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Executive Summary In 1995 Congress repealed the National Maximum Speed Limit (NMSL) of 55 mph (89 km/h), returning to the states the responsi- bility for setting speed limits on major highways. Since then, 49 state legislatures have taken the opportunity to raise speed limits on Interstate highways--and, in some cases, on other major roads-- often to levels that had been in effect before the NMSL was estab- lished in 1974. Some states are reexamining methods for determining appropriate speed limits. Several are monitoring the effects of changes in speed limits on driving speeds and safety outcomes. In this study current practice in setting speed limits on all roads-- not just major highways--is reviewed, and guidance to state and local governments on appropriate methods of setting speed limits and related enforcement strategies is provided. The study is intended for a broad audience of those involved in decisions about speed limits-- state and local legislators, traffic engineers, and law enforcement and judicial officials, as well as the interested general public. OVERVIEW OF CURRENT PRACTICE Speed limits are one of the oldest strategies for controlling driving speeds. Connecticut imposed the first maximum speed limit of 8 mph (13 km/h) in cities in 1901. Since that time, primary responsi- bility for setting speed limits has remained with state and local gov- 1
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MANAGING SPEED 2 ernments. Nationally mandated speed limits such as the NMSL are exceptions to the rule. The current framework for speed regulation was developed in the 1920s and 1930s. Each state has a basic statute that requires driv- ers to operate vehicles at a speed that is reasonable and prudent for existing conditions. Speed limits are legislated by road class (e.g., Interstate highway) and geographic area (e.g., urban district). They generally apply to all roads of a particular class throughout a jurisdic- tion. However, state and most local governments have the authority to change the limits by establishing speed zones for highway sections where statutory limits do not fit specific road or traffic conditions, and to determine alternative maximum speed limits in these zones. Legislated speed limits are established by state legislatures, city councils, or Congress on the basis of judgments about appropriate trade-offs among public safety, community concerns, and travel effi- ciency. Legislated limits are established for favorable conditions-- good weather, free-flowing traffic, and good visibility. Drivers are expected to reduce speeds as these conditions deteriorate. Speed lim- its in speed zones are determined administratively. The most com- mon approach sets the limit on the basis of an engineering study, which takes into consideration such factors as operating speeds of free-flowing vehicles, crash experience, roadside development and roadway geometry (e.g., curvature, sight distance), and parking and pedestrian levels to make a judgment about the speed at which the limit should be set. In many speed zones, it is common practice to establish the speed limit near the 85th percentile speed, that is, the speed at or below which 85 percent of drivers travel in free-flow con- ditions at representative locations on the highway or roadway section. This approach assumes that most drivers are capable of judging the speed at which they can safely travel. REGULATION OF DRIVING SPEEDS If most drivers are assumed to be capable of making reasonable judgments about appropriate driving speeds, why are speed limits even necessary? The primary reason for regulating individual choices is the signif- icant risks drivers can impose on others. For example, a driver with a
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3 Executive Summary higher tolerance for risk may decide to drive faster, accepting a higher probability of a crash, injury, or even death in exchange for a shorter trip time. This driver's decision may not adequately take into consid- eration the risk his choices impose on the other road users. Even a driver traveling alone who is involved in a single-vehicle crash may impose medical and property damage costs on society that are not fully reimbursed by the driver. The imposition of risks on others that are not adequately considered when the activity of a person or a firm affects their welfare is a primary reason for government intervention in many areas besides traffic safety, such as environmental protection and product safety. Another reason for regulating speed derives from the inability of some drivers to correctly judge the capabilities of their vehicles (e.g., stopping, handling) and to anticipate roadway geometry and roadside conditions sufficiently to determine appropriate driving speeds. This reason may not be as relevant for experienced motorists driving under familiar circumstances. However, inexperienced drivers or experi- enced drivers operating in unfamiliar surroundings may underesti- mate risk and make inappropriate speed choices. Even drivers familiar with a particular road can make inappropriate decisions because of fatigue or other factors. A final reason for regulating speed, which is related to the issues of information adequacy and judgment, is the tendency of some driv- ers to underestimate or misjudge the effects of speed on crash prob- ability and severity. This problem is often manifested by young and inexperienced drivers and may be a problem for other drivers. The risks imposed on others and the adequacy of information about appropriate driving speeds vary by road class. For example, the risks imposed on others by individual driver speed choices are likely to be relatively small on rural Interstate highways where free-flowing traffic creates fewer opportunities for conflict with other road users. Moreover, under normal conditions, drivers typically have adequate information to determine appropriate driving speeds because these highways are usually built to the highest design standards, access is limited, and roadside activity is minimal. In contrast, the risks imposed on others by individual driver speed choices may be large on urban arterials where roadside activities are numerous and traffic vol-
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MANAGING SPEED 4 umes are high for extended periods of the day, increasing the proba- bility of conflict with other road users. These differences are impor- tant factors for consideration in setting appropriate speed limits on different types of roads. THE SAFETY CONNECTION AND THE ROLE OF SPEED LIMITS Drivers' speed choices impose risks that affect both the probability and severity of crashes. Speed is directly related to injury severity in a crash. The probability of severe injury increases sharply with the impact speed of a vehicle in a collision, reflecting the laws of physics. The risk is even greater when a vehicle strikes a pedestrian, the most vulnerable of road users. Although injury to vehicle occupants in a crash can be mitigated by safety belt use and airbags, the strength of the relationship between speed and crash severity alone is sufficient reason for managing speed. Speed is also linked to the probability of being in a crash, although the evidence is not as compelling because crashes are complex events that seldom can be attributed to a single factor. Many driver attri- butes and behavioral factors besides speed affect the probability of crashes--driving under the influence of alcohol or other drugs, age, attitudes toward risk, and experience of the driver--but speed has been shown to play an important role. The concept of speed itself is complex. It can relate to the speed of a single vehicle or to the distribution of speeds in a stream of traffic. Crash involvement on Interstate highways and nonlimited-access rural roads has been associated with the deviation of the speed of crash-involved vehicles from the average speed of traffic. Crash involvement has also been associated with the speed of travel, at least on certain road types. For example, single-vehicle crash involvement rates on nonlimited-access rural roads have been shown to rise with travel speed. The primary purpose of speed limits is to enhance safety by reduc- ing the risks imposed by drivers' speed choices. Speed limits enhance safety in at least two ways. By establishing an upper bound on speed, they have a limiting function; the objective is to reduce both the
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5 Executive Summary probability and the severity of crashes. Speed limits also have a coor- dinating function. Here the intent is to reduce dispersion in speeds (i.e., lessen differences in speed among drivers using the same road at the same time) and thus reduce the potential for vehicle conflicts. A related function of speed limits is to provide the basis for enforce- ment and sanctions for those who drive at speeds excessive for con- ditions and endanger others. In setting speed limits, decision makers attempt to establish a rea- sonable balance between risk (safety) and travel time (mobility) for a road class or specific highway section. Thus, the posted speed limit should inform motorists of maximum driving speeds under favorable conditions that decision makers consider reasonable and safe for a road class or highway section. EFFECTIVENESS OF SPEED LIMITS The principal objective of speed limits is improved safety, but sim- ply posting a speed limit does not guarantee the desired change in driving speeds or a reduction in crashes or crash severity. Recent changes in speed limits in the United States provide an opportunity to study these effects. In 1987 Congress allowed states to raise speed limits from 55 to 65 mph (89 to 105 km/h) on qualifying sections of rural Interstate highways. In the immediately following years, most states that raised limits observed increases on the order of 4 mph (6 km/h) in average speeds and 85th percentile speeds, and increases in speed dispersion of about 1 mph (2 km/h). These speed changes were generally associated with statistically significant increases in fatalities and fatal crashes on the affected highways--a plausible finding because of the strong link between even modest increases in speed at higher speeds and increased crash severity. Although they provided compelling evidence of higher fatalities on Interstate highways, most studies did not examine the issue of broader network effects, such as potential effects on safety from any traffic diversion or redeployment of enforcement personnel. A more limited num- ber of studies that attempted to look at such system effects reported mixed results. One study that examined effects on non-Interstate rural highways found evidence of spillover effects in higher fatalities
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MANAGING SPEED 6 on these roads. Two other studies that examined system effects on a county- and statewide basis reported evidence of offsetting reduc- tions in fatalities that resulted in neutral and even positive systemwide net safety outcomes. Additional research and analysis are needed to determine the extent and size of such systemwide effects. Studies have been conducted following repeal of federal maximum speed limits in 1995; many of them focused on Interstate highways. Most found results similar to the speed limit changes in 1987: mod- est increases in average speeds and 85th percentile speeds and, in some cases, speed dispersion on highways on which speed limits were raised. Although not consistent across all states, most studies indi- cated an increase in fatalities on highways on which speed limits were raised. Most studies did not explore any possible system effects, and the results should be considered preliminary because they are gener- ally based on 1 year of data or less. Most of the recent U.S. literature has focused on the effects of rais- ing speed limits on Interstate highways. In the future, however, cir- cumstances could warrant reductions in speed limits on some Interstates and other major highways. An earlier Transporta- tion Research Board study (1984) of the effects of the national 55-mph (89-km/h) speed limit found that the lower limit reduced both travel speeds and fatalities, although driver speed compliance gradually eroded. The report provides a comprehensive review of studies that examined the effects of lowering speed limits on major highways. In contrast to the extensive analysis of speed and safety changes on Interstate highways, few studies have examined the effects of chang- ing speed limits on lower-speed, nonlimited-access highways. Those that were identified found little effect on driving speeds or crash rates when speed limits were raised to near the 85th percentile speed or lowered to near the 35th percentile speed in selected speed zones on rural roads and on urban and suburban arterials. The results, however, cannot be generalized to speed zones on all nonlimited- access highways. Further, the lack of observed changes in driving speeds may be explained to the extent that changes in posted speed limits simply legalized existing driver behavior, that is, changed
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7 Executive Summary compliance levels rather than speed behavior. Nevertheless, the find- ings suggest the difficulty of altering behavior merely by changing the sign. ROLE OF ENFORCEMENT AND SANCTIONS Managing speeds through speed limits requires a system of speed laws and a process for establishing reasonable speed limits as well as enforcement, sanctions, and public education, ideally all working together. Enforcement is an integral part of such a system. Even if reasonable speed limits are established through legislation or engi- neering studies and most drivers comply within a small tolerance, enforcement is still necessary to ensure the conformity of a minority of drivers who will obey traffic regulations only if they perceive a credible threat of detection and punishment for noncompliance. The main difficulty with the traditional approach to speed enforcement--radar enforcement using a mobile or stationary police vehicle--is its short-lived temporal and spatial effect on deterring speeding. Maintaining the deterrence effect requires a level of enforcement intensity and expense that has proven difficult to sustain because of competing enforcement priorities and limited resources available for speed enforcement. Targeted enforcement combined with focused publicity campaigns can boost the effectiveness of traditional enforcement methods. Automated enforcement, particularly photo radar, has been shown to be efficient and effective where it has been used for speed control, particularly on high-volume arterials. Photo radar could also be cou- pled with variable speed limit systems on urban Interstate highways where high traffic volumes can make traditional enforcement meth- ods hazardous. Alternatives to enforcement to achieve desired driv- ing speeds on local roads include physical measures known as "traffic calming" (e.g., speed humps, roundabouts, and raised intersections). Redesigning roads to achieve greater congruity between driver per- ceptions of appropriate travel speeds and the cues provided by the road itself (e.g., narrowing lanes) may also influence motorists' speeds. A proper mix of these approaches can enable police to lever- age their resources and deploy them efficiently.
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MANAGING SPEED 8 Traffic court judges are also important participants in effective speed enforcement. They may overturn speeding violations if they think the speed limits are unreasonable or reduce fines if they believe the sanctions are too harsh. If judges are lenient in their treatment of speeding offenses and routinely dismiss speeding citations, the incentive for the police to enforce the speed limits may be reduced. Thus it is important that traffic court judges--as well as the police and motorists--perceive that speed limits are reasonable and enforceable. GUIDANCE On the basis of its review of the purpose and methods of setting and enforcing speed limits, the committee offers the following guidance to responsible decision makers. Its primary focus is on the effects of speed limit policies on safety rather than on travel time, energy consump- tion, or environmental pollution. The committee attempted to be as specific as possible, but the relevant studies and the data on which the guidance is based fall short of providing sufficient support for quanti- fying with much precision the effects of changes in speed limits on driving speeds and safety. General Framework for Establishing Speed Limits The current general approach--legislated speed limits and administra- tively established speed zones--is sound. It balances the desirability of uniform speed limits (legislated limits for broad road classes) with the need for exceptions (speed zones) to reflect local differences. The practice of establishing speed limits to reflect a reasonable balance between travel speeds and risks under favorable operating conditions is also sensible. Making Decisions About Appropriate Speed Limits Decisions about legislated speed limits reflect trade-offs and judg- ments about the relative importance of safety, travel time, and feasi- bility of enforcement. Legislators should consult with traffic engineers, law enforcement officials, judges, public health officials,
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9 Executive Summary and the general public in their deliberations. Consultation, however, cannot ensure that all parties will reach consensus, particularly where multiple interests are involved, such as residents and commuters on residential streets. In addition to safety, final selection of a speed limit should meet the requirements of enforceability and acceptance by the community at large. Provision should also be made to monitor driv- ing speeds and crash experience, and the decision should be reviewed periodically because road conditions, vehicle safety features, driving attitudes, and behavior change over time. Determination of appropriate speed limits in speed zones should be made on the basis of an engineering study. Traffic engineers nor- mally conduct the study; consultation with law enforcement officials should be standard practice. Elected officials and citizen groups may also become involved when community concerns have been raised about driving speeds. Speed zones should be reviewed periodically-- with greater frequency where conditions are changing rapidly, such as developing suburban areas--to determine whether a change in speed limits or boundaries of the speed zone is warranted. Methods of Setting Speed Limits Legislated Speed Limits The strong link between speed and crash severity supports the need for setting maximum speed limits on high-speed roads (e.g., Interstate highways, freeways, high-speed rural multi- or two-lane roads) to place an upper bound on travel speeds to reduce crash probability and sever- ity. The committee refrained from recommending a specific numeric limit, however. Road conditions vary too widely to justify a "one-size- fits-all" approach. Roads, even those in the same class, are not all built to the same design standards, nor are traffic volumes uniform. In determining appropriate speed limits for each road type, deci- sion makers should be guided by both the likely risks imposed on others by individual driver speed choices and the availability of infor- mation to enable drivers to make appropriate speed choices. They should take enforcement practicality into consideration. Decision makers should also request technical information on the following
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MANAGING SPEED 10 four factors to help guide their determination of appropriate legis- lated speed limits for a specific road class: · Design speed, that is, the design speed of a major portion of the road, not of its most critical design features (e.g., a sharp curve); · Vehicle operating speed, measured as a range of 85th percentile speeds taken from spot-speed surveys of free-flowing vehicles at representative locations along the highway; · Safety experience, that is, crash frequencies and outcomes; and · Enforcement experience, that is, existing speed tolerance (i.e., allowance for driving above the posted speed limit) and level of enforcement. The weight given to these factors, particularly those related to speed, depends on the type of road. For example, on many rural Interstate highways, vehicle operating speeds should be a major fac- tor in setting speed limits. Design speeds provide little additional information because restrictive design features are not generally pres- ent on these highways; typically drivers can anticipate conditions and determine appropriate driving speeds. In addition, risks to other road users are small compared with other highways. Finally, maintaining high levels of enforcement is difficult on long stretches of rural Interstate. In contrast, design speeds should carry greater weight in the determination of speed limits on nonlimited-access rural roads where restrictive roadway geometry is likely to play an important role in defining an appropriate driving speed. Poor safety records on these roads support lower speed limits, but the limits must be reasonable for conditions; enforcement is limited because of extensive rural road mileage. Safety and enforcement considerations should be given higher pri- ority than design speeds or vehicle operating speeds on many urban roads, particularly residential streets. Intersections and traffic signals play a more critical role than design in limiting speed. Driver mis- judgment about appropriate driving speeds poses high risks to vul- nerable road users (e.g., pedestrians and bicyclists) on many urban roads. Neighborhood pressures may result in setting very low speed limits on residential streets, but often they are not enforced--or
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11 Executive Summary enforcement tolerances are large--and compliance is poor even by some neighborhood residents. Thus, where low speeds are desirable, speed limits must be enforced, or alternatives such as traffic calming should be considered for certain residential streets. More detailed guidance for each of seven road classes plus one category for special speed zones (e.g., school and work zones) is provided in Chapter 6. Speed Limits in Speed Zones Determination of appropriate speed limits in speed zones should be made on the basis of an engineering study. The most common factor considered in setting speed zone limits is the 85th percentile traffic speed. Setting the speed limit at or near this level can be desirable on some roads because it (a) enables the police to focus their enforce- ment efforts on the most dangerous speed outliers and (b) is gener- ally at the upper bound of a speed range where crash involvement rates are lowest on certain road types, according to some studies that have examined the relationship between speed and crash probability. Setting the speed limit primarily on the basis of the 85th per- centile speed is not always appropriate. Potential safety benefits may not be realized on roads with a wide range of speeds (i.e., the spread between the slowest and fastest drivers). Basing the speed limit on a measure of unconstrained free-flowing travel speed is not appropri- ate for urban roads with a mix of road users and high traffic volumes and levels of roadside activity. Traffic engineers should consider an expert-system approach, discussed in Chapter 3, which offers a sys- tematic and consistent method of determining speed limits in these speed zones. Differential Speed Limits No conclusive evidence could be found to support or reject the use of dif- ferential speed limits for passenger cars and heavy trucks. More research and evaluation of the effects of differential speed limits on driving speeds and safety outcomes are needed in states that have adopted them. Motorists do not appear to decrease speed at night when lower nighttime speed limits are in effect. However, compelling evidence
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MANAGING SPEED 12 could not be found to support the elimination of nighttime speed limits in states that have adopted them. Variable Speed Limits Technology is available to support speed limits that change with con- ditions, but more experimentation and evaluation are needed to determine the effectiveness of these systems from a safety and traffic efficiency perspective and to learn where variable speed limits can be deployed most usefully. The current high cost of variable speed limit systems restricts their use to Interstate highways and freeways with high traffic volumes or to selected segments of major roads where weather (e.g., fog, visibility) is a frequent problem. Once their effec- tiveness is more clearly established, broader application of variable speed limit systems should be considered on urban Interstate high- ways in the United States because they are well suited to addressing temporal changes in traffic volumes, speed, and density on these highways. Enforcement and Other Speed Management Strategies Policy makers can affect the level of enforcement through resource allocation, but they must recognize that if drivers believe that a speed limit is unreasonable, enforcement will be difficult and expensive. If a low speed limit is posted on a road designed for higher speeds, enforcement problems will be considerable. This occurred on many Interstate highways under the NMSL. When speed limits were raised by 10 mph (16 km/h) on sections of qualified rural Interstate highways in 1987, average traffic speeds increased much less than the change in the speed limit immediately following the change. Apparently many drivers were already exceeding the old speed limits because speeds had crept up since the NMSL went into effect. Strategic deployment of traditional enforcement methods on roads and at times when speed-related incidents are most common or where road conditions are most hazardous can help focus resources on potential problems. The relative infrequency of crashes, however, can make it difficult to show systematic safety improvements from
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13 Executive Summary targeted enforcement strategies. Planned patrols at varying times and locations can extend deterrence effects temporally and spatially from particular locations, but only after an initial period of continuous enforcement. Patrols must be visible and sufficiently frequent to cre- ate a credible deterrent. Police can improve compliance by combining enforcement initiatives with high-profile public information cam- paigns. Publicity must be followed up by enforcement actions, how- ever, if the approach is to successfully deter speeding. Changing fundamental attitudes about speeding requires a long-term sustained effort. Automated enforcement, particularly photo radar, can be used to complement traditional enforcement methods, particularly where roadway geometrics or traffic volume makes traditional methods dif- ficult or hazardous. Photo radar is controversial. Its successful intro- duction requires adoption of legal changes (e.g., resolution of constitutional privacy issues, vehicle owner versus driver liability), funding, and public education. It should be deployed selectively at first--at locations that are hazardous and difficult to patrol with tra- ditional methods and where speeding is a problem--to ensure essen- tial public support. In the near term, speed limits should be set at levels that are largely self-enforcing or at the lowest speed the police are able to enforce. Speed limits alone will not be effective in all situations. Keeping driving speeds at desired levels in urban areas poses a particular chal- lenge. Traffic calming can be used judiciously on residential streets, but community as well as resident support is important for its suc- cess. Systemwide effects must also be considered so that the traffic and safety problems will not simply migrate to other streets. Road redesign has the potential to achieve greater consistency between desired and actual operating speeds. Unfortunately, strict application of current roadway design procedures does not ensure speed consis- tency. Because of the size of the U.S. road network and the pace of rehabilitation, road redesign is a long-term strategy, and more under- standing concerning the overall safety benefits of alternative designs is needed. The approach can yield satisfactory solutions, but addi- tional study of the relationships between operating speeds and road- way geometric elements is necessary.
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MANAGING SPEED 14 CONCLUDING COMMENT Most states chose to raise speed limits on major highways following repeal of the NMSL. The effects of their decisions on driving speeds and safety outcomes, in particular, should be closely monitored. Efforts to mitigate any adverse safety effects through enforcement should be redoubled, and initiatives to promote safety belt use and reduce driving while intoxicated--measures with large and proven safety benefits--should be continued. Technological advances may offer additional techniques for con- trolling driving speeds on all types of roads. For example, technology can help establish limits that are more sensitive to actual changes in road conditions and thus provide drivers with better information. Such technology can be installed in the vehicles and highways of the future to monitor and control speed. Finally, it can help improve the efficiency and effectiveness of enforcement. Further development, experimentation, and evaluation are needed for many technologies to realize their potential. The issue of appropriate driving speeds, however, will persist as long as there are individual drivers making choices about risk and time efficiency. Ultimately, decisions about appropriate speed limits depend on judgments about society's tolerance for risk, valuation of time, and willingness to police itself. These judgments, in turn, should be reviewed periodically in the light of changes in vehicles and highway conditions and shifts in public perceptions of safety and attitudes toward risk. REFERENCE ABBREVIATION TRB Transportation Research Board TRB. 1984. Special Report 204: 55: A Decade of Experience. National Research Council, Washington, D.C., 262 pp.
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