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1 Introduction In 1995 Congress rescinded the National Maximum Speed Limit (NMSL) of 55 mph (89 km/h), which had been in effect since 1974 following the oil shortages experienced in the preceding year.1 The National Highway System Designation Act of 1995 returned to the states the prerogative of setting speed limits on major highways. Following repeal of the NMSL, 49 state legislatures have raised statutory speed limits, often to levels that were in effect before pas- sage of the NMSL (Figure 1-1, Table 1-1). The 55-mph (89-km/h) speed limit was retained only in the predominately urban District of Columbia and Hawaii. The highest speed limits are in western states where congestion is relatively low and the highway infrastructure of more recent construction than in many eastern states. One state-- Montana--advocated that drivers themselves determine the safe speed at which to travel. Consequently, Montana does not post day- 1 In 1987 Congress had relaxed the 55-mph (89-km/h) speed limit, allowing states to raise the limit to 65 mph (105 km/h) on qualified sections of rural Interstate highways. 15
WASH. N.H. ME. MONT. N.D. VT. MICH. ORE. MINN. MASS. WISC. IDAHO N.Y. S.D. CT. WY. R.I. PA. IOWA N.J. NEB. NEV. OH. DEL. ILL. UTAH IND. W. MD. VA. COLO. D.C. VA. CALIF. KANS. MO. KY. N.C. TENN. ARIZ. OK. S.C. ARK. N.M. GA. MISS. ALA. LA. TEX. FLA. HAWAII ALASKA Legend: Basic law 65 mph 55 mph (Hawaii and the 75 mph District of Columbia) 70 mph Figure 1-1 Maximum speed limits on Interstate highways as of June 10, 1998 (data from National Highway Traffic Safety Administration and Federal Highway Administration regional offices, state legislatures, and other sources).
Table 1-1 Maximum Speed Limits by State as of June 10, 1998 (Data from National Highway Traffic Safety Administration and Federal Highway Administration Regional Offices, State Legislatures, and Other Sources) Current Maximum Pre-NMSL Speed Limit (mph) Maximum Interstate Primary State (mph) Highway Highway 70 70 55 Alabama Alaska 70 65 55 75 a Arizona 75 55 Arkansas 75 70 (65) 55 70 b (55) California 70 65 (55) 70 75 55 Colorado 65 c Connecticut 60 55 65d Delaware 60 50 55 e (50) District of Columbia 60 50 70 d (65) Florida 70 55 70 f Georgia 70 55 Hawaii 70 55 55 Idaho 70 75 65 65 g (55) Illinois 70 55 Indiana 70 65 (60) 55 65 b Iowa 75 55 Kansas 75 70 70 Kentucky 70 65 55 Louisiana 70 70 65 Maine 70 65 55 Maryland 70 65 55 Massachusetts 65 65 55 Michigan 70 70 (55) 55 65 h Minnesota 65 70 Mississippi 70 70 65 70 i Missouri 70 65 Basic Law j Basic Law j,k (65) Basic Law j,k (60) Montana 75 l Nebraska 75 60 Basic Law j Nevada 75 70 (55) New Hampshire 70 65 55 65 m 55 m New Jersey 70 75 a 60 n New Mexico 70 New York 55 65 55 (continued on next page)
Table 1-1 (continued) Current Maximum Pre-NMSL Speed Limit (mph) Maximum Interstate Primary State (mph) Highway Highway 70 o North Carolina 70 55 65 p North Dakota 75 70 Ohio 70 65 (55) 55 70 q Oklahoma 70 65 (55) Oregon 75 65 (55) 55 65 d Pennsylvania 65 65 65 d Rhode Island 60 55 South Carolina 70 65 55 65 r South Dakota 75 75 Tennessee 75 70 65 70 (65) s 70 (60) s Texas 70 Utah 70 75 55 Vermont 65 65 50 Virginia 70 65 55 Washington 70 70 (60) 60 (55) West Virginia 70 70 65 Wisconsin 70 65 55 75 t Wyoming 75 65 Note: Figures in parentheses are speed limits for heavy trucks. Primary highways are part of the federal-aid highway system--highways that are eligible for federal highway funds. The Federal-Aid Primary System comprises interconnecting main roads important to interstate, statewide, and regional travel, consisting of rural arte- rial routes and their extensions into or through urban areas (see text box). NMSL = National Maximum Speed Limit. 1 mi = 1.609 km. a Urban Interstates remain 55 mph (89 km/h). b Rural freeways and expressways only. Other freeways and expressways are at 65 mph (105 km/h). Other city, county, and state roads may go to 65 mph on the basis of engineering and traffic surveys. c Speed limit increases on suitable multilane limited-access highways will be imple- mented by Oct. 1, 1998. Differential speed limits may be enacted. d Only certain segments. e Only part of Woodrow Wilson Bridge eligible for 55 mph (89 km/h). f Urban Interstates are 65 mph (105 km/h). Speed limits of some divided highways without controlled access are 65 mph based on engineering and traffic studies. g Only some urban Interstates are at 65 mph (105 km/h).
19 Introduction h Approximately 120 mi (193 km) of non-Interstate freeways and expressways will remain at 55 mph (89 km/h) as well as all two-lane state highways. i State can raise any road to 70 mph (113 km/h) with safety study. j A speed that is reasonable and prudent for conditions but with no numeric limit. k No maximum numeric posted daytime limit for passenger vehicles; daytime speed is "reasonable and proper" for conditions. Nighttime speeds for passenger vehicles are 65 mph (105 km/h) on the Interstate and 55 mph (89 km/h) on all other roads. Speed limits for heavy trucks are 65 mph day and night on the Interstate, and 60 mph (97 km/h) day and night on all other roads except for triple truck combinations, which are limited to 55 mph day and night on all roads. l Urban Interstates remain at 60 mph (97 km/h). Speed limits on four-lane express- ways are 65 mph (105 km/h) with some exceptions. m In January 1998, state legislation was passed raising speed limits to 65 mph (105 km/h) on approximately 400 mi (640 km) of limited-access highways for an 18- month trial period. n 70 mph (113 km/h) on four-lane roads with shoulders; 65 mph (105 km/h) on two-lane roads with shoulders; and 60 mph (97 km/h) on two-lane roads without shoulders. o An additional 340 mi (550 km) of non-Interstate controlled access are 70 mph (113 km/h). p 55 mph (89 km/h) on two-lane highway at night, and 55 mph (89 km/h) on gravel roads, day and night. q 75 mph (121 km/h) on rural segments of turnpike [50 mph (80 km/h) minimum], 65 mph (105 km/h) on urban segments [40 mph (64 km/h) minimum], 60 mph (97 km/h) on urban Interstates, and 55 mph (89 km/h) on state roads and other high- ways at night. r 65 mph (105 km/h) on major two-lane highways. Forty counties have decided to retain 55 mph (89 km/h). s For cars, 70 mph (113 km/h) in the daytime, 65 mph (105 km/h) at night. For trucks, 65 mph on Interstates in daytime, 60 mph (97 km/h) on primary roads in the daytime, 55 mph (89 km/h) at night. The Texas Transportation Commission approved speed limits lower than the state maximum of 70 mph on about half of the state's farm-to-market system. t 60 mph (97 km/h) on urban Interstates; 65 mph (105 km/h) on four- and two-lane roads; some secondary and mountain roads remain at 55 mph (89 km/h).
MANAGING SPEED 20 time speed limits for passenger vehicles on major highways, but driv- ers must travel at a reasonable and proper speed for conditions. In many states where speed limits were raised, public officials are closely monitoring related changes in vehicle speeds and safety outcomes. SCOPE OF STUDY AND CHARGE With such a major policy shift, this study was conceived as an oppor- tunity to review current practice in setting speed limits on all roads, not just major highways, and to provide guidance to state and local governments on both appropriate speed limit and enforcement poli- cies. Of course, speed limits are only one strategy for managing vehi- cle speeds. Redesigning roads to achieve desired operating speeds2 and retrofitting neighborhood streets with speed humps and traffic circles are alternative approaches to reducing traffic speeds. These strategies are often considered on streets where compliance with local speed limits is poor. This study touches briefly on such strategies, but they have been widely covered elsewhere. The primary focus here, in response to the study charge, is on reg- ulating speed through speed limits and enforcement. More specifi- cally, the interdisciplinary committee of experts convened to conduct the study has, in response to the charge, · Reviewed research to establish what is known about various methods of setting speed limits; the role of speed in safety; the role of road design and function in setting and enforcing speed limits; the effectiveness of speed limits, particularly with regard to safety; enforcement of and compliance with speed limits; and social benefits and costs of speed limits (i.e., trade-offs among safety, travel effi- ciency, and other factors that affect driver speed choices); · Considered the effects of new and emerging technologies for speed management and speed enforcement and expected changes in the highway environment (e.g., growing numbers of older drivers); and · Provided its judgment concerning appropriate changes to cur- rent practice on the basis of its findings. 2 Technical terms are defined in a glossary, which can be found in Appendix E.
21 Introduction Setting speed limits is complex and frequently controversial. The process is often viewed as a technical exercise, but the decision involves value judgments and trade-offs that are frequently handled through the political process by state legislatures and city councils. Speed limits represent implicit trade-offs among safety, efficiency of travel, and feasibility of enforcement. These trade-offs, in turn, reflect societal norms about appropriate driver behavior, tolerable lev- els of risk,3 and acceptable levels of enforcement. In this study alter- native methods of establishing speed limits (e.g., legislated mandates, engineering studies) are considered in terms of what is known about the trade-offs among safety, efficiency, and enforceability. What is known and what is not known from available studies and data con- cerning the effects of changes in speed limits on driving speeds, safety, and travel times are also reviewed. RESPONSIBILITY FOR SETTING SPEED LIMITS With two exceptions--during World War II and more recently with the NMSL of 55 mph (89 km/h)--setting speed limits in the United States has been a responsibility of state and local governments. Every state has a basic speed statute, which requires drivers to operate their vehicles at a speed that is reasonable and prudent for existing conditions and hazards.4 State statutes authorize maximum speed limits that may vary by highway type (e.g., Interstate highways) or location (e.g., urban district) (NHTSA 1997a, vi).5 Generally, statutory limits apply throughout a political jurisdiction (ITE 1992, 347). 3 The term "risk" as it is used in this report includes both the probability of being in a crash and the severity of the crash. 4 This basic structure is contained in the most recent version of the Uniform Vehicle Code, which provides a model set of motor vehicle laws to encourage uniformity in state traffic regulation (National Committee on Uniform Traffic Laws and Ordinances 1992, 82). 5 There are two types of maximum speed limits: (a) absolute limits and (b) prima facie limits. An absolute speed limit is a limit above which it is unlawful to drive regardless of roadway conditions, the amount of traffic, or other influencing factors. A prima facie speed limit is one above which drivers are presumed to be driving unlawfully but where, if charged with a violation, they may contend that their speed
MANAGING SPEED 22 States and, in most cases, local governments have the authority to establish speed zones where statutory limits do not fit specific road or traffic conditions. Alternative maximum legal speed limits are established by administrative action in the speed zone, typically on the basis of an engineering study, and become effective when the lim- its are posted and properly recorded (ITE 1992, 347). Speed limits are set to inform motorists of appropriate driving speeds under favor- able conditions. Drivers are expected to reduce speeds if conditions deteriorate (e.g., poor visibility, adverse weather, congestion, warning signs, or presence of bicyclists and pedestrians), and many state statutes reflect this requirement. Speed control regulations--both legislated and administratively established maximum speed limits--provide the legal basis for adju- dication and sanctions for violations of the law. State and local offi- cials may also post advisory speed signs, which do not have the force of law but warn motorists of suggested safe speeds for specific condi- tions at a particular location (e.g., turn, intersection approach) (ITE 1992, 347). PURPOSE OF SETTING SPEED LIMITS The primary reason for setting speed limits is safety. In setting speed limits, decision makers attempt to strike an appropriate societal bal- ance between travel time and risk for a road class or specific highway section. Thus, the posted legal limit informs motorists of maximum driving speeds that decision makers consider reasonable and safe for a road class or highway section under favorable conditions. In addi- tion, speed limits provide the basis for enforcement. Well-conceived speed limits provide law enforcement officers and courts with an indication of appropriate speeds for favorable conditions and thus help target enforcement and sanctions on those who drive at speeds that are excessive for conditions and likely to endanger others. Speed was safe for conditions existing on the roadway at that time and, therefore, that they are not guilty of a speed limit violation. Approximately two-thirds of the states have absolute speed limits and one-third have prima facie limits or limits of each type (ITE 1992, 347).
23 Introduction limits have also been established for fuel conservation, as they were following the oil crisis in 1973. Finally, speed limits could be enacted to improve air quality--motor vehicles emit more pollutants at high speeds--but speed limits are very rarely established solely for envi- ronmental goals. The broad objectives of speed limits are not always easy to achieve in practice. For example, the basic premise of a speed limit--that it communicates information about a driving speed that decision mak- ers have determined appropriately balances risk and travel efficiency for a particular road section--assumes both that a safe and reasonable speed can be defined and that there is a cause-and-effect relationship between speed limits (as opposed to speed) and safety (ITE 1993, 1). Neither of these assumptions is self-evident. Drivers, neighborhood residents, traffic engineers, law enforce- ment officials, and legislators may differ as to what constitutes a rea- sonable balance between risk and travel efficiency. For example, local governments frequently receive requests to lower speed limits from neighborhood residents who seek to reduce speeding on local streets. Traffic engineers may not find the reduction to be justified by an engineering study. Drivers themselves--depending on their age, risk tolerance, trip purpose, and familiarity with particular roads--may not agree on what speed best balances risk with travel efficiency. However, without some consensus concerning appropriate driving speeds among drivers, the law enforcement community, and the courts, the imposition of speed limits alone is not likely to have much effect on driver speeding behavior. Moreover, as the experience with the 55-mph (89-km/h) NMSL shows, even when there is a high degree of driver compliance, public attitudes can change over time. Initially there were high levels of sup- port for the 55-mph speed limit, reflecting the national sense of cri- sis because of fuel shortages. Support eroded, however, as the crisis eased and fuel became more plentiful. In 1986, the year before Congress relaxed the 55-mph limit on rural Interstates, the Federal Highway Administration reported that 76 percent of vehicles exceeded 55 mph on these highways (FHWA 1987, p. 183). Attitudes toward appropriate speed limit levels could change again. For example, as the driving population ages or if aggressive driving
MANAGING SPEED 24 incidents, of which speeding is a component, become more perva- sive,6 some segments of the population could favor lower limits. Or, if vehicles and roads become safer, motorists could favor higher lim- its, at least on the safest roads. OVERVIEW OF ISSUES RELATED TO SETTING SPEED LIMITS Safety Safety is the most important reason for managing speed through the imposition of speed limits. Traffic safety has significantly improved on U.S. highways over the last several decades. In 1996, the most recent year for which data are available, the fatality rate per 100 mil- lion vehicle-mi (100 million vehicle-km) of travel remained at its historic low of 1.7 (1.1) as compared with 2.5 (1.6) in 1986 and 3.2 (2.0) in 1976 (NHTSA 1997b, 15). Since 1992, however, the num- bers of fatalities and injuries have slowly crept up, although the fatal- ity rate has remained constant (NHTSA 1997b, 15). Speeding contributes to motor vehicle crashes, but many other driver-related factors affect traffic safety: driving under the influence of alcohol or other drugs, safety belt use, age and attitudes toward risk, and expe- rience of the driver. The relationship among speed limits, driver speed choice, and safety on a given road is complex. Setting appropriate speed limits and related enforcement strategies is the first step in a chain of events that may affect crash probability and crash severity (Figure 1-2). Presumably, well-conceived speed limit and enforcement policies will 6 According to Ricardo Martinez, administrator of the National Highway Traffic Safety Administration (NHTSA), aggressive driving is defined as driving behavior that endangers or is likely to endanger people or property (AASHTO Journal 1997, 8). It includes such driving behaviors as honking, gesturing and screaming, tailgating, run- ning lights, weaving through traffic, improper lane changes, speeding, shoulder run- ning, and even shooting (Highway and Vehicle Safety Report 1997, 3). Incidents of aggressive driving are linked with irresponsible driving behavior, reduced levels of traf- fic enforcement, and increased congestion and travel, especially in urban areas (AASHTO Journal 1997, 8).
25 Introduction Driver Characteristics Vehicle (e.g., age, attitude Characteristics toward risk) and Performance Speed Limits Driver Choice of Distribution Crash Probability and Speed and Route of and Enforcement Level Speeds Severity Other Driver, Vehicle, Roadway, and Environmental Roadway Factors Traffic Flow Characteristics, Characteristics Weather, and Time of Day Figure 1-2 Systems view of relationships among speed limits, enforce- ment levels, and safety. affect a driver's choice of speed. They may also influence route selec- tion. For example, a driver may divert to a road with a higher speed limit if such an alternative is available. However, driver decisions about speeds and route choice are influenced by many other factors including the characteristics of the driver, the vehicle, the road, and traffic flow; weather; and time of day (Figure 1-2). Speed choices made by individ- ual drivers determine the aggregate distribution of traffic speeds on a particular road section, which in turn affects both the probability and severity of crashes.7 Finally, over time the safety record of a particular road may influence drivers' speeds and, in the long term, could result in a change in the speed limit or in enforcement strategies. A cause-and-effect relationship between speed limits and safety is not straightforward. In this study, what is known about the relation- ships among speed, crash probability, and crash severity is examined first to help identify the importance and role of speed in traffic safety. Then, the effects of speed limits and changes in these limits as a 7 Of course, in those situations in which there is only one vehicle on the road, then it is the speed level alone--not the distribution of speeds--that affects crash probability and severity.
MANAGING SPEED 26 strategy for managing driving speed are reviewed to help determine the effect on driver behavior and safety outcomes. Finally, the impli- cations of these reviews for methods of setting speed limits as they relate to safety are discussed. Road Class The relationships among speed limits, speed, and safety differ by road class (see text box). Congested traffic on city streets with low posted speed limits creates numerous opportunities for vehicle con- flicts (e.g., stopped and turning vehicles, traffic entering the street). Numerous crashes may result, but typically they are not severe unless a pedestrian or a cyclist is struck. Driving on divided, limited-access highways with substantially higher speed limits under free-flowing traffic conditions offers an environment with less potential for vehi- cle conflicts. When crashes do occur, however, they are more likely to involve injuries or fatalities. If crashes are aggregated over both road types, speeds appear to be inversely related to crashes (i.e., as speed increases, crashes decline). However, if crash type and road type are separated, the true relationships are revealed (i.e., driving slowly on congested urban roads is associated with large numbers of crashes that often involve minor injury or property damage only, whereas driving on high-speed freeways is associated with fewer but more severe crashes). Where possible in this study, the relationships between speed limits, speed, and safety are analyzed by road type, and the methods of setting speed limits for different road classes are discussed. Driver Perception of Risk The willingness of drivers to heed and comply with speed limits is influ- enced by their perception of the riskiness of driving in general and of speeding in particular. According to a nationwide survey conducted for Prevention Magazine by Princeton Survey Research Associates, nearly 57 percent of drivers surveyed say they do not observe the speed limit, that is, they do not always drive at or below the speed limit (1996, 7). Notwithstanding the limitations of self-reported behavior, one could legitimately ask why so many drivers exceed posted speed limits.
27 Introduction U.S. Highways by Road Class (AASHTO 1994, 1015; TRB 1984, 2022) Interstate highways and freeways are a type of principal arterial high- way characterized by multiple lanes with traffic separated by direc- tion, controlled access (i.e., limited access), and grade separation (rather than intersections). These highways, which can accommo- date the highest travel speeds, generally provide direct service between cities and larger towns either between or within states and generate a large proportion of longer trips. Most long-distance commercial and recreational travel occurs on the rural portions of this system. The urban segments also serve local traffic in and around major metropolitan areas. Other arterial highways serve as traffic "arteries" by carrying traf- fic to and from urbanized areas. These highways also carry large traffic volumes at relatively high speeds, but access is not controlled (i.e., nonlimited access), most intersections are at grade, and access to abutting property is permitted. In rural areas, arterial highways provide for through-travel movement between and within counties. Arterial highways provide for major circulation within metropolitan areas. Collector highways collect and disperse traffic between rural and urban arterial highways and lower-level roads. These highways have at-grade intersections, limited sight distance, and other design limitations. They carry lower traffic volumes at lower speeds. Local roads and neighborhood and residential streets account for the vast majority of road mileage but carry the smallest traffic volumes at low speeds. The primary function of these roads is to provide access to residential areas, individual farms, and businesses; through traffic is discouraged. Pedestrians, bicyclists, and parked vehicles may use these facilities.
MANAGING SPEED 28 In general, motorists do not perceive driving as a life-threatening activity. Millions of Americans drive each day and most complete their trips safely, thus reinforcing the individual driver's perception that the risks involved in driving are low. With about 180 million licensed drivers, each driving an average of 13,800 mi (22 200 km) per year, driver involvement in a crash,8 on the average, is one every 131,300 driver-mi (211 300 driver-km), or once every 9.5 years of driving. Driver involvement in a fatal crash is considerably less--one every 44 million driver-mi (71 million driver-km), or about once every 3,200 years of driving.9 Motorists have different tolerances for risk, and they travel under a variety of conditions, some of which are more conducive to serious crashes. For example, the probability of driver involvement in a fatal crash is considerably higher on two-lane rural roads, on weekend nights when alcohol consumption is a key factor, and for young (under 25) and older (65 or over) drivers and vehicle occupants. Nevertheless, the common perception, even among drivers who have been in a crash, is that such incidents are rare, unpredictable events largely outside rea- sonable human control--a view reinforced by the frequent direct feed- back of crash-free motor vehicle trips (Evans 1991, 311). Many motorists believe not only that the personal risk of driving is low but also that they themselves are less likely than others to expe- rience a crash. Most drivers rank their own driving skills and safe driving practices as better than average (Evans 1991, 322324; Williams et al. 1995, 119; Svenson 1981, 146). Drivers may perceive their driving capabilities to be above average, but their actual judgments may not be as good as they believe. Drivers often underestimate the risks of traveling at high speeds. Younger drivers, for example, frequently say "I can handle the speed; my reflexes are good." In addition, drivers often misjudge the speed 8 Estimates of driver involvement in motor vehicle crashes provided by the National Safety Council--18,900,000 driver involvements in 1996--were used for this calcula- tion (National Safety Council 1997, 78). Driver involvements in police-reported crashes, estimated at 12,185,000, are considerably lower (NHTSA 1997b, 94). 9 NHTSA is the primary source for the figures on driver involvements in fatal crashes and vehicle miles traveled (NHTSA 1997b, 15, 94).
29 Introduction at which they are traveling. They have limited capacity to estimate the relative speeds of vehicles in both car-following and car- overtaking situations, and after prolonged travel at higher speeds they are apt to perceive moderate speeds to be even lower than they really are (Várhelyi 1996, 3839; Recarte and Nunes 1996, 291). Given the tendency of drivers to underestimate or misjudge the effects of speed in driving, it is not surprising that speeding is often ranked by motorists as less serious than other traffic offenses, such as driving while intoxicated or running red lights (Várhelyi 1996, 3336). In part because of driver underestimation or misjudgment of the effects of speed in driving, most drivers in the United States do not interpret speed limits as rigid thresholds that must be observed. In addition, drivers do not always concur that speed limits are reason- able for conditions. Thus, they have come to expect enforcement "tol- erances" of up to 10 mph (16 km/h), and even greater on roads on which posted speed limits are well below average traffic speeds (TRB 1984, 149). For all these reasons, many drivers appear more con- cerned with "going with the flow," or going below the enforcement threshold, than with the risk of a crash or of detection for exceeding the speed limit. If consistently applied and enforced, speed limits can be an impor- tant means of conveying useful information to drivers about appro- priate driving speeds. However, as with most information, drivers will heed the message to the extent that it is perceived as important, rel- evant, and consistent with their prior beliefs (Bettman et al. 1991, 1819). Of course, speed limits have the force of law and, if enforced, can influence behavior. The more drivers perceive speed limits to be credible and reasonable for conditions, and enforced, the more likely they will be obeyed. FACTORS AFFECTING DETERMINATION OF APPROPRIATE SPEED LIMITS Those responsible for determining appropriate speed limits--state and local legislators and traffic engineers, often with input from law enforcement officials and community groups--must define limits that are appropriate for different road classes and users.
MANAGING SPEED 30 Roadway Function and Use Different vehicle speeds and speed limits are appropriate for different road classes. New highways are planned with a speed in mind, known as the design speed, to accommodate the intended function of a par- ticular facility and its expected level of service, subject to the con- straints of terrain, development, and other environmental factors (Krammes et al. 1996, 8).10 Once selected, the design speed influences many critical design decisions, such as the amount of banking on hor- izontal curves and the length of vertical curves. The design criteria rec- ommended for these critical features by the American Association of State Highway and Transportation Officials (AASHTO) have consid- erable built-in safety margins; they are often based on worst-case sce- narios and performance characteristics of older vehicles (e.g., locked-wheel braking on wet pavements) (Krammes et al. 1996, 14). Speed limits are often set on the basis of operating speeds deter- mined by spot speed surveys of a sample of free-flowing vehicles traveling under favorable weather and visibility conditions at a par- ticular location on a highway.11 This speed can exceed design speeds. The disparity, however, is not necessarily cause for concern because of the built-in safety margins in the design values.12 In addition, many highway features are constructed with more than minimum design val- ues so that the design speed may actually apply to only a small number of critical features on a road segment. As a result, the design speed of a highway is likely to underestimate the "maximum safe speed" over much of its length (Krammes et al. 1996, 14). 10 See glossary for definitions used in this section. 11 The speed limit is often set at the 85th percentile of the speed distribution, that is, the speed at or below which 85 percent of drivers are operating their vehicles. (See glossary and Chapter 3 for a more detailed discussion.) 12 Concerns about liability when posted speed limits based on vehicle operating speeds exceed the design speed of a highway segment, however, may lead to a redefinition of terms to bring about greater consistency among posted speed limits, operating speeds, and design speeds (Fitzpatrick et al. 1997, 59). A recently initiated multiyear study conducted under the auspices of the National Cooperative Highway Research Program will examine the relationships among design speeds, operating speeds, and posted speeds, and will develop appropriate alternatives to speed-based geometric cri- teria for use in project design.
31 Introduction According to AASHTO-recommended design criteria, the highest-level roads--new freeways and expressways built to expedite through traffic--should be designed for vehicular speeds of 68 to 75 mph (110 to 120 km/h) where environmental conditions are good and traffic volumes are light (AASHTO 1994, 63). Speed limits for these roads are as high as 75 mph in some states (Table 1-1) and may exceed design speeds at certain locations. At the other end of the spectrum, AASHTO-recommended crite- ria for local streets serving residential areas suggest designs that accommodate speeds between 19 and 31 mph (30 and 50 km/h), and speed limits are typically established in this range (AASHTO 1994, 429). Certain factors, such as development and street grid patterns with numerous intersections, have a greater influence on actual vehi- cle speeds than design speed, which has limited practical significance on these streets (AASHTO 1994, 429). Differences in speed limits by road class also reflect differing objectives with respect to road users. Travel efficiency is a priority on rural Interstate highways and freeways, which are restricted to motorized vehicles, have limited access, and are intended primarily for through traffic. Thus, speed limits are set at the higher end of the traffic speed distribution on these highways. By comparison, access is the primary consideration for motor vehicles on residential streets. Motorists share local streets with pedestrians and bicyclists, who are more vulnerable than vehicle occupants in a collision. Thus, travel efficiency is not the primary consideration in neighborhood travel, and speed limits often correspond to speeds at the lower end of the speed distribution.13 These distinctions may not be as clear on other road classes. For example, travel efficiency is still an important goal on urban Interstate highways, but in many metropolitan areas congestion lim- its travel speeds for several hours of the day. Through traffic must share urban Interstates with local traffic, whose frequent entries and exits at closely spaced interchanges can interrupt traffic flow, creating 13 Speed limits in many urban areas actually correspond to the 30th percentile speed in the speed distribution (Tignor and Warren 1989, 2).
MANAGING SPEED 32 the potential for vehicle conflicts. Thus these highways may be posted with speed limits lower than design considerations alone would warrant. Nonlimited-access, two-lane rural roads and subur- ban arterials are examples of other road types that have multiple objectives and multiple road users. Many two-lane rural roads can accommodate through travel at high speeds. However, the potential for conflict at intersections and driveways, and between motor vehi- cles and farm equipment on the road, often requires setting speed limits lower than if travel efficiency were the primary objective. Similarly, the exposure of pedestrians and cross-street traffic to through traffic operating at high speeds on arterials in rapidly devel- oping suburban areas may also warrant lowering speed limits on these roads. Vehicle and Driver Characteristics In some states, differential speed limits are established for major vehicle classes with different operating characteristics--primarily heavy trucks and vehicles towing trailers (Table 1-1).14 Differential speed limits reflect longer stopping distances for such vehicles than for passenger vehicles. Some analysts argue that differential speed limits exacerbate actual speed differences among vehicles, creating the potential for conflict by encouraging passing and overtaking maneuvers, thereby degrading safety. State experience with differen- tial speed limits is reviewed in this study. Drivers have different capabilities to operate vehicles safely at higher speeds. For example, speeding appears to be a factor in the high fatality rate of the youngest segment of the driving population. Drivers aged 15 to 20 years old are involved in more speeding-related fatal crashes than any other age group (NHTSA 1997c, 2). Numerous studies have documented the high risk-taking behavior of young drivers, which involves tailgating and driving at speeds well in excess of the speed limit ( Jonah 1986; Evans and Wasielewski 1983; Wasielewski 1984 in Evans 1991, 104, 137). Driving inexperience 14 A variation of this approach is differential speed limits for day and night. Historically, crash probability was higher at night than during the day.
33 Introduction may also play a role (Evans 1991, 104). However, without knowing the incidence of speeding by age group in the general population of drivers, it is not possible to determine the extent to which speeding contributes to the probability of fatal crashes for the youngest drivers. Speeding is not a major factor in the high fatality rates of older drivers. As a group, drivers 65 years of age and older have the second- highest fatality rates of all driver groups but the lowest involvement in speeding-related fatal crashes (NHTSA 1997c, 2). Many older drivers have reduced capacity to handle speed, though, which increases their probability of having a crash. For example, on the average, they have longer reaction times and lessened visual acuity (TRB 1989, 7273). Their tendency to misjudge the speed of oncom- ing vehicles when turning at intersections and to drive at lower speeds than the prevailing traffic, in addition to their frailty, con- tributes to their high fatality and injury rates (NHTSA 1997b, 21). These and other differences in the behavior and abilities of differ- ent driver population groups that relate to speed are difficult to man- age through speed limit policies. Highways and speed limits must accommodate a broad spectrum of drivers. Minimum speed limits have been established on some high-speed roads that may deter slow drivers as well as vehicles that cannot maintain adequate speed levels. In certain communities speed limits have been reduced in areas where large concentrations of the elderly reside. Speed limits alone, however, are insufficient to address the highway safety problems of these spe- cial populations. Other strategies for addressing their speed-related problems are briefly discussed in the report. ORGANIZATION OF REPORT Many of the issues raised in this introductory section are addressed in detail in the following chapters. In Chapter 2 the relationship between speed and safety is investigated in more depth to help iden- tify the role of speed in crash causation and injury severity. The rela- tionship of speed to travel time and vehicle operating costs is also considered because drivers make trade-offs among safety, travel time, and other trip-related costs in deciding what speed to travel. Having laid the groundwork for the importance of speed on traffic safety and
MANAGING SPEED 34 travel efficiency, Chapter 3 is focused on speed limits--the primary method for managing speed addressed in this study. The theoretical justification for speed limits is elaborated, the strengths and weak- nesses of the primary methods of setting speed limits are described, and what is known about the effects of speed limits on driving speeds and safety is summarized. Speed enforcement and adjudication issues are examined in Chapter 4, including the relevance of deterrence theory for speed enforcement and the potential for application of automated enforcement technologies to augment traditional enforce- ment methods. In Chapter 5, other speed management strategies are discussed briefly, including highway design and infrastructure approaches, highway- and vehicle-related technologies, and inter- ventions for special driver populations. Finally, the committee's guid- ance on appropriate strategies for both setting and enforcing speed limits is provided in Chapter 6. REFERENCES ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials FHWA Federal Highway Administration ITE Institute of Transportation Engineers NHTSA National Highway Traffic Safety Administration TRB Transportation Research Board AASHTO. 1994. A Policy on Geometric Design of Highways and Streets. Washington, D.C. AASHTO Journal. 1997. Hearing Addresses Aggressive Driving. Vol. 97, No. 29, July 18, pp. 79. Bettman, J.R., E.J. Johnson, and J.W. Payne. 1991. Consumer Decision Making. In Handbook of Consumer Behavior (T.S. Robertson and H.H. Kassarjian, eds.), Prentice-Hall, Englewood Cliffs, N.J., pp. 5084. Evans, L., and P. Wasielewski. 1983. Risky Driving Related to Driver and Vehicle Characteristics. Accident Analysis and Prevention, Vol. 15, No. 3, pp. 121136. Evans, L. 1991. Traffic Safety and the Driver. Van Nostrand Reinhold, New York, 405 pp. FHWA. 1987. Highway Statistics 1986. FHWA-PL-89-002. U.S. Department of Transportation. Fitzpatrick, K., R.A. Krammes, and D.B. Fambro. 1997. Design Speed, Operating Speed and Posted Speed Relationships. ITE Journal, Vol. 67, No. 2, Feb., pp. 5259.
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