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114 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS The conclusions of the research are as follows: 1. NAFTA may lead to increased truck volumes using U.S. highways, but it is unlikely that new truck types not currently considered in highway geometric design will be entering the United States. Although trucks larger and heavier than currently permitted in the United States do operate in both Canada and Mexico, any trucks entering the United States are required to comply with current federal and state laws governing truck size and weight. The creation of a NAFTA inter- national access network of roads is being considered, but the proposed criteria for the truck sizes that would operate on that network do not differ substantively from current U.S. limits applicable in many states. 2. A substantial number of three- and four-axle single- unit trucks in the current truck fleet are larger than the two-axle single-unit design vehicle presented in the Green Book. 3. The WB-15 [WB-50] design vehicle is no longer com- mon in the U.S. truck fleet. 4. The WB-19 [WB-62] design vehicle shown in the Green Book has a KCRT distance of 12.3 m [40.5 ft]. The laws of many states allow KCRT distances up to 12.5 m [41 ft]. 5. The WB-20 [WB-65] design vehicle shown in the Green Book involves neither the best nor worst case of the rear tandem axles of the truck. 6. Where trucks larger than the WB-19 [WB-62] design vehicle operate with the rear axles pulled forward to a KCRT distance of 12.5 m [41 ft], their offtracking and swept path width are the same as that of the WB-19 [WB-62] design vehicle. Pulling the axles forward to a KCRT distance of 12.5 m [41 ft] is required by many states and, even when not required, is preferred by truckers to increase the maneuverability of such a vehicle. 7. In states where combination trucks with semitrailers longer than 16.2 m [53 ft] are permitted to operate, they constitute only 0.5 to 4 percent of all trucks. 8. Combination trucks, known as Rocky Mountain Dou- bles, with one 14.6-m [48-ft] semitrailer and one 8.7-m [28.5-ft] full trailer, operate in 20 states (mostly in the western United States), including 3 states where Turn- pike Doubles are not permitted and 6 states where triple-trailer trucks are not permitted. In such states, Rocky Mountain Doubles may be the largest combi- nation trucks that can legally operate. 9. TRB Special Report 267 (13) has recommended that single-semitrailer trucks with six axles, including a rear tridem axle, be permitted to operate with gross vehicle weight up to 40,900 kg [90,000 lb]. Implementation of this recommendation would not have any effect on geo- metric design because single-semitrailer trucks with six axles have offtracking and swept path width that are slightly less than a comparable five-axle truck. 10. Design vehicles that might be needed in the Green Book at some future time include the following: ⢠A combination truck with a single 17.4-m [53-ft] semitrailer, designated the WB-22 [WB-71] design vehicle; ⢠A combination truck with two 10.1-m [33-ft] trailers, designated the WB-23D [WB-77D] design vehicle; ⢠A Turnpike Double combination truck, with two 16.1-m [53-ft] trailers, designated the WB-37D [WB-120D] design vehicle; and ⢠A B-Train double combination with one 8.5-m [28-ft] trailer and one 9.6-m [31.5-ft] trailer. None of these vehicles currently operate with sufficient frequency to warrant adoption as a design vehicle, although in TRB Special Report 267 (13), the WB-23D [WB-77D] has been proposed for wider operation. Therefore, the dimensions and turning performance of these vehicles have been documented in the research, but no recommendation has been made to include these design vehicles in the Green Book. Their incorpora- tion in the Green Book should be considered if truck size and weight laws are changed to permit such vehi- cles to operate more widely and if they are actually present in sufficient numbers to warrant their consid- eration as design vehicles. 11. Rear swingout is the phenomenon by which the rear outside corner of a truck follows a path outside the rear outside axle of the truck during a turn. Rear swing- out increases as the distance from the rear axle to the rear of the truck, known as rear overhang, increases. However, turning plots show that, while the outside rear corner of the trailer follows a path outside the rear
115 trailer wheels, it is inside the swept path of the truck. For this reason, rear swingout is rarely a concern to other vehicles, unless they are making a parallel turn. None of the current Green Book design vehicles or the new design vehicles recommended in this report for inclusion in the Green Book have rear swingout that exceeds 0.21 m [0.69 ft] for a turn with a radius of 15 m [50 ft], even with the rear axles pulled forward to maintain a KCRT distance of 12.5 m [41 ft]. 12. Trucks the size of the WB-19 [WB-62] or larger have swept path widths so great that the truck cannot make a 90-deg right turn from one two-lane road to another while remaining within a 3.6-m [12-ft] lane for turn- ing radii of 23 m [75 ft] or less. Trucks making such turns at locations with curb return radii of 23 m [75 ft] or less must either encroach on the roadway shoulder (or curb line) or on an opposing lane. 13. The minimum rollover threshold for trucks is gener- ally in the range from 0.35 to 0.40 g. This minimum rollover threshold generally applies to trucks fully loaded with uniform density cargo. 14. Antilock brake systems improve the braking distances of trucks by reducing the variability in driver control efficiency observed with conventional braking sys- tems. An antilock brake system applies the vehicle brakes and then releases them, as needed, to prevent wheel lock-up, which may lead to loss of control. Trucks with antilock brakes require longer braking dis- tances than passenger cars, but the braking distances of passenger cars and trucks on wet pavement are nearly the same. 15. Antilock brake systems are now available on nearly all truck tractors. Field observations during 2002 found that antilock brake systems are available on approxi- mately 43 percent of trailers. Based on the expected service life of trailers, it can be expected that within 10 years nearly all trailers will be equipped with antilock brake systems. 16. The current Green Book design criteria for passing sight distance are such that a truck can safely pass a passenger car on any crest vertical curve where a pas- senger car can safely pass a truck. 17. The current Green Book criteria for intersection sight distance were recently updated and include explicit adjustment factors for trucks. There is no indication of a need for further changes in these design criteria. 18. Current Green Book design criteria for sight distance at railroad-highway grade crossings appear to be appro- priate for the current truck fleet. 19. The 85th-percentile weight/power ratios of trucks in the current truck fleet range from 102 to 126 kg/kW [170 to 210 lb/hp] for the truck population using free- ways and from 108 to 168 kg/kW [180 to 280 lb/hp] for the truck population using two-lane highways. 20. Analysis indicates that the minimum lengths of accel- eration lanes presented in the Green Book may be sufficient to accommodate average trucks but not to accommodate heavily loaded trucks. No change is recommended at this time because there is no indica- tion that trucks are encountering specific problems on acceleration lanes designed in accordance with the Green Book criteria. 21. The current Green Book criteria for lane width and pavement widening on horizontal curves appear to be appropriate for the current truck fleet. 22. The current Green Book criteria for horizontal curve design provide an adequate margin of safety against skidding and rollover by trucks traveling at the design speed. The lowest margins of safety are for horizontal curves with design speeds of 30 km/h [20 mph] or less. It is important that the design speed for such curves be selected based on consideration of likely operat- ing speeds because exceeding the design speed of a 30-km/h [20-mph] curve by as little as 13 km/h [8 mph] could lead to skidding on a wet pavement or rollover. 23. The current Green Book criteria for cross-slope breaks and vertical clearances appear to be appropriate for the current truck fleet. The recommendations of the research are as follows: 1. A design vehicle representing a three-axle single-unit truck should be added to the Green Book. 2. The WB-15 [WB-50] design vehicle should be dropped from the Green Book. 3. The KCRT distance for the WB-19 [WB-62] design vehicle should be increased from 12.3 to 12.5 m [40.5 to 41 ft]. 4. The WB-20 [WB-65] design vehicle should be dropped from the Green Book, and the WB-20 [WB-67] design vehicle, which represents the worst-case place- ment of the rear axles for a truck with a single 16.2-m [53-ft] semitrailer, should be retained. 5. Where trucks larger than the WB-19 [WB-62] design vehicle operate with the rear axles pulled forward to a KCRT distance of 12.5 m [41 ft], design elements such as intersection geometrics should be based on the WB-19 [WB-62] design vehicle. However, where the overall length of the vehicle is the basis for design, such as for sight distance at railroad-highway grade crossings, the length of the actual design vehicle should be used. 6. A design vehicle representing a Rocky Mountain Dou- ble combination should be added to the Green Book. 7. Based on the comparable braking distances for pas- senger cars and trucks on wet pavement, there does not appear to be any need for a change in the Green Book design criteria for stopping sight distance. The Green Book expresses a concern that stopping sight distance
116 for trucks may be particularly critical at the end of a long downgrade. Computer simulation research to assess truck braking capability for a superelevated horizontal curve at the end of a downgrade would be desirable. 8. There is no indication that a change in passing sight dis- tance criteria is needed to better accommodate trucks. Although passing maneuvers involving trucks require longer distances than passing maneuvers involving only passenger cars, there is no indication that trucks encounter any particular safety problems in passing zones marked with current criteria. 9. Where trucks the size of the WB-19 [WB-62] or larger are present and make right turns in substantial num- bers, curb return radii larger than 23 m [75 ft] are rec- ommended. In many cases, such radii can best be pro- vided in conjunction with a channelized right-turn roadway. The offtracking and swept path width of the specific selected design vehicle should be considered in developing the channelization geometrics. 10. The design of double- and triple-left-turn lanes requires consideration of the swept path width of left-turning trucks. Although this issue can be addressed in the design process with computer modeling of truck paths, it is recommended that a table showing the swept path widths of various design vehicles making left turns with radii of 22.9 to 47.7 m [75 to 150 ft] be added to the Green Book for use by designers. 11. Additional guidance should be provided in the Green Book on the maximum entry speeds and diameter of the inscribed circle for roundabouts of specific site categories for specific design vehicles. 12. A truck speed profile model (TSPM) has been devel- oped in the form of a spreadsheet that can be used to estimate the truck speed profile on an upgrade for any specified truck weight/power ratio, initial speed, and vertical alignment. This spreadsheet is recommended for design application as an alternative to the charts for critical length of grade currently presented in the Green Book, which are based on a single value of truck weight/power ratio, a single value of initial speed, and a uniform (constant percent) grade. 13. Additional research is recommended to determine whether trucks encounter any specific safety problems on acceleration lanes designed in accordance with Green Book criteria.
