temperament Health and medical condition Fatigue Alcohol or drug use Vehicle Configuration and hitching Size Weight Power Cab style Body type Chassis design Crashworthiness Conspicuity Inspection and maintenance Human engineering Driver's cab environment Integrity of design and manufacture Cargo Weight Density Hazardousness Restraint and liquid slosh Loading pattern Environment Visibility Time of day Pavement surface condition Weather Temperature Traffic Volume Composition Speed Density Roadway Roadside features Geometric features Cross-sectional features Pavement surface features Roadway type Location Traffic control Intersections and interchanges Regulation and enforcement replacing? The evidence on physical differences between twins and tractorsemitrailers is analyzed in the sections on handling and stability and traffic operations that follow.
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From page 127... ...
116 TWIN TRAILER TRUCKS truckload carriers is identical in these respects to the less-than-truckload tractor-semitrailer traffic being replaced; however, the specialized nature of this industry segment complicates the comparison of accident rates for twins and other vehicles. Understanding the origins of observed differences in the safety performance of twins and of other combination trucks compared with that of other vehicles (related to how they are used and who uses them as well as to the mechanical characteristics of the vehicles)
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From page 128... ...
Safety 117 Traffic operations studies: These studies measure interactions among vehicles in the traffic stream, such as following distances, closure rates, speed differences, conflicts, and passing behavior. As with truck performance evaluations, the results can only suggest possible accident impacts.
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From page 129... ...
118 TwIN TRAILER TRUCKS offset inward from those of the leading wheels (Figure 4-1)
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From page 130... ...
Safety 119 Oscillatory sway: Trailers of articulated vehicles can oscillate in response to small perturbations while traveling on a straight road. Sway usually does not jeopardize vehicle control but may distress the driver of the truck or other motorists.
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From page 131... ...
120 TwIN TRAILER TRUCKS The amount of offtracking (measured in terms of the maximum lateral distance that the path of the rear tires diverges from the path of the front tires) is not influenced by vehicle width, although its consequences may be slightly exaggerated because of reduced clearances resulting from wider trucks.
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From page 132... ...
Safety 121 operating condition that may become rather prevalent with twins, however, involves the coupling of an empty trailer behind a full one to facilitate trailer movement between terminals. Under such conditions, the wheels on the empty rear trailer are severely overbraked and controllable braking is hampered.
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From page 133... ...
122 TWIN TRAILER TRUCKS or rolling over. Because the yaw stability level is lower on the shorterwheelbase tractors that are typically employed with twins, there may be a mildly greater potential for such unstable yaw response with twin trucks.
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From page 134... ...
Safety 123 Summary Points: Handling and Stability Twin trailer truck versus 45-ft semitrailer Twins show a pc response to evasive steering and impaired sensory feedback, superior low-speed offtracking; therefore, the rate of rear-tt rollover accidents is likely to be higher and the rate of intersec accidents to be lower. 48-ft versus 45-ft semitrailer 48-ft semitrailers exhibit w low-speed offtracking; therefore, the rate of intersection accid is likely to increase.
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From page 135... ...
124 TWIN TRAILER TRUCKS Aerodynamic buffeting: The likelihood of potentially hazardous aerodynamic buffeting as small vehicles approach and pass large trucks is increased as the size of the truck increases. Blockage of view: Large trucks can block the view of nearby motorists, thereby increasing the accident risk.
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From page 136... ...
Safety 125 Freeway Merging and Lane Changing The increases in truck length and weight that are anticipated are not likely to have a significant adverse effect on merging and lane changing. Past studies of the effects of trucks on merging and lane changing are inconclusive, although there are prior arguments that suggest that increased truck size and weight may have detrimental effects on the ease and safety of these maneuvers.
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From page 137... ...
126 Twm TRAILER TRUCKS Aerodynamic Buffeting The aerodynamic forces acting on a moving vehicle are suddenly altered when it overtakes or passes a large truck traveling in an adjacent lane. Unless appropriate steering adjustment is quickly made, the small vehicle may suffer a rapid and potentially hazardous lateral displacement.
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From page 138... ...
Safety 127 Summary Traffic operations are influenced by the presence of large trucks. Findings of prior studies, although not as conclusive as might be desired, indicate that twins,48-ft semitrailers, and 102-in, wide trucks can have generally adverse but marginal effects on traffic operations compared with 45-ft semitrailers.
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From page 139... ...
128 TwIN TRAILER TRUCKS conducted for this study concerning the relative severity of twin trailer truck accidents compared with accidents of other combination vehicles. Appendix F gives a literature review of truck accident rates; Appendix G gives the literature review of accident severity and an analysis of BMCS accident reports.
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From page 140... ...
Safety 129 not so narrow or unusual as to invalidate generalization from the reported results to other situations? Of the 14 studies reporting relative accident rates for twins and tractorsemitrailers that are reviewed in Appendix F, five stood out as studies that were the most nearly free from obvious methodological flaws, that made reasonable attempts to minimize the obscuring effects of the operating environment, and that reflect accident experience under conditions that are typical of many roads throughout the nation (Table 4-2)
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From page 141... ...
TABLE 4-2 Summary of Comparative Accident Rate Studies Involvement-Rate Ratio: Twins to Tractor-Semitrailers Vehicles and Years of Fatal Casualty All Study Roads Represented Data Accidents Accidents Accidents Chirachavala Nationwide, intercity van and O'Day (5) trailers, ICC-authorized carners 1977 - - 0.98° Glennon (6)
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From page 142... ...
Safety 131 and validation of similar day and night distribution of operations and similar driver age and operational experience indicate exceptional control for operating environment effects. Because this was a carrier-sponsored study, it might be subject to some skepticism; however, it did withstand close scrutiny in litigation.
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From page 143... ...
132 TWIN TRAILER TRUCKS or in different parts of the country according to state restrictions on twins use. Therefore, some significant differences almost certainly exist between the operating environments of twins and tractor-semitrailers in these data.
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From page 144... ...
Safety 133 and the urban exposure estimates are especially uncertain because entering and exiting traffic within the segments is presumably highest in urban areas (urban segments were each up to 70 mi long)
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From page 145... ...
134 TWIN TRAILER TRUCKS proportion of twins accidents were single-vehicle accidents (that is, involved no vehicle other than the truck) , a factor partially explaining the reduced overall severity of twins accidents.
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From page 146... ...
Safety 135 Summary Points: Accident Involvement Rate and Severity Twin trailer truck versus 45-ft semitrailer Twins accider rates in general appear slightly higher than those for tractor-semi trailers. There is no significant difference in accident severity.
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From page 147... ...
136 Twm TRAILER TRUCKS impacts of twins described in the preceding section are reasonable. Because this report includes estimates of both current travel and accident frequencies for twins and tractor-semitrailers, in principle it would appear to be possible to compute accident rates for each vehicle type and measure the safety impacts of twins directly.
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From page 148... ...
TABLE 4-3 Annual Multitrailer Fatal Accident Involvements, 1975-1985 Region 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985" California No. of involvements 83 101 89 95 134 89 94 73 105 96 88 Fraction of all combination-truck involvements .359 .461 .346 .352 .425 .295 .330 .289 .345 .267 .273 Other Western and Mountain No.
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From page 149... ...
138 TWIN TRAILER TRUCKS dents to 4.9 percent. Outside California, multitrailers involved in fatal accidents increased from an annual average of 62 (1.8 percent of all combinations involved in fatal accidents)
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From page 150... ...
TABLE 4-4 Annual Twin Trailer Truck Accident Involvements Reported to the Bureau of Motor Carrier Safety, 1976- 1985 Region 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 California No. of involvements 266 272 315 248 223 245 182 179 202 207 Fraction of all combination-truck involvements .266 .245 .234 .185 .186 .187 .147 .154 .147 .155 Other Western and Mountain No.
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From page 151... ...
140 Twm TRAILER TRUCKS bination involvements) to 1,250 in 1984 (4.1 percent)
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From page 153... ...
142 Twm TRAILER TRUCKS accidents, the separation appeared to have occurred as a consequence of some other event (for example, rollover of the rear trailer) during the accident, and did not appear to have contributed to causing the accident.
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From page 154... ...
TABLE 4-5 Involvement Rates for Single-Trailer and Multitrailer Combination Trucks and Other Vehicles from FHWA Accident Rate Monitoring Vehicle Miles of Travel (X 106) Fatal and Injury Involvements Involvement Rate (per 108 )
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From page 155... ...
144 TWIN TRAILER TRUCKS The results supply some further confirmation that serious safety problems are not arising from expanded use of twin trailer trucks. However, the involvement rates reported for both single-trailer and multitrailer combinations appear to be extremely low when compared with other sources of accident rate estimates, and the quantity of data is still small.
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From page 156... ...
Safety 145 TABLE 4-6 Truck Driver Opinion Survey: Respondent Profile All Amount of Twins Experience Respondents Low Medium High Age° (years) 49 51 49 47 Driving experiences (years)
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From page 157... ...
146 Twmi TRAThER TRUCKS TABLE 4-8 Truck Drivers' Comparisons of Ease of Operation of Twins and Tractor-Semitrailers Respondents Selecting "Semis Easier"' (%) Low Twins Medium Twins High Twins All Experience Experience Experience Operating on slick pavement 88 93 90 79 Operating under severe crosswinds 87 90 84 85 Controlling truck in emergency braking 86 92 88 78 Hitching and unhitching a trailer 85 100 83 73 Stopping quickly in emergency 81 90 79 72 Operating empty truck 80 87 81 73 Passing slow vehicles on two-lane road 71 80 69 64 Operating on very rough pavement 69 84 64 60 Operating truck having poorly adjusted brakes 65 79 72 43 Operating truck having worn or mismatched tires 63 68 65 56 Controlling loaded truck on long steep downgrade 63 67 66 54 Changing lanes on freeway 57 74 53 44 Passing slow vehicles on freeway 54 69 48 43 Operating on steep, curved off ramps 53 75 49 33 Merging onto freeway 45 60 38 37 Maintaining speed on long steep upgrade 37 51 34 26 Operating on city streets 33 61 24 14 Turning sharp corners 11 23 9 2 "The choices were "Twins Easier," "No Difference," "Semis Easier," and "Don't Know." sharp corners.
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From page 158... ...
Safety 147 TABLE 4-9 Truck Drivers' Comparisons of Frequency of Adverse Situations Respondents Selecting "More Often with Twins"6 (%) Low Twins Medium Twins High Twins All Experience Experience Experience Trailer swaying on open roadway 88 90 89 84 Magnified trailer movement in response to abrupt steering 81 80 84 80 Poor trailer load distribution interferes with handling 78 85 82 66 Following motorists hesitate to pass on two-lane roadways 73 82 66 70 Difficulty detecting quickly when trailer starts unusual motion 72 77 71 66 Hitch malfunction 67 76 61 64 A trailer feels like it is about to roll over 63 60 66 64 The truck feels like it is about to jacknife 62 60 67 59 Difficulty seeing rear of the truck 59 66 65 48 Brakes malfunction 51 53 56 43 Vehicles approaching on two-lane roads encroach on shoulder 43 58 39 33 Passing motorists pull in too quickly 39 53 29 33 "The choices were "More Often with Semis," "No Difference," "More Often with Twins," and "Don't Know." handling and stability research.
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From page 159... ...
148 TWIN TRAILER TRUCKS placing the heavier trailer in front) , proper alignment of all units in the combination, and adjustment, maintenance, or specification of brakes.
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From page 160... ...
Safety 149 The one available accident rate study that used accident and travel experience of a single trucking company and compared twins and tractorsemitrailers operating under identical conditions (6) tends to bear out twins' higher accident rate, although the difference measured was not statistically significant.
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From page 161... ...
150 TwIN TRAILER TRUCKS The extent to which carriers adopt the new equipment, The greater capacity of twins and larger tractor-semitrailers compared with the vehicles they are replacing and the extent to which carriers can utilize the increased capacity in practice, The effect of designated network restrictions on the road mileage between points served by the larger trucks, and Any new freight traffic attracted from rail to truck because of the efficiency gains from use of the larger trucks. For the most part, the carriers using twins will operate them exactly as they operated the tractor-semitrailers replaced by twins; however, net changes in the operating environment of combination-truck travel may occur because of two factors: Restrictions on routes where the longer trucks can travel, diverting some truck traffic away from poorer roads (though at the expense of increasing trip mileage)
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From page 162... ...
Safety 151 possible ratios of twins accident involvement rates to tractor-semitrailer rates established in this chapter, the net changes in annual highway accident losses by 1990 will be as follows: Relationship of Twins Involvement Rate to Tractor-Semitrailer Rate Net Change According to Percentage of TractorSemitrailer Miles Replaced by Twins 8 Percent 4 Percent Twins have same rate as tractor-semitrailers Fatalities (no.) - 30 - 15 Injuries (no.)
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From page 163... ...
152 TWIN TRAILER TRUCKS These estimates of safety impacts of changes in twins traffic depend on the following assumptions: 1. Combination-truck travel in 1990 is projected as fo11ows Percentage of Tractor-Semitrailer Miles Switched to Twins 8 4 No shift Miles (X 106)
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From page 164... ...
Safety 153 the uncertainties in the underlying accident rate and travel estimates suggest that a more complicated approach is not warranted. The final assumption in the foregoing -- that the accident rate for the travel diverted from tractor-semitrailers to twins is the same as the average for all tractor-semitrailer travel -- is particularly imprecise and may have the effect of exaggerating the impact of the shift to twins.
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From page 165... ...
154 TWIN TRAILER TRUCKS One-third of the additional twins travel will be on urban roads, and most of this will be on urban Interstates. As the literature review sum- marized earlier in this chapter showed, there are virtually no data or accident research results to support any inference about whether the relative safety of twins and tractor-semitrailers on urban roads is any different than that on rural roads.
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From page 166... ...
Safety 155 Summary Taken together, the evidence shows that twins probably have slightly more accident involvements per mile traveled than tractor-semitrailers operated under identical conditions at highway speeds. However, the 9 percent reduction in truck miles in operations that are converted from tractorsemitrailers to twins approximately offsets the higher rate of accident involvements per mile that twins exhibit, with the net result of very little change in the number of accidents.
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From page 167... ...
156 TwIN TRAILER TRUCKS Safety Comparison of Doubles versus Tractor-Semitrailer Operation. Bureau of Motor Carrier Safety, FHWA, U.S.
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From page 168... ...
5 Highway Condition, Operations, and Design Impacts The nationwide introduction of new, larger combination trucks, as authorized by the Surface Transportation Assistance Act of 1982, could potentially harm U.S. highways by accelerating their deterioration, reducing their operating efficiency, and requiring more costly designs and construction.
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From page 169... ...
158 Twmi TRAILER TRUCKS action between these trucks and other vehicles. In some circumstances, the effective highway capacity may be reduced and vehicle delay increased so that overall highway efficiency declines.
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From page 170... ...
Highway Condition, Operations, and Design 159 1. Load and vehicle characteristics Total gross vehicle weight (GVW)
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From page 171... ...
160 TwIN TRAILER TRUCKS imply fewer trips, which tends to compensate for any increase in pavement wear per trip, but pavement design relationships suggest that the incremental effect on pavement deterioration increases sharply as the load increases. For instance, when a single axle is loaded to 20,000 lb, it will do more than 12 times the damage than if it were loaded to 10,000 lb.
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From page 172... ...
Highway Condition, Operations, and Design 161 that is equivalent in pavement wear to one application of a different load and axle combination. Depending on the type of pavement distress selected, load-equivalency factors vary (1)
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From page 173... ...
162 TwIN TRAILER TRUCKS design procedures, such as those of the Asphalt Institute (4) for asphalt pavements and those of the Portland Cement Association (5)
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From page 174... ...
2.0 01 -J U, w 1.5 0 0 0 00 load spread o single axles 0.5 0 w U- U- U, Highway Condition, Operations, and Design 163 D 10 .
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From page 175... ...
164 TWIN TRAILER TRUCKS 40 percent on flexible pavements and about 55 percent on rigid pavements (Appendix I)
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From page 176... ...
Highway Condition, Operations, and Design 165 Therefore, even if twins carried the same loads and had the same axle arrangement as the tractor-semitrailers they replace, they would increase pavement wear somewhat because of less uniform loading. Effect of Axle Arrangement Both twins and the tractor-semitrailers they replace rely on four axles under the trailers (or trailer)
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From page 177... ...
166 TWIN TRAILER TRUCKS basis of the AASHTO load-equivalency factors, a 60,000-lb twin trailer truck will have a 40 percent greater effect on flexible pavement wear than a 60,000-lb tractor-semitrailer with the same weight distribution on its axles. On rigid pavements, however, twins reduce the effect on pavement wear by 18 percent (Appendix I)
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From page 178... ...
Highway Condition, Operations, and Design 167 TABLE 5-1 Average ESALs for Twins and Tractor-Semitrailers ESALs by Pavement Average Gross Type Truck Type Vehicle Weight (Ib) Flexible Rigid Van Trailer Carrying Full or Partial General Freight Payload Tractor-semitrailer 53,900 0.80 1.15 Twin trailer 60,100 1.67 1.50 Empty Van Trailer Tractor-semitrailer 29,700 0.11 0.09 Twin trailer 30,800 0.14 0.09 Nora: Data are from TRB summaries of most recent state weigh station data reported to FHWA up to 1982; ESALs calculated for each axle are weighted by using AASHTO design procedures (3)
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From page 179... ...
168 TwIN TRAILER TRUCKS portion of the eastern region in which twins were prohibited before 1983. With regard to road class, the increase is concentrated on Interstates, which account for about 40 percent of the total increase.
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From page 180... ...
Highway Condition, Operations, and Design 169 ment rehabilitation costs, 70 percent can be attributed to load as measured by ESALs, a figure that matches the conclusions of the last FHWA highway cost-allocation study.5 Thus, of the $1.6 billion, about half, $800 million, is spent on load-related Interstate pavement rehabilitation. FHWA statistics indicate that the total annual loading on rural Interstates amounts to about 50 billion ESAL-mi, for an average load-related pavement rehabilitation cost of 1.6 cents/ESAL-mi.
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From page 181... ...
170 TWIN TRAILER TRUCKS accelerate pavement deterioration, principally because their added cubic capacity and tare weight will tend to increase average.GVWs. No differential effects due to load distribution or axle configuration are expected.
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From page 182... ...
Highway Condition, Operations, and Design 171 Because most recently constructed bridges and culverts were designed to withstand forces greater than those produced by trucks loaded to the legal GVW limit, imposition of higher (or different) loads through modest changes in legal truck weights and configurations will not appreciably increase the risk of collapse (catastrophic failures of new bridges have occurred only when bridges were grossly underdesigned or improperly constructed)
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From page 183... ...
172 Twm TRAILER TRUCKS Bridge Superstructures Longitudinal bridge girders, trusses, or arches and transverse floor beams that form the superstructure are designed to withstand forces that would be induced by hypothetical truck loading patterns. Provided that actual forces from in-service trucks do not exceed these design levels, no practical effect on bridge service life exists, unless truck volumes are very much greater than those expected at the time of design.
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From page 184... ...
Highway Condition, Operations, and Design 173 constructed with less than 2 ft of cover, an uncommon practice on Interstate and major primary highways. Detailed explanations of these findings are presented in Appendix J
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From page 185... ...
174 TWIN TRAILER TRUCKS surfaces. In this study available evidence was examined concerning the possibility of such impacts on the following: Full-strength paved shoulders, constructed with the same load-bearing capacity as that of adjacent lanes; Gravel, turf, and thinly paved shoulders, constructed without the load-bearing capacity of adjacent lanes; and Roadside appurtenances, such as signs, guardrails, and curbs.
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From page 186... ...
Highway Condition, Operations, and Design 175 HIGHWAY OPERATIONS A primary consideration in the provision and upgrading of streets and highways, in addition to structural, geometric, and roadside features, is the ability to accommodate travel without excessive congestion and delay. Each highway is limited in its ability to efficiently handle traffic by design features such as the number and width of lanes.
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From page 187... ...
176 TwiN TRAILER TRUCKS tractor-semitrailers versus twins, 45-ft versus 48-ft semitrailers, or trucks 96 in. wide versus those 102 in.
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From page 188... ...
Highway Condition, Operations, and Design 177 on superior highways, where large trucks have generally been found to be less disruptive. In summary, the increased weight of twins, 48-ft semitrailers, and 102in.
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From page 189... ...
178 Twir' TRAILER TRUCKS on design standards and practices to see whether they are changing or are likely to produce different designs to better accommodate twins and other wider and longer vehicles. Historically state highway agencies have assumed primary responsibility for adopting highway standards, but generally individual states have been heavily influenced by recommended policies developed by AASHTO.
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From page 190... ...
Highway Condition, Operations, and Design 179 can be tailored to specific truck characteristics and loadings, it probably will not change immediately because of the use of twins. Bridge Design Practices Neither design standards and procedures nor the resultant designs for bridges and other highway structures need change because of twins or other new vehicle types.
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From page 191... ...
180 TWIN TRAILER TRUCKS when turning (19)
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From page 192... ...
Highway Condition, Operations, and Design 181 Further discussion of specific relationships between geometric design and truck characteristics is presented in Appendix L Speéial Highway Rehabilitation or Reconstruction Activities About one-fifth of the state highway agencies surveyed indicated that they anticipated special rehabilitation or reconstruction work on the designated network or access routes to that network.
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From page 193... ...
182 TwIN TRAILER TRUCKS Noise Emissions Vehicle noise emissions generally increase with increases in GVW, the number of wheels (or axles) , and speed.
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From page 194... ...
Highway Condition, Operations, and Design 183 Summary Points: Environment Twins versus 45-ft semitrailers Twins will reduce exposure noise and air pollutant emissions nationwide because of vehic mile reduction. 48-ft versus 45-ft semitrailers 48-ft semitrailers will redu exposure to noise and air pollutant emissions nationwide becat of vehicle-mile reduction.
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From page 195... ...
184 TWIN TRAILER TRUCKS H.S. Cohen and J.H.
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From page 196... ...
6 Long-Term Monitoring Government decisions concerning truck size and weight limits, truck safety regulation, and truck taxation have important consequences. These decisions directly influence the productivity of the $100 billion-a-year trucking industry, the public's return on its $30 billion annual expenditure on highway construction and maintenance, and the safety of the roads on which large trucks travel.
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From page 197... ...
186 TWIN TRAILER TRUCKS cial studies on the characteristics and impacts of heavy-truck traffic that would be valuable for helping to answer current policy questions relating to truck size and weight. These recommendations for long-term monitoring are an integral part of the committee's response to Congressional direction for the twin trailer truck study.
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From page 198... ...
Long-Term Monitoring 187 required are defined. The basic uses of truck data, the pending and likely future truck size and weight regulatory issues that will require analysis, and the categories of data required for these applications are identified.
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From page 199... ...
188 TwIN TRAILER TRUCKS Although the multiple applications of truck data must be kept in mind in designing any new data programs, in this chapter data needs for evaluating existing or new truck size and weight limits are primarily addressed. Current Truck Size and Weight Issues Truck size and weight limits have changed frequently in the past, almost always in the direction of allowing larger sizes, and this trend appears likely to continue.
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From page 200... ...
Long-Term Monitoring 189 Data Requirements Analyzing the costs and benefits of a change in truck size and weight limits requires predicting the impact of the change on roadway wear, congestion and the demand for new capacity, road design requirements, the frequency of traffic accidents, and the cost of transporting freight. To predict these effects, the analyst needs, first, information about current truck travel characteristics and an estimate of how travel will change under the new limits and, second, an understanding of the relationships between truck travel and the impact categories -- highway performance, safety, and freight costs.
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From page 201... ...
190 TWIN TRAILER TRUCKS industry impacts of size and weight regulations; however, the available data are incomplete. The major existing programs include the following: The FHWA-sponsored state Vehicle Classification Count and Truck Weight Study Program (12)
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From page 202... ...
Long-Term Monitoring 191 Bureau of Motor Carrier Safety accident reports (20) , which are required of all interstate motor carriers for accidents involving injuries or property damage over $2,000; Fatal Accident Reporting System (21)
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From page 203... ...
192 TWIN TRAILER TRUCKS Trucking Industry Efficiency The major source reporting data on costs and finances in the trucking industry is the Interstate Commerce Commission, which collects financial and operating data on regulated trucking firms. The data for individual companies are available to the public.
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From page 204... ...
Long-Term Monitoring 193 destination, and route traveled. Data-related difficulties included the following: The total number of combination-truck miles was uncertain for the same reason as that in the cost allocation study.
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From page 205... ...
194 Twns TRAILER TRUCKS states a program for conducting classification counts and weighing at a statistically valid sample of sites linked to roadway and other traffic data at the same sites. Data specifications are compatible with capabilities of automatic counting and weighing devices.
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From page 206... ...
Long-Term Monitoring 195 the federal level on data programs relevant to truck size and weight issues, and larger amounts are spent by state governments. Truck size and weight issues -- for example, relationship of size and weight to accident rates or to pavement wear -- are certainly complex, but analyses of these questions have historically been constrained simply by absence of data or flaws in research design rather than by these inherent complexities.
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From page 207... ...
196 TwIN TRAILER TRUCKS Federal Truck Data Collection Responsibility As truck size and weight policy becomes increasingly a federal concern, the U.S. Department of Transportation (DOT)
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From page 208... ...
Long-Term Monitoring 197 The National Highway Traffic Safety Administration and FHWA, to ensure that the former's large-scale safety monitoring programs (the National Accident Sampling System and the Fatal Accident Reporting System) are applicable to the safety analysis needs of the federal highway programs; DOT and the states (possibly through participation of the American Association of State Highway and Transportation Officials)
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From page 209... ...
198 TwIN TRAILER TRUCKS Extended Monitoring of Twins FHWA should continue the short-term twin trailer truck monitoring activities begun in this study for at least 4 years until the trucking industry has more fully adjusted to the new federal size limits. Specifically, tracking of twins traffic by carrier class, body type, and road class and of twin trailer truck accident frequency should continue by using existing federal data programs and the special programs in place in several states.
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From page 210... ...
Long-Term Monitoring 199 twins safety alone is not warranted. Although the evidence on the relative safety of twins and tractor-semitrailers is not absolutely conclusive, this narrow issue does not have the highest priority among truck safety questions.
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From page 211... ...
200 Twm TRAILER TRUCKS P.M.
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From page 212... ...
7 Summary and Conclusions This concluding chapter will draw together the estimates presented earlier of the various benefits and costs of nationwide legalization of twins in the three areas of trucking productivity, safety, and pavements and other highway impacts. The summary will show that although greater use of twins has reduced the cost of shipping goods by truck, it has also increased some public costs, particularly road pavement wear.
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From page 213... ...
202 TwIN TRAILER TRUCKS twins were prohibited in 14 states, all in the East. However, they were common in other parts of the country and accounted for about 4 percent of all miles of combination-truck travel.
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From page 214... ...
Summary and Conclusions 203 another vehicle, the rear trailer may exhibit an exaggerated side-to-side motion often described as a crack-the-whip effect. This phenomenon is known technically as rearward amplification and may result in rollover of the rear trailer.
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From page 215... ...
204 TwIN TRAILER TRUCKS less uniformly than tractor-semitrailer loads, and they have four single axles under their trailers rather than the two pairs of closely spaced axles (tandem axles) that support the semitrailer in a tractor-semitrailer.
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From page 216... ...
Summary and Conclusions 205 TABLE 7-1 The Effects of Nationwide Legalization of Twin Trailer Trucks Use and productivity gains by 1990 Twins travel 7.8 billion mi/year -- 10.8 percent of all combination truck travel; up from 3.7 percent in 1982; less-than- truckload carriers account for nearly all growth Net combination-truck 500 million milyear-0.7 percent of total travel savings combination-truck travel Freight cost savings $500 million/year (from reduced freight handling and reduced mileage) -0.5 percent of intercity truck freight costs Highway safety impacts Handling of twins Severe rear-trailer sway in response to abrupt steering maneuvers Driver's ability to sense impending trailer instability impaired Rear wheels deviate more from track of front wheels at highway speed; tracking is better than that of tractor-semitrailer at city street speeds Accident rates Twins probably have slightly higher rates than similarly operated tractor-semitrailers on rural roads Accident severity No difference between twins and tractor-semitrailers under similar conditions Systemwide safety Reduction in travel offsets slightly higher accident rate; therefore net overall effect on safety is small Impacts on pavement and other highway features Pavement Twins cause 90 percent more pavement wear on asphalt pavement and 20 percent more on portland cement concrete than tractor-semitrailers carrying the same freight Systemwide pavement cost $50 million/year increase in pavement rehabilitation cost by 1990-2 percent of total rehabilitation expenditures Wear of structures, traffic Impacts all negligible flow, and geometric design requirements from twins will not all accrue to trucking companies.
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From page 217... ...
206 Twir. TRAILER TRUCKS previously drove a tractor-semitrailer, the accident risk is slightly greater unless the driver is well aware of the particular handling characteristics of twins and is skilled at allowing for these characteristics.
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From page 218... ...
Summary and Conclusions 207 be a serious concern. Public awareness of truck safety and media attention devoted to the topic are intense, and thewidely held perception that trucks are a major and growing part of the highway accident problem has been a principal basis for opposition to liberalized truck size rules.
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From page 219... ...
208 Twm TRAILER TRUCKS the late 1970s and recently both correspond to periods of economic expansion when more inexperienced drivers are hired, older equipment is used more, and traffic volumes are increasing. Thus the number of combination-truck accident involvements reported in 1984, although above the previous year's level, was about the same as in the peak year of 1979.
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From page 220... ...
Summary and Conclusions 209 available throughout the industry so that other carriers can give full consideration to adopting them. In the longer term, alternative dolly designs for coupling twin trailers by using double drawbars could provide an opportunity for improving rear-trailer stability.
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From page 221... ...
210 TwmT TRAILER TRUCKS also stipulating that the axles be full width to assure that the potential stability benefits would accrue. Regulators should consider, as a general rule, that changes to truck size regulations that allow increased vehicle productivity be coupled to requirements to upgrade the safety of the vehicles as well, using the potential productivity gain as an incentive for safety gains.
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From page 222... ...
Summary and Conclusions 211 Monitoring Truck Use and Its Effects Efforts to improve truck safety will be hindered if reliable information is not available to identify safety problems, analyze their causes, and measure the success of actions taken to solve them. Chapter 6 contains recommendations for enhancing capabilities for monitoring truck use and impacts.
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From page 223... ...
Glossary of Trucking and Highway Terms A number of special terms referring to trucks, the trucking industry, and highways that are used frequently in this report are defined. Trucking terms are not standardized, so, for example, a given piece of equipment may be known by different names in different parts of the country or among different groups involved in the industry.
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From page 224... ...
213 Straight truck Three-axle 41014 tractor-semitrailer Five-axle tractor-semitrailer Twin trailer truck #=UjV% Straight truck with full trailer FIGURE G-1 Truck types. nection of the dolly is usually a single eye that attaches to the pintle hook on the rear of the front trailer.
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From page 225... ...
214 FIGURE G-2 Dolly being connected to rear trailer. FIGURE G-3 Converter dollies: A-dolly (most common in United States)
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From page 226... ...
215 DOUBLE TRAILER TRUCK or DOUBLE. A combination truck with two trailers, usually both semitrailers.
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From page 227... ...
216 I-. 60'-O" J-12'-O" I- 48'-0" L1i FIGURE G-4 Typical dimensions of a tractor-semitrailer with 48ft semitrailer.
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From page 228... ...
65'-O" I 27-O" 217 bles with seven or more axles. The most common pre-1983 twin trailer truck consisted of a two-axle cab-over-engine tractor drawing two single-axle semitrailers, each 27 ft long and coupled by a singleaxle dolly (Figure G-5)
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From page 229... ...
218 67'-O" 8'-0" _- I- 28'-0" :y.oL 28'-0" -I 10-O"_ -I- 21'-2" I- 8'-6" 1- 22'-6" 62'-2" FIGURE G-6 Typical dimensions of twin trailer truck permitted under 1983 federal law. [NOTE: Use of conventional tractor adds 3 to 7 ft to total length.]
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From page 230... ...
219 in quality. Eighty percent of non-Interstate primary miles are twolane undivided roads, and 32 percent have lane widths less than the 12-ft Interstate standard.
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From page 231... ...
220 for more efficient line-haul travel or where, in the reverse operation, consolidated shipments destined for smaller terminals are sorted and routed.
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From page 232... ...
221 aspects of trucking businesses. Since the Motor Carrier Act of 1980, regulation in all these areas has been considerably relaxed.
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From page 234... ...
Appendix A Sections of Highway Laws Relating to Truck Size and Weight SURFACE TRANSPORTATION ASSISTANCE ACT OF 1982 Public Law 97-424, 97th Congress, Jan.
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From page 235... ...
224 eluding enforcement tolerances, and a tandem axle weight of thirtyfour thousand pounds, including enforcement tolerances, and with an overall maximum gross weight, including enforcement tolerances, on a group of two or more consecutive axles produced by application of the following formula: = 500 LN (N 1 + 12N + 36) where W equals overall gross weight on any group of two or more consecutive axles to the nearest five hundred pounds, L equals distance in feet between the extreme of any group of two or more consecutive axles, and N equals number of axles in group under consideration, except that two consecutive sets of tandem axles may carry a gross load of thirtyfour thousand pounds each providing the overall distance between the first and last axles of such consecutive sets of tandem axles is thirty-six feet or more: Provided, That such overall gross weight may not exceed eighty thousand pounds, including all enforcement tolerances, except for those vehicles and loads which cannot be easily dismantled or divided and which have been issued special permits in accordance with applicable State laws, or the corresponding maximum weights permitted for vehicles using the public highways of such State under laws or regulations established by appropriate State authority in effect on July 1, 1956, except in the case of the overall gross weight of any group of two or more consecutive axles, on the date of enactment of the Federal-Aid Highway Amendments of 1974, whichever is the greater.
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From page 238... ...
227 institute any civil action for injunctive relief as may be appropriate to assure compliance with the provisions of this title. Such action may be instituted in any district court of the United States in any State where such relief is required to assure compliance with the terms of this title.
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From page 239... ...
228 TANDEM TRUCK SAFETY ACT OF 1984 Public Law 98-554, 98th Congress, Oct.
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From page 240... ...
229 termination, the Secretary shall consider any possible alternative route that serves the area in which such segment is located. The Secretary shall make such determination within a period of 120 days after the date of receipt of notification from a Governor under paragraph (1)
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From page 241... ...
230 "(3) The Governor shall transmit with such notification specific evidence of safety problems that supports such determination and the results of consultation regarding any alternative route under paragraph (2)
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From page 242... ...
231 (b) Section 411(c)
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From page 243... ...
232 (2) by striking out the period at the end thereof and inserting in lieu thereof the following: "and for any truck tractor-semitrailer combination in which the semitrailer has a length not to exceed 28 1/2 feet and which generally operates as part of a vehicle combination described in section 411(c)
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From page 244... ...
Appendix B Implementation of the Designated Network A summary of the extent, conditions, and administration of the network of roads open to twins and longer and wider semitrailers is given. The characteristics of this network of roads have a dominant influence on the productivity, safety, and highway impacts of the new federal size limits.
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From page 245... ...
234 When enacting legislation to comply with the federal limits, several states chose to uniformly apply the federal limits on all state-administered roads or on all public roads (subject in many states to local government restrictions) , presumably motivated by the complexity of administering two distinct sets of truck size limits for different collections of roads.
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From page 246... ...
235 TABLE B-i Federally Designated Network Other Federal-Aid Primary (FAP) Roads Designatedb Percent of All Region Interstate Miles0 Miles Other FAP Miles California 2,400 1,900 17 Other Western and Mountain 9,200 27,800 72 Central and Plains 16,700 87,400 74 East No pre-1983 twins 8,700 7,800 15 Pre-1983 twins 5,600 11,000 32 U.S.
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From page 247... ...
236 ; 90-100 percent FIGURE B-i Fraction of non-Interstate federal-aid primary road miles included in federally designated network. Adding the state-designated routes to the federally designated network increases from 15 to 23 the number of states with virtually all federal-aid primary road miles open to twins 65 ft long and 102 in.
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From page 248... ...
TABLE B-2 Federally Designated and Additional State-Designated Through Roads Open to Twins 102 in. Wide and 65 ft Long by Road System, 1985 Miles (X 1,000)
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From page 249... ...
TABLE B-3 Federally Designated and Additional State-Designated Through Roads Open to Twins 102 in. Wide and 65 ft Long by Region, 1985 Miles (x 1,000)
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From page 250... ...
239 FIGURE B-2 Fraction of all public roads open to twins 102 in. wide and 65 ft long in 1985.
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From page 251... ...
240 LL_Z_ u out u percent t•J 90- 100 percent FIGURE B-3 Fraction of all public roads practically open to combinations with a semitrailer 102 in. wide and 48 ft long in 1985.
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From page 252... ...
TABLE B-4 Characteristics of the Federally Designated Network and Primary Portions of State-Designated Networks Open to Twins Non-Interstate Lane Width Non-Interstate Rigid Pavement (% of miles) |
From page 253... ...
242 The fraction of designated miles with lanes less than 12 ft wide is fairly uniform among the regions, but lanes less than 11 ft wide are more prevalent (as a fraction of total designated miles) in the East (Table 134)
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From page 254... ...
243 ADMINISTRATION Federal Federal responsibilities for administration of the designated network are limited to the initial selecting of routes, modifying of the network, enforcing state compliance with the federal law's reasonable-access requirement, and overseeing state activities relating to the roads that are funded with federal aid. Additions to the federal network require approval of the state in which they are located, and states can petition to have segments of the Interstate deleted.
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From page 255... ...
244 With the creation of the federal network, each state was faced with the problem of administering two sets of size and weight rules, each applicable to separate portions of its highway system, unless the state chose to open all its roads to the federally authorized vehicles. Thirteen states have chosen the latter course, and 37 are enforcing two sets of limits.
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From page 256... ...
245 Special maintenance activities on the designated networks are described in Chapter 5; provisions for access routes are described in the following section. STATE ACCESS PROVISIONS The 1983 act provided that no state could deny reasonable access between the designated network and terminals and facilities for food, fuel, repair, and rest.
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From page 257... ...
TABLE B-5 State Access Provisions Region and State General Access Limit (miles) Other Provisions East: no pre-1983 twins Alabama 1 As of May 1985 the state legislature had taken no action on reasonable access; access is from (temporary)
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From page 258... ...
Region and State General Access Limit (miles) Other Provisions New York 0.3 Special requests for access beyond limit Tennessee - Special requests (shortest reasonable route)
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From page 259... ...
TABLE B-5 continued Region and General Access State Limit (miles) Other Provisions Arizona Unlimited - California 0.5 0.5 mi for services from designated interchanges; terminal access by request to local agency Colorado Unlimited - Hawaii Unlimited - Idaho Unlimited Kingpin to rear axle restriction of 39 ft for all semitrailers except on federally designated network Montana Unlimited - Nevada Unlimited Local jurisdiction restrictions may apply, but none is in effect New Mexico 20 - Oregon - Application to local agency (restrictions are rare in practice)
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From page 260... ...
TABLE B-6 States Classified by Type of Access Rule Employed Type of Access Rule Region Other Western California and Mountain Central and Plains East No Pre-1983 Twins Pre-1983 Twins All miles or virtually all Arizona Kansas Mississippi miles open to federally Hawaii Nebraska authorized vehicles Montana Ohio Texas Some restrictions in Colorado Arkansas North Carolina Maryland principle, but very Nevada Indiana broad access rule or Oregon Oklahoma very localized Utah South Dakota restrictions only Washington Wyoming Access by distance limit California Illinois Connecticut New York and by special Iowa Georgia Florida application North Dakota Maine Pennsylvania - - Rhode Island - South Carolina Vermont Virginia West Virginia Access within a specified Alaska Louisiana Alabama Kentucky ditance limit only - New Mexico Michigan -Missouri Wisconsin Access by ispecial - Idaho Minnesota District of Columbia Tennessee application only Massachusetts Delaware New Hampshire New Jersey
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From page 261... ...
250 To whom the access privilege applies: In eight states only the person who applies for access has permission to use the route; in the other states with access application procedures, once access is approved, the route is open to all vehicles. Effective duration of access permission: In four states access permission may be for a limited time only, and in the District of Columbia access is by trip permit only.
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From page 262... ...
251 Access routes granted through special application tend to be short, averaging 3.3 mi each in the five states for which data are available. Access routes are most commonly arterial roads in rural areas, although, as would be expected, in states where the designated network is extensive, access routes are more likely to be local streets and roads in urban areas.
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From page 263... ...
252 to use them as much as possible, taking circuitous routes to avoid gaps in the network and sometimes violating restrictions. Initial disputes between states or local governments and truck operators in many areas have been resolved over 3 years of experience with the new size and weight rules through negotiation and increased familiarity and acceptance of twins and larger tractor-semitrailers.
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From page 264... ...
253 The restrictions have slowed introduction of twins in some areas, but this impact is lessening with time. Even in the Northeast, where restrictions are the most stringent and twins use is lightest, carriers cite the characteristics of the freight markets they serve rather than state regulations as the primary factor determining how much they use twins.
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From page 265... ...
Appendix C Use Characteristics and Cost Impacts: Literature Review The economic strength of the incentives to individual trucking firms of switching from tractor-semitrailers to twin trailer trucks is an important determinant of the potential growth in the use of twins. The literature is reviewed from the aspects of (a)
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From page 266... ...
255 Two 28-ft trailers can hold four "marks," whereas a semitrailer can hold only two. (A mark is a single shipment, usually weighing 5,000 to 10,000 ib, that can be delivered without handling over a loading dock.)
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From page 267... ...
256 dividual stores rely on twins partly because of the operating efficiencies due to reduced handling. Cost Advantages Shipping rates in the West and in firms with higher percentages of twins were not lower than those in comparable regions or firms, and as a result the Fay/TDS researchers were unable to document shipper savings.
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From page 268... ...
257 JUNG (3) Jung gave a brief essay on the economic benefits of twin trailer trucks and pointed to their two basic advantages: the flexibility of reducing the handling of small shipments and the greater cubic capacity for lighterdensity products.
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From page 269... ...
TABLE C-i Fully Allocated Line-Haul Cost Estimates, 1981(4) Cost Estimates (cents/mile)
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From page 270... ...
259 and an overall length of 65 ft would be permitted nationwide. The analysis was made for specific traffic corridors and within specific regions in order to identify the effects on carrier costs in those regions with either lower permitted gross vehicle weights (73,280 ib)
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From page 271... ...
260 Cost Advantages By estimating the increase in average payload weight and relying on the line-haul cost estimates in Table C- 1, Kolins estimated that in the Northeast, Southeast, and Southwest, the combined effect of increasing gross vehicle weights to 80,000 lb and allowing twin trailer trucks nationwide would be to reduce line-haul costs between 16.6 and 19.5 percent/tonmile (Table C-2)
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From page 272... ...
261 portation. The network model has one node in each state except in California, Texas, Pennsylvania, and New York, which each have two.
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From page 273... ...
262 In addition, the shift of commodities to various alternative truck types was assumed to parallel the distribution of such shipments in areas of the country where they were already permitted. Thus the distribution of shipments by truck type in the West was used as a basis for estimating the shifts of commodities to twin trailer trucks nationwide.
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From page 274... ...
263 Cost Reductions The cost advantage of shifting to twins was estimated by first developing average carrier costs from 1977 Interstate Commerce Commission statistics. Private carrier costs were estimated with data collected from individual.
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From page 275... ...
264 size and weight limits effective in 1981 would not filter through the fleet until 1985. The use of 1985 as a base year served mainly to provide a hypothetical future year that allows some time for the industry to adapt to changes in federal and state regulations.
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From page 276... ...
265 miles of travel to 14.5 percent. The share of general commodity LTL travel would increase to 66.4 percent (Table C-4)
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From page 277... ...
The forecasts were produced by developing a network data base of freight shipments, estimating the cost of each shipment for alternative truck configurations, and assigning the shipments to the least-cost legal configuration (assuming that carrier cost savings would be passed on to shippers)
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From page 278... ...
267 Carriers Benefitting The General Accounting Office estimated that 44 percent of all interstate ton-miles by LTL carriers before 1982 was constrained by the physical size of trailers. This estimate was derived from the projections of travel and commodity density by the Truck Size and Weight Study (10-13)
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From page 280... ...
Me J Merge!
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From page 281... ...
Appendix D Truck Handling and Stability: Literature Review A review and evaluation are presented of research on the safety-related handling and stability properties of large trucks that may be different for twins or for longer, wider semitrailers than for conventional combination trucks. The properties reviewed are Offtracking, Response to steering, Sensory feedback, Braking, Oscillatory sway, Rollover in steady turns, Yaw stability in steady turns.
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From page 282... ...
271 offtracking. At low speeds, the rear tires track inward, toward the center of the curved path.
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From page 283... ...
272 40.0 S 30.0 Triple (COE) ZDouble (COE)
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From page 284... ...
273 apparently has not been documented in the literature, presumably highspeed (outward) offtracking is very slight for all the vehicles considered here on a properly superelevated curve at normal operating speeds.
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From page 285... ...
274 assumed to be traveling at constant velocity, were subjected to a stepped steering input, and the lateral dynamic response (primarily the yaw angle of the rear trailer body) was simulated.
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From page 286... ...
275 Ervin et al.
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From page 287... ...
276 Direct comparisons of twins and tractor-semitrailers, of longer and shorter tractor-semitrailers, and of wider and narrower combination trucks have yet to be undertaken. Nevertheless, sensory feedback is almost certainly more seriously impaired for twins than for tractor-semitrailers.
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From page 288... ...
277 An FHWA research program has provided limited data for comparing the relative braking performance of twins and tractor-semitrailers. Field tests of in-use vehicles simulated emergency stopping from a speed of 20 mph: the measure of merit was the distance traveled from the point of brake application to the point of rest.
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From page 289... ...
278 TABLE D-1 Stopping Distances from Utica Testing (13) Twins Tractor-Semitrailer Gross Gross • Vehicle Stopping Vehicle Stopping Braking Pavement Speed Weight Distance Weight Distance System Condition (mph)
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From page 290... ...
279 Ervin et al.
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From page 291... ...
280 use of radial or worn tires, short trailer wheelbases, loading that produces either an empty rear trailer or one that is more heavily loaded than the leading trailer, and high truck speeds (17)
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From page 292... ...
28 1 YAW STABILITY IN STEADY TURNS Yaw instability or divergence describes a condition in which the heading of a turning vehicle increasingly diverges from the desired direction of travel, typically resulting in spinout for straight trucks and jackknifing for articulated vehicles. The process can be described in the following way.
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From page 293... ...
282 and Dimensions, by the Road and Transportation Association of Canada, may shed further light on the handling properties of twins and the effect of dolly design. REFERENCES Review of Safety and Economic Aspects of Increased Vehicle Sizes and Weights.
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From page 294... ...
283 W.R.J. Mercer, J.R.
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From page 295... ...
Appendix E Traffic Operations: Literature Review The efficiency and safety of the flow of traffic on a road may be altered by the presence of large trucks. The literature is reviewed that examines the effects of trucks on eight important factors influencing traffic flow: speed, passing, freeway merging and lane changing, splash and spray, aerodynamic buffeting, blockage of view, lateral placement, and level of service and capacity.
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From page 296... ...
285 The magnitude of truck speed reduction on upgrades is dependent not only on truck performance but also on driver skill, entry speed, and the length and severity of grade (6,8-11)
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From page 297... ...
286 nounced -- loaded tractor-semitrailers traveled approximately 13 mph faster than loaded twins. Differences in the speed performance of twins and tractor-semitrailers are attributed to possible differences in their weights, aerodynamic properties, and available horsepower.
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From page 298... ...
287 In summary, large trucks travel more slowly than other vehicles on both upgrades and the more severe downgrades. Although the upgrade effect reflects reduced performance due primarily to larger weights, speed reductions on downgrades result from a deliberate attempt by the driver to maintain vehicle control.
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From page 299... ...
288 the primary factor is the reduction of passing opportunities due to the presence of opposing traffic, grades are frequently more severe on twolane highways and sight-distance restrictions assume a critical role in passing behavior. Extensive investigations of the behavior of automobiles in passing large trucks on two-way, two-lane rural highways have been reported by Troutbeck (23-25)
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From page 300... ...
289 of the faster-moving vehicles are delayed, both because of reduced opportunity to change lanes and because of temporary lane blockage as increasing numbers of trucks pass other trucks. When there is an acceptable gap, trucks have been observed to pass slower-moving vehicles even though other traffic in the passing lane will be delayed (28)
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From page 301... ...
290 passing times and distances due to small length differentials in comparison with conventional tractor-semitrailers are likely to be offset by a reduction in the number of trucks necessary to transport a given cargo quantity. The primary detrimental effect of both twins and 48-ft tractor-semitrailers derives from performance decrements resulting from larger weight/power ratios.
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From page 302... ...
291 corresponding percentages for twins and triples were 15 and 45 percent, respectively (29)
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From page 303... ...
292 to be superior to the tractor-semitrailer by each of seven observers, but it was judged to be the best among the 11 different trucks that were evaluated. On the basis of tests conducted at Madras, Oregon, in 1974, the amount of spray generated by twins has also been reported to be less, by approximately 20 percent, than that generated by a conventional tractor-semitrailer (33)
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From page 304... ...
293 AERODYNAMIC BUFFETING The aerodynamic forces acting on a vehicle are suddenly altered when the vehicle passes a large truck traveling in an adjacent lane, whether in the same or the opposite direction. Unless appropriate steering adjustment is made, a potentially hazardous lateral displacement may result.
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From page 305... ...
294 accident information is available with which to assess the possible gravity of this problem. The few studies that have examined truck blockage of view have focused on model development and an analysis of the effects of sign and signal location (37-40)
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From page 306... ...
295 Vehicle body clearance Body clearances are increased as lane width increases (44) and with increases in shoulder width and improvement in shoulder type (42)
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From page 307... ...
296 that shoulder width is ample if the truck clears the pavement edge by a minimum distance of 1 ft, preferably 2 ft (46)
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From page 308... ...
297 nature of the highway element (for example, intersections versus openroad conditions, number of lanes, merging sections, and gradient)
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From page 309... ...
Using a deterministic model, Huber has demonstrated the effects of vehicle length on passenger-car equivalents for free-flowing, multilane conditions (49)
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From page 310... ...
299 TABLE E-4 Effect of Gradient and Truck Type on Passenger-Car Equivalents of Trucks: Extended Four-Lane Freeway Upgrades with 15 Percent Trucks (48) Passenger-Car Equivalent by Type of Truck Gradient Light Medium Heavy (%)
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From page 311... ...
300 REFERENCES J.C. Glennon.
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From page 312... ...
301 Special Speed Monitoring Survey: Trucks and Buses. Voluntary Truck and Bus Fuel Economy Improvement Program, NHTSA, U.S.
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From page 313... ...
302 Splash and Spray Characteristics of Trucks and Truck Combinations. Research Committee Report 5.
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From page 315... ...
Appendix F Accident Rates of Twins: Literature Review Descriptions are presented of 15 studies comparing accident rates of twin trailer trucks or other double trailer trucks with rates of tractor-semitrailers, and the collective results of these studies are summarized. Then a review is given of research aimed at identifying factors in the operating environment (the characteristics of roads, carriers, and drivers)
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From page 316... ...
305 permit requirements placed on doubles use might have influenced their performance. A second deficiency of this study, in addition to the limited exposure of doubles, was that doubles were not identified specifically by the toll records.
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From page 317... ...
306 Some large Common carriers reported accident rates for their intercity tractors that were very much in line with the predicted levels. Other Common carriers, however, reported rates for intercity tractors that were so low as to strain credibility.
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From page 318... ...
Vehicle Involvement Rate by Type of Exposure Direct Induced Vehicle Tractor- Tractor- Weight (ib) Twins Semitrailer Twins Semitrailer 25,001-60,000 0.83 1.8 0.95 1.2 60,001 ormore 1.4 2.1 0.99 1.0 The authors were unable to determine which of the two techniques yielded the more accurate estimates.
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From page 319... ...
308 within which the majority of twins was registered. Campbell concluded that, except for data for 1977 model-year tractors, the Campbell and Carsten study (3)
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From page 320... ...
accidents per million vehicle miles for tractor-semitrailers and twins, respectively. The total number of accidents for each vehicle type exceeded 1,000, and the difference in accident rates between the two vehicle types was judged to be not significant.
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From page 321... ...
310 The primary weakness of this study results from the data that were employed. The BMCS accident data are carrier reported and may be subject to reporting bias.
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From page 322... ...
311 TABLE F-i Accident Data for Consolidated Freightways MatchedPair Analysis (9)
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From page 323... ...
312 types on the basis of their relative appearance at 15 vehicle counting stations located throughout the state. On the basis of their complete analysis, the investigators concluded that there was no statistically significant difference in the total accident rates but that the operation of twins resulted in a significantly larger number of deaths per million vehicle miles of travel ("As Reported" data in Table F-2)
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From page 324... ...
313 population of twins in California prevents unqualified extrapolation of the findings of this and other California studies to other regions. BMCS Report (12)
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From page 325... ...
314 0.6 S . Ia, 0.4 - 1 0 1 0 t LeI S + Twins accident rate I c., C 0 2 - larger and utilization It smaller (I)
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From page 326... ...
315 biases and by imprecision in the attempt to control for confounding effects of different operational environments. Further, there is doubtless a regional bias because twins have been operated more extensively in midwestern and western states than elsewhere.
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From page 327... ...
316 Fatal Casualty All Accidents Accidents Accidents Rural segments (12 sites) 1.06 1.11 1.24 Urban segments (6 sites)
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From page 328... ...
317 Perhaps the major strength of the study by Graf and Archuleta is its attempt to control for possible confounding effects of the operating environment by focusing on the 18 specific road segments and presenting comparative accident and exposure statistics for each segment. In addition, the sample size was relatively large, and care greater than that normally exercised was taken to assure accurate exposure estimates.
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From page 329... ...
318 Zeiszler (15) In many ways, the Zeiszler study was similar to the later study by Yoo, Reiss, and McGee (11)
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From page 330... ...
319 A major finding was that twins had a significantly larger total accident involvement rate than tractor-semitrailers. However, no statistically significant differences were evident in the involvement rates for casualty accidents.
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From page 331... ...
320 principal finding relative to the contradictory conclusion of the Transportation Systems Center. The two independent reviews concurred, however, in their documentation of deficiencies sufficiently serious to suggest that the BioTech study did not meet the standards of reasonable scientific inquiry.
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From page 332... ...
321 were often limited both in accuracy and quantity. Thus, selectivity is demanded in judging which of the results from prior studies are most useful.
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From page 333... ...
TABLE F-4 Summary of Comparative Accident Rate Investigations Data Source Date Ratio of Vehicle Involvement Rate of Twins Versus That of Tractor-Semitrajiers Fatal Casualty All Accidents Accidents Accidents Comments Indiana Toll Road Commission 1966-1970 - - 0.49 Limited data, five-axle twins not (1) evaluated Consolidated Freightways, west- 1964-1968 - - 0.60 Insufficient documentation, unvali- em states (2)
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From page 334... ...
Ratio of Vehicle Involvement Rate of Twins Versus That of Tractor-Semitrailers Fatal Casualty All Data Source Date Accidents Accidents Accidents Comments Nationwide interstate carriers 1969-1980 1.20" 1.0'8" (12) Rural roads, California (13)
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From page 335... ...
324 Results of these five studies indicate a much smaller difference between the accident involvement rates of twins and tractor-semitrailers than does the range of results from the entire collection of studies. Of the remaining six studies identified in Table F-4, only three presented findings in apparent contradiction with the foregoing -- Fleischer and Philipson (4)
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From page 336... ...
325 The death rate and casualty-accident involvement rate are greater for rural highways than for urban highways (21)
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From page 337... ...
326 A larger proportion of twins mileage has been with van trailers and a smaller proportion with tanker and platform trailers (8) ; and Twins have carried a larger share of mixed cargo (general freight)
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From page 338... ...
327 in the annual vehicle utilization (miles per vehicle per year)
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From page 340... ...
329 DOT-HS-803-418. Traffic Safety Center, University of Southern California, Los Angeles, Feb.
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From page 341... ...
Appendix G Accident Severity Literature Review and Analysis of Bureau of Motor Carrier Safety Accident Reports In addition to accident involvement rates, relative accident severity is an essential component of comparisons of the accident patterns of twins and tractor-semitrailers. Even given a situation for which the total accident involvement rates are equivalent, one or the other of the two configurations may be more frequently involved in more severe types of accidents and would therefore be labeled as the more "hazardous" of the two.
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From page 342... ...
331 The second approach expresses the average consequences of an accident, for example, the average number of deaths per accident. For either of these two collective measures, the preferred base is the complete set of accidents, that is, the set that contains accidents of all severity levels.
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From page 343... ...
TABLE G- 1 Comparative Accident Severity: Deaths per Accident or Fatal Accident Frequency Accident Severity by Truck Type Tractor- Base Data Type of Severity Measure Twins Semitrailer Source 1969, nationwide, BMCS Average number of deaths per Q.047 0.044 Scott and O'Day accident (all seventies)
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From page 344... ...
333 TABLE 0-2 Comparative Accident Severity: Injuries, Casualties, or Vehicles per Accident Type of Severity Base Data Measure Accident Severity by Truck Type Tractor- Twins Semitrailer Source 1969, nationwide, BMCS 1969, nationwide, BMCS 1972, California, rural roads and incorporated freeways 1972, California, rural roads and incorporated freeways 1976 -- 1980, nationwide, Consolidated Freightways 1974, California, rural roads and urban freeways 1969-1980, nationwide, BMCS special survey, 10 carrier periods °Approximate. Average number of injuries per accident with automobiles (all seventies)
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From page 345... ...
TABLE G-3 Comparative Accident Severity: Nonfatal Injury or Casualty Frequency • Base Data Type of Severity Measure Accident Severity by Truck Type Tractor- Twins Semitrailer Source 1977, nationwide, BMCS, Fraction of accidents (all severities) 0.48 0.73 Chirachavala and O'Day (8)
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From page 346... ...
335 This last point is worthy of further mention because operational environment most likely exerts a strong influence on relative accident severity. Chirachavala et al.
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From page 347... ...
336 TABLE G-5 Influence of Roadway, Environment, and Accident Type on Injury Ratio (13) Fatality Ratio by Accident Type Collision with Single-Vehicle Accident Automobile Roadway Tractor- Tractor- Type Environment Twins Semitrailer Twins Semitrailer Rural, Dry undivided Day 0.75 1.01 2.35 2.55 Night 0.74 1.31 2.32 3.29 Wet/snowy 0.78 0.98 2.44 2.46 Rural, Dry divided Day 0.48 1.14 1.28 2.54 Night 1.44 1.28 3.79 2.86 Wet/snowy 0.98 0.80 2.57 1.77 Urban Dry Day 0.27 0.50 2.01 2.63 Night 0.28 0.54 2.12 2.86 Wet/snowy 0.64 0.55 4.77 2.91 ANALYSIS OF BMCS REPORTS The basis for this investigation was accident reports submitted to the Bureau of Motor Carrier Safety by motor carriers of property in interstate or foreign commerce.
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From page 348... ...
337 Commerce Commission. To control for operating environment influences, the data were partitioned into 32 categories, representing all combinations of four regions1 (California, Other Western and Mountain, Central and Plains, and East)
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From page 349... ...
TABLE G-6 Accidents Involving Five-Axle Vans Cartying General Freight and Operated by ICC-Authorized Carriers Region Highway Type No. of Accidents by Land Use Rural - Tractor- Twins Semitrailer Urban Twins Tractor- Semitrailer Central and Four or more lanes, divided 365 4,560 85 2,066 Plains Four or more lanes, undivided 6 167 14 631 Two lanes 92 1,864 28 1,076 Ramps 26 407 20 354 Other Western Four or more lanes, divided 243 588 23 120 and Mountain Four or more lanes, undivided 7 19 8 40 Two lanes 85 196 22 65 Ramps 10 32 8 29 California Four or more lanes, divided 102 179 35 89 Four or more lanes, undivided 3 4 8 41 Two lanes 26 38 13 29 Ramps 10 14 9 13 East Four or more lanes, divided 8 4,723 2 1,827 Four or more lanes, undivided 1 200 2 500 Two lanes 14 1,865 2 1,338 Ramps 1 425 1 288 SourcE: Bureau of Motor Carner Safety accident data, 1976-1981.
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From page 350... ...
TABLE G-7 Severity of Accidents Involving Twins and TractorSemitrailers: All Highway Types, Locations, and Regions Accident Severity Base Measure Twins Tractor-Semitrailer All Avg.
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From page 351... ...
TABLE G-8 Comparison of Observed and Expected Frequencies of Injury Accidents Involving Twins Land Use and Highway Type - Region Fraction of Tractor-Semitrailer Accidents Involving Injury Twins Accidents No Involvin Injury Total g No. Observed Expected Rural Four or more lanes, California 0.637 102 63 64.9 divided Central and Plains 0.602 365 197 219.6 East 0.618 8 2 4.9 Other Western and Mountain .
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From page 352... ...
Land Use and Highway Type Region Fraction of Tractor-Semitrailer Accidents Involving Injury Twins Accidents No. Involvin In Total g ji"' No.
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From page 353... ...
TABLE G-9 Observed and Expected Frequencies of Injury and Fatal Accidents Involving Twins Injury Accidents Fatal Accidents t Total Number Percen Number Percent No. of Differ- Clii- _____________ Differ- Chi- Variable Category Accidents Observed Expected ence° Squared Observed Expected ence" Squared All All 1,279 707 764 75 8.74' 78 - 92 • 15.0 1.78 Location Rural 999 531 589 9.9 11.15" 70 80 12.2 1.00 Urban 280 176 175 -0.4 0.01 8 12 33.9 1.27 Region California 206 119 132 9.6 2.26 16 14 -11.1 0.13 Central and Plains 636 365 389 6.2 3.63 30 39 23.7 2.23 East 31 12 19 36.8 6.62" 0 2 100.0 2.58 Other Western and Mountain 406 211 225 6.0 1.35 32 36 10.4 0.30 Highway Four or more lanes, type divided 863 483 525 8.0 6.94" 47 57 18.0 1.57 Four or more lanes, undivided 49 33 29 -13.4 1.13 3 4 21.0 0.17 Two lanes 282 148 158 6.6 1.30 28 28 0.4 0.00 Rampi 85 43 52 17.0 3.08 0 2 100.0 2.28 "100[(expected - observed)
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From page 354... ...
TABLE G-10 Observed and Expected Frequencies of Multivehicle Accidents Involving Twins Variable Category Total No. of Accidents Number Observed Number Expected Percent Differencea ChiSquared AU All 1,279 563 692 18.7 Location Rural 999 392 492 20.4 33.10" Urban 280 171 200 14.5 13•44b Region California 206 136 128 -- 6.0 0.90 - Central and Plains 636 268 365 26.6 58.74" East 31 9 18 49.9 10.84" Other Western and Mountain 406 150 181 17.1 7.25" Highway Four or more lanes, divided 863 371 484 23.4 51.23" type -Four or more lanes, undivided 49 33 34 2.7 0.08 Two lanes 282 120 136 11.5 2.94 Ramps - 85 39 39 -- 0.2 0.00 100[(expected - observed)
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From page 355... ...
TABLE G- 11 Observed and Expected Frequencies of Injury and Fatal Single-Vehicle Accidents Involving Twins Injury Accidents Fatal Accidents Variable Categoiy Total No. of Accidents Number Observed Expected Percent Differ- ence° Chi- Squared Number Observed _____________ Expected Percent Differ- ence° ChiSquared All All 716 338 393 14.0 13.79" 28 35 20.9 1.28 Location Rural 607 292 338 13.7 11.37" 26 32 18.0 0.83 Urban 109 46 55 15.8 2.46 2 4 46.2 0.74 Region California 70 29 40 27.8 5.19" 5 5 28 0.00 Central and Plains 368 184 196 5.9 1.35 7 14 50.8 3.49 East 22 7 12 40.4 408b 0 1 100.0 1.26 Other Western and Mountain 256 118 145 18.9 8.36" 16 15 -7.7 0.06 Highway Four or more lanes, type divided 492 242 274 11.6 6.62" 21 24 14.3 0.40 Four or more lanes, undivided 16 8 8 -6.3 0.05 1 2 45.0 0.39 Two lanes 162 72 84 14.5 3.05 6 8 27.5 0.52 Ramps 46 16 27 41.8 9.95" 0 1 100.0 0.77 °100[(expected - observed)
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From page 356... ...
TABLE G-12 Observed and Expected Frequencies of Injury and Fatal Multivehicle Accidents Involving Twins Variable Category Total No. of Accidents Injury Accidents Number __________________ Observed Expected Percent Differ- ence Chi- Squared Fatal Accidents Number __________________ Observed Expected Percent Differ- ence° ChiSquared All All 563 369 356 -3.6 1.06 50 50 -0.2 0.00 Location Rural 392 239 240 0.6 0.02 44 42 -4.6 0.08 Urban 171 130 116 -12.3 4.77' 6 8 23.5 0.40 Region California 136 90 89 -0.8 0.01 11 9 -19.9 0.25 Central and Plains 268 181 180 -0.7 0.02 23 22 -6.1 0.08 East 9 5 6 21.0 0.94 0 1 100.0 0.72 Other Western and Mountain 150 93 81 -15.2 3.07 16 18 13.0 0.28 Highway Four or more lanes, type divided 371 241 241 0.2 0.00 26 28 8.4 0.17 Four or more lanes, undivided 33 25 20 -15.4 3.36 2 2 -2.9 0.00 Two lanes 120 76 71 -25.5 0.60 22 18 -21.7 0.82 Ramps 39 27 23 -6.4 1.18 0 2 100.0 1.37 lOO[(expected - observed)
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From page 357... ...
TABLE G- 13 Observed and Expected Frequencies of Automobile-Truck Accidents Involving Twins Resulting in Injury or Death to Automobile Occupant Total No. f Injury Accidents Fatal Accidents Automobile- Number Percent Number Percent Truck Differ- Cli- ____________ Differ- Chi- Variable Category Accidents Observed Expected ence° Squared Observed Expected ence" Squared All All 417 276 278 0.8 0.04 40 38 -5.8 0.12 Location Rural 280 172 181 5.2 1.17 34 31 -8.0 0.18 Urban 137 104 97 -7.6 1.65 6 6 4.7 0.01 Region California 95 64 66 2.4 0.09 8 6 -34.0 0.46 Central and Plains 206 142 143 0.9 0.03 19 18 -6.8 0.09 East" - - - - - - - - - Other Western and Mountain 116 70 69 -0.9 0.01 13 14 7.3 0.04 Highway Four ormore lanes, type divided 265 172 180 4.2 0.81 18 20 11.0 0.72 Four or more lanes, undivided 26 22 19 -18.3 2.06 2 1 -88.9 0.79 Two lanes 95 63 61 -3.2 0.13 20 15 -31.2 1.37 Ramps 31 19 19 -0.1 0.00 0 1 100.0 1.12 100[(expected - observed)
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From page 358... ...
347 differed with respect to the infliction of injury or death on automobile occupants. Differences between truck types were small and in no case statistically signficant (Table G- 13)
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From page 359... ...
348 J.C. Glennon.
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From page 360... ...
Appendix H Driver Survey Questionnaire TRUCK DRIVER OPINION SURVEY OPERATION AND SAFETY OF TWIN TRAILER TRUCKS Transportation Research Board I DRIVER Your age: (years)
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From page 361... ...
350 b. Other combination behind-the-wheel training: (hours)
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From page 362... ...
351 ifi. EMPLOYMENT What best describes 'your employment status?
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From page 363... ...
Total number of axles on the tractor and trailer (count a tandem axle as two axles) : (number)
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From page 364... ...
353 Controlling loaded truck on long, steep downgrade Maintaining speed on long, steep upgrade Operating on city streets Turning sharp corners Operating on steep, curved off-ramps 1. Operating on slick pavement Operating on very rough pavement Operating under severe crosswinds Operating truck having poorly adjusted brakes Operating truck having worn or mismatched tires Operating empty truck Hitching and unhitching a trailer 5.
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From page 365... ...
354 C Difficulty detecting quickly when a trailer starts an un- usual motion A trailer appears like *
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From page 366... ...
355 VII. OTHER TRUCKS Have you operated a tractor-semitrailer with a 48-foot semitrailer?
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From page 367... ...
Appendix I Pavement Effects of Different Vehicle Weights, Weight Distributions, and Axle Arrangements -- An Example In this example an illustration is given of how changes in gross vehicle weight (GVW) , weight distribution, and axle arrangement can individually alter a truck's effect on pavement wear and pavement life.
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From page 368... ...
357 ESALs by Axle or Axle Group 1 2 3 Pavement Type Single Tandem Tandem Total Flexible 0.10 0.52 0.27 0.89 Rigid 0.07 0.79 0.37 1.23 2. Base plus 6,000 lb [weight distribution between tandem axles (ratio of front tandem load to rear tandem load)
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From page 369... ...
358 - Total Adjusted Percent ESALs Adjustment Total increase Pavement Type (unadj.) Factor ESALs over Base Flexible 1.27 0.91 1.16 30 Rigid 1.92 0.91 1.15 42 Base with less uniform weight distribution (GYW and axle ar-.
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From page 370... ...
359 ESALs by Axle or Axle Group Percent Pavement 1 2 3 4 5 Increase Type Single Single Single Single Single Total over Base Flexible 0.10 0.38 0.37 0.20 0.20 1.25 40 Rigid 0.07 0.32 0.320.150.151.01(18)
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From page 371... ...
weight distribution and axle arrangement alter the incremental impact of the added 6,000 lb of weight as well as the impact of the original 60,000 lb. Also, dividing a tandem axle unevenly between the two single axles to match typical weight distributions of twins increases the effect on pavement wear.
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From page 372... ...
361 REFERENCE Guide, for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, D.C., 1986 (in preparation)
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From page 373... ...
Appendix J Effects of Twins and Other Large Trucks on Bridges and Culverts Bridges are complex structures that are designed in widely differing ways and constructed with a variety of materials. Generally, every bridge consists of three principal elements: a substructure (the piers, abutments, and other foundation structures that support the bridge from the ground)
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From page 374... ...
363 rations and loads but are intended to envelop the effects of all in-service truck types loaded within legal axle and gross vehicle weight (GVW) maximums.
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From page 375... ...
364 loading pattern or greater, and about 65 percent of the bridges on federalaid primary highways (excluding Interstates) are designed to accommodate at least the HS 15-44 or HS 20-44 loading patterns (Table J-l)
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From page 376... ...
365 limits such overstress use may be permissible but will increase the longterm possibility of fatigue failure, therefore reducing useful service life. EFFECTS ON BRIDGE SUBSTRUCTURES The methods of Jung and Witecki and Whiteside et al.
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From page 377... ...
366 -S 20-44 60 oc3e - - I 0\'O_ - 50 L/ 30.0 __ __-' HS15-4 LU -- I - - U)
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From page 378... ...
114 § 98 x cc 82 I U)
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From page 379... ...
368 These comparisons indicate that the forces induced by multiple twin trailer trucks are close to, but somewhat less than, the forces created by multiple tractor-semitrailers. For transverse floor beams, fully loaded twins, like fully loaded tractorsemitrailers, induce forces well below those of the HS 20-44 and HS 1544 standard loading patterns.
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From page 380... ...
369 W = maximum load carried on any group of two or more consecutive axles and Bm = equivalent base length (the distance over which W must be equally distributed to generate the same maximum bending moments in main bridge beams,. girders, and trusses)
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From page 381... ...
370 allocation study concluded that bridge deck repair costs should be assigned to vehicle classes without regard to weight or axle configuration (7)
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From page 382... ...
371 only does the difference between single and tandem axles diminish but also the dead load effects of the overburden become dominant (Figure J3)
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From page 383... ...
372 tractor-semitrailers or twin trailer trucks. The changing mix of the truck fleet is therefore unlikely to adversely affect culvert performance and serviceability.
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From page 384... ...
Appendix K Effects of Twins and Other Large Trucks on Highway Shoulders and Roadside Appurtenances The possible effects of twins and other STAA trucks on highway shoulders and roadside appurtenances are discussed. EFFECTS ON HIGHWAY SHOULDERS Two shoulder types are considered: (a)
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From page 385... ...
374 Gravel, Turf, and Thinly Paved Shoulders When paved shoulders and adjacent lanes are not constructed in the same manner, the added destructive effect of use by twins for parking may be sufficient to accelerate shoulder deterioration. The extent to which this phenomenon will occur depends on the paved shoulder design and the frequency of heavy truck parking.
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From page 386... ...
375 rather than the clearance with an opposing vehicle, making a shoulder encroachment more likely. Similar adjustments to shoulder clearances might be made on two-lane and multilane highways in passing situations, but again available research results do not indicate what the frequency of resulting shoulder encroachments might be.
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From page 387... ...
Appendix L Relationships Between Geometric Design and Large-Truck Characteristics Summarized in this appendix are relationships between large trucks and 13 key highway design features. PAVEMENT WIDENING ON CURVES To account for offtracking, in which a vehicle's rear wheels follow a different path from that of the front wheels, and driver difficulty in steering, the American Association of State Highway and Transportation Officials (AASHTO)
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From page 388... ...
377 consequence, reconsideration of these design values may be warranted to account for the tractor with 48-ft semitrailer. MINIMUM DESIGN FOR SHARPEST TURNS At-grade intersections must sometimes be designed to permit low-speed turning in the minimum possible space.
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From page 389... ...
378 turn without encroaching on adjacent lanes, medians, or shoulders, but larger openings may result in erratic maneuvering by smaller vehicles. Although not prescribing minimum openings, AASHTO design policy recognizes that designers must balance these factors on the basis of anticipated traffic characteristics and indicates that some lane encroachments by large trucks may be permissible.
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From page 390... ...
379 As a result, no change in AASHTO minimum passing sight distances is expected unless AASHTO adopts a new rationale for these distances, but the introduction of longer trucks does modestly increase actual safe passing sight distance requirements for maneuvers involving these longer trucks. Typically, a twin trailer truck with 28-ft trailers is about 5 ft longer than a tractor-semitrailer with a 45-ft semitrailer.
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From page 391... ...
380 ommend specific values for lane length. The introduction of longer trucks, twins and tractors with 48-ft semitrailers may tend to increase the need for longer turning lanes at locations with substantial truck traffic.
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From page 392... ...
381 SHOULDER WIDTH Shoulders provide a place for vehicles to park during emergency situations. As shoulders increase in width, the possibility that a stopped vehicle will interfere with traffic flow or cause a safety hazard decreases.
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From page 393... ...
382 C.M. Walton and 0.
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From page 394... ...
Study Committee Biographical Information KENNETH W HEATHINGTON, Chairman, is Associate Vice President for Research for the University of Tennessee Statewide Administration, Executive Director of the University of Tennessee Research Corporation, and Professor of Civil Engineering.
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From page 395... ...
384 He received a bachelor's degree in civil engineering from the University of Minnesota. After joining the California Department of Transportation as an engineer, he held the positions of Assistant Director of Financial Management, Chief of the Division of Project Development, and Chief of the Division of Highways with the department.
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From page 396... ...
385 Motor Carrier Advisory Committee, and the National Hazardous Materials Advisory Committee.
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From page 397... ...
386 RONALD L HUTCHINSON is a consulting engineer.
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From page 398... ...
387 JOHN K MLADINOV is Executive Deputy Commissioner of the New York State Department of Transportation.
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From page 399... ...
MR Administrator, State Highway Administrator and Chief Administrative Officer of the Division of Highways, and Deputy Secretary.
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From page 400... ...
The Transportation Research Board is a unit of the National Research Council, which serves the National Academy of Sciences and the National Academy of Engineering. The Board's purpose is to stimulate research concerning the nature and performance of transportation systems, to disseminate the information produced by the research, and to encourage the application of appropriate research findings.
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Key Terms
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