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6 Guidance on Setting and Enforcing Speed Limits In this chapter the committee offers its guidance on appropriate methods of setting and enforcing speed limits. Speed limits represent trade-offs between risk and travel time for a road class or specific highway section that reflect the decision makers' attempt to achieve an appropriate balance between the societal goals of safety and mobility. The process of setting speed limits is often viewed as a technical exercise, but the decision involves value judgments and trade-offs that are frequently handled through the political process in state legislatures and city councils. Thus, the guidance offered here is directed toward a broad audience of those involved in decisions about setting and enforcing speed limits: state and local legislators, traffic engineers, law enforcement and judicial officials, and the interested general public. The guidance attempts to be as specific as possible, recognizing that decision makers are looking for practical advice. However, it stops short of recommending numeric speed limits. Road conditions vary too widely to justify a "one-size-fits-all" approach. There is no 186
187 Guidance on Setting and Enforcing Speed Limits single "right" answer in setting appropriate speed limits because pol- icy makers in different communities may legitimately disagree on the priority given to the factors--safety, travel time, enforcement expen- ditures, community concerns--that affect decisions about speed lim- its. Moreover, the available studies and data fall short of providing decision makers with an adequate basis on which to quantify with much precision the effects of changes in speed limits on such critical factors as driving speeds, safety, and travel time by road class. Technical input on how these factors should be weighed in different situations, however, can help guide the decision. Thus, the focus here is to identify the critical decision factors, elaborate what is known and what is not known about their importance by road class, and comment on the decision-making process itself. The primary focus is on the effects of speed limit policies on safety, rather than on travel time, energy consumption, or environmental pollution. The chapter is organized as a series of questions and answers. It begins with a general discussion of the rationale, purpose, and current methods of setting speed limits both for broad road classes (legislated limits) and specific road sections (speed zones). The decision process for determining speed limits is then discussed. Next, a roadway clas- sification scheme is offered, which distinguishes seven different road classes and a category for special zones (i.e., school and work zones); advice is provided on the key factors for consideration in determin- ing appropriate speed limits for each category. The role and limits of current enforcement and adjudication methods in regulating driving speeds are then discussed. Finally, the potential for technology to improve methods of determining and enforcing speed limits is con- sidered. Key points are highlighted in bold in the text that follows. WHY REGULATE DRIVING SPEEDS? The argument can be made that most motorists drive in a reasonable and prudent manner, selecting their driving speeds so as to arrive at their destinations safely. If this is so, why not leave it up to the indi- vidual driver to determine an appropriate speed? There are three principal reasons for regulating drivers' individual speed choices: (a) externalities, that is, the risks and uncompensated costs imposed
MANAGING SPEED 188 on others because of individual driver choices about appropriate driv- ing speeds; (b) inadequate information that limits a motorist's ability to determine an appropriate driving speed; and (c) driver misjudg- ment of the effects of speed on crash probability and severity. The first reason derives from differences in drivers' risk tolerances. For example, in selecting an appropriate driving speed, an older driv- er is apt to assign more weight to the risks of mortality and injury to himself and others than to travel time; other drivers might assign greater weight to travel time. Even the same driver may make differ- ent trade-offs between travel time and safety depending on trip urgency, trip length, and familiarity with the road. The problem arises because individual drivers' decisions about speed may be made without adequate consideration of the effect of their choices on the safety of 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 potential costs imposed on others is a primary reason for regu- lating speed. The second reason for regulating speed is the inability of drivers to judge vehicle capabilities (e.g., stopping, handling) and to adequately anticipate roadway geometry and roadside conditions to determine appropriate driving speeds. Drivers are generally able to modify their driving speeds appropriately as traffic volumes increase and weather conditions deteriorate. Unless they are familiar with conditions, how- ever, they may not be as aware of appropriate speeds on roads with poor geometrics or high levels of roadside activity. 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 effects of externalities and the availability of information enabling motorists to anticipate conditions and select appropriate driving speeds differ significantly 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.
189 Guidance on Setting and Enforcing Speed Limits In addition, under normal conditions drivers tend to 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.1 In contrast, the risks imposed on others by individual driver speed choices may be large on urban arterials where traffic volumes are high for extended periods of the day, roadside activities are numerous, and potential for conflict with entering vehicles and with vulnerable bicyclists and pedestrians is great. These differences are important factors for consideration in setting appropriate speed limits on different types of roads. WHAT IS THE PURPOSE OF SPEED LIMITS? The primary purpose of speed limits is to regulate driving speeds to achieve an appropriate balance between travel time and risk for a road class or specific highway section. Speed limits have also been imposed for fuel conservation when national maximum speed limits were established on major highways during World War II and again in 1974 following the oil crisis of the preceding year.2 Safety--more specifically, avoidance of crashes and mitigation of crash outcomes--is the most important reason for imposing speed limits. Many factors besides speed affect traffic safety--driving under the influence of alcohol or other drugs, safety belt use, roadway geometry, and weather--but speed has been shown to play an impor- tant role. Speed is directly related to injury severity in a crash through the change in velocity (Delta-V) that occurs in a crash. The probability of severe injury increases sharply with the impact speed through its relation with Delta-V. The energy release is proportional to the 1 Not all rural Interstate highways are constructed to the highest design standards, however. Some predate the Interstate construction program and were upgraded to minimum Interstate standards. 2 The conservation effects, however, were not large. For example, Bloomquist (1984) estimated that the 1974 National Maximum Speed Limit (NMSL) reduced fuel con- sumption by 0.2 to 1.0 percent. Originally enacted as a temporary fuel conservation measure, Congress made the 55-mph (89-km/h) speed limit permanent because of the apparent safety benefits (TRB 1984, 15).
MANAGING SPEED 190 square of the impact speed; the higher the impact speed, the greater the potential Delta-V. The risk of severe injury is even greater when a vehicle strikes a pedestrian, the most vulnerable of road users; mor- tality risk for the pedestrian rises rapidly as impact speeds increase, with the rapid rise beginning at very low speeds. The strength of the relationship between speed and crash severity alone is sufficient rea- son for managing speed. Speed is also linked to the probability of being in a crash, although the evidence3 is not as compelling because crashes are complex events that seldom can be attributed to a single factor. In addition, the association between speed and crash probability varies by road type. Crash involvement has been associated with the devi- ation of a driver's speed from the average speed of traffic regardless of whether the deviation is above or below the average traffic speed. Evidence of increased crash probability from traveling above the average speed is found on many different road types, including Interstate highways, nonlimited-access rural roads, and urban arteri- als. Evidence of increased crash probability from traveling below the average speed is found primarily on Interstate highways near inter- changes where traffic slows to merge or exit and on rural roads where vehicles slow at intersections or when negotiating turns. Crash involvement has also been associated with a driver's selection of speed on certain road types. For example, on nonlimited-access rural roads, single-vehicle crash involvement rates have been shown to rise with travel speed. Speed limits are intended to enhance safety in at least two ways. They have a limiting function. By establishing an upper bound on speed, their purpose is to reduce both the probability and the sever- ity of crashes. They also have a coordinating function; here the intent is to reduce dispersion in driving 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. 3 The reader is directed to Chapter 2 for a more complete review of the studies that link speed and crash probability. In the interest of brevity, the specific references to these studies are not repeated here. This protocol is also followed in other places in the text where the reader is directed to specific chapters for more detailed information.
191 Guidance on Setting and Enforcing Speed Limits Speed limits are also established to provide motorists with a com- mon set of rules about appropriate driving speeds. The purpose is to encourage uniform driving behavior and an orderly flow of traffic. Setting speed limits requires making implicit trade-offs among road user safety, travel time, practicality of enforcement, and other factors that may affect motorists' decisions about appropriate driving speeds. Research can help inform the decision maker who must deter- mine an appropriate speed limit, but the decision ultimately reflects value judgments about acceptable levels of risk, the value of time, and acceptable levels of enforcement. WHAT INFORMATION SHOULD SPEED LIMITS CONVEY TO THE DRIVER? A speed limit sign should convey two basic messages: (a) the maxi- mum speed for a reasonable and prudent driver traveling in free- flowing traffic with good visibility and under fair weather conditions, and (b) the speed that will be enforced within some tol- erance for minor measurement error. Traditionally, speed limits have been set to inform motorists of appropriate driving speeds under favorable conditions. Drivers are expected to reduce speed if conditions deteriorate (e.g., poor visibil- ity, adverse weather, congestion, warning signs, or presence of bicy- clists and pedestrians). In the future, variable speed limits may make possible posted speed limits that vary with conditions.4 However, until the technology becomes more widely available and less costly, speed limits should inform the driver of the maximum appropriate driving speed under favorable conditions. Minimum speed limits have also been established on some high-speed roads to reduce dis- persion in speeds. In this case, the speed limit informs the driver of the minimum appropriate speed under favorable conditions. 4 Another alternative is to use prima facie limits more widely. Prima facie limits enable drivers charged with a violation for exceeding the speed limit to contend that their speed was safe for conditions existing at the time. Prima facie limits provide greater flexibility to drivers to determine an appropriate speed for conditions and place a greater burden of proof on the enforcement community that a violation has occurred.
MANAGING SPEED 192 Speed limits should also inform drivers who exceed the maximum (or fall below the minimum) limit that, with some tolerance for minor measurement error, they can expect a citation. In other words, speed limits should mean what they say. HOW SHOULD SPEED LIMITS BE SET? The approach currently in wide use to set speed limits is sound, that is, speed limits are legislated by broad road class (e.g., Interstate high- way) and geographic area (e.g., urban district). Where statutory lim- its do not fit specific road or traffic conditions, speed zones may be established administratively, and speed limits for that highway sec- tion may be reduced from or raised above the statutory limit.5 The system appropriately balances the desirability of uniform speed limits (legislated limits for broad road classes) with the need for exceptions (speed zones) to reflect local differences. Establishing Legislated Speed Limits by Road Class Decision Process for Determining Legislated Speed Limits Legislated speed limits by road class are determined by state legisla- tures and city councils for state and local roads, respectively. Legislators should seek the advice of traffic engineers, law enforce- ment officials, judges, public health officials, and the general public in determining appropriate speed limits, and provision should be made to monitor and enforce whatever decision is reached. Consultation, however, does not ensure that all parties will reach consensus or that tensions will be resolved between different inter- ests, such as between commuters and residents on appropriate speed limits on residential streets. The decision process requires trade-offs and judgments concerning the relative importance of safety and travel time and the feasibility of enforcement. There is no single "right" speed limit, but, in addition to satisfying safety, the final 5 In those states with absolute speed limits, speed zone limits cannot be raised above the maximum absolute limit for that road class.
193 Guidance on Setting and Enforcing Speed Limits selection of a speed limit should meet the requirements of enforce- ability and acceptance by the community at large. Nor should the process stop there. Roadway conditions, vehicle safety features, driving behavior, and attitudes change over time. The change in driver attitudes toward and compliance with the 55-mph (89-km/h) NMSL is an example of how driver support for a legis- lated speed limit can erode over time. Legislated speed limits should be reviewed periodically and revised when necessary on the basis of monitoring data on actual driving speeds and safety outcomes. Roadway Classification Scheme The committee identified seven road classes plus a category of spe- cial zones (e.g., school zones, work zones) as the basis on which to differentiate speed limits by road type. The categorization scheme covers most major road classes and is expressed in terminology appropriate for the general reader. The committee's classification scheme differs from the more technical highway functional classifi- cation system developed by the American Association of State Highway and Transportation Officials (AASHTO), a classification well known to the engineering community (Table 6-1). Data on mileage, travel volume, and safety, which are useful in making deci- sions about appropriate speed limits by road class, are only available by the AASHTO functional classification scheme. Thus they are provided here, although there is not always a one-to-one corre- spondence between the AASHTO and the committee road classi- fication systems. Guidance on Setting Legislated Speed Limits by Specific Road Class In determining appropriate speed limits by road class, decision mak- ers should be guided by both the likely risks imposed on others by individual driver speed choices and by the adequacy of cues provided by the roadway to help drivers anticipate conditions and make appropriate speed choices. They should also take enforcement prac- ticality into consideration. Table 6-2 summarizes differences between road classes on the basis of these general considerations.
Table 6-1 Road Class Categories and Characteristics (FHWA 1997) Fatal Crashes Fatalities a Injuries c Annual Travel in Road Class Mileage Road Class Injury (Percent of (Percent 100 MVM Fatality (Percent of (AASHTO (Percent (Committee Rate b Rate b Total) of Total) (Percent of Total) Total) Definition) of Total) Definition) 2,324.5 Rural Interstate Rural Interstate 32,818 2,388 2,850 1.23 91,200 39 (9.4) (0.8) (6.4) (6.8) (2.4) 5,093.5 Urban Interstate Urban Interstate 22,240 3,701 4,137 0.81 444,600 87 (20.5) Other freeway and (0.6) (9.9) (9.9) (12.0) expressway d 3,788.1 Rural multilane/high- Other principal 235,490 8,450 9,801 2.59 392,700 104 (15.3) speed two-lane arterial, rural (6.0) (22.6) (23.4) (10.5) Minor arterial, rurale 3,777.2 Urban/suburban Other principal 52,973 4,977 5,434 1.44 731,900 194 arterial, urban f (15.2) multilane (1.3) (13.3) (13.0) (19.6) 2,410.4 Rural lower-speed Major collector, 705,311 6,477 7,234 3.0 395,000 164 (9.7) two-lane rural (18.0) (17.4) (17.3) (10.6) Minor collector, rural g h 4,269.9 Not defined Minor arterial, 176,940 4,557 5,092 1.19 778,000 182 (17.2) urban (4.5) (12.2) (12.1) (20.8) Collector, urban Not defined h 1,077.7 Local roads, rural 2,119,154 3,956 4,280 3.97 253,800 236 (4.3) (54.1) (10.6) (10.2) (6.8)
Urban residential street Local street, urban 574,524 2,080.8 2,845 3,079 1.48 646,600 311 (14.7) (8.4) (7.6) (7.3) (17.3) Rural unpaved road N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. Note: Safety statistics are for 1996, the most recent year available. Year-to-year-variations by road class are small. AASHTO = American Association of State Highway and Transportation Officials; MVM = millions of vehicle miles; FHWA = Federal Highway Administration; N.A. = not applicable; 1 mi = 1.609 km. a Includes vehicle occupant and nonoccupant fatalities. b Fatality and injury rates are expressed as fatalities (or injuries) per 100 MVM. c Includes vehicle occupant and nonoccupant injuries of all types, not just incapacitating. Estimates or 1995 data are used for 10 states plus the District of Columbia because of incomplete reporting at the time of publication. d The category "other freeway and expressway" includes roads with less than full control of access. e AASHTO defines this road category as serving major statewide travel. The roads are designed for relatively high-speed travel and minimum interference to through movement (AASHTO 1994, 11). f This category also includes a substantial share (nearly 40 percent of the total mileage) of two-lane roads. The major purpose of these roads is traffic circulation, not access. g AASHTO defines this road category as serving intracounty travel where travel distances are shorter and speeds are more moderate than on arterial routes (AASHTO 1994, 11). This category includes a small amount of four-lane divided highway mileage. h The committee did not cover every possible road type. For example, the categories of "urban/suburban multilane roads" and "urban residential streets" were thought to adequately capture the major urban road types. Hence, a separate category was not defined for urban minor arterial and collector roads.
Table 6-2 Major Factors for Consideration in Setting Speed Limits by Road Class Available Information for Road Class Effects of Externalities Motorist Speed Choice Enforcement Practicality Rural Low--free-flowing traffic; Good--roads usually constructed Selective, targeted enforcement has Interstates limited access; pedestrians to highest design standards; short-term positive effects, but and bicyclists not generally drivers can generally anticipate coverage of complete mileage is pres-ent; the best safety conditions difficult record of all rural road classes Urban Somewhat greater than on rural Not as good as rural Interstates Mileage is easier to cover, but Interstates Interstates because higher because of driver difficulties apprehending speeders in high- traffic volumes and closer anticipating traffic bottlenecks traffic volumes on multilane seg- interchange spacing increase or incidents, particularly during ments can be difficult potential for vehicle conflict, congested periods but still relatively low Rural Greater than on rural Fair to good, depending on road- Coverage of very extensive mileage multilane/ Interstate highways because way geometry and extent of is difficult high-speed many highways are not access control--speed zones two-lane divided and access is not can be used with lower speed always limited, resulting in limits where driver anticipation at-grade intersections with of appropriate speeds is likely potential for vehicle con- to be poor flicts
Urban/ Greater than on rural multi- Wide variety of conditions. Less difficult from a coverage per- suburban lane roads because of more Generally poorer than for rural spective; some room to pull over multilane roadside activity (commer- multilane because of greater violators cial and residential develop- roadside activity, access points, ment) and presence of some and cross traffic that complicate vulnerable road users (e.g., determination of appropriate bicyclists) speeds. Speed adaptation can be a problem traveling from rural to suburban and urban areas Guidance on Setting and Enforcing Speed Limits Rural Greater than on rural multi- Poor--lower design standards Adequate coverage of low-volume lower- lane because there are more generally; motorists sometimes mileage is infeasible speed opportunities for vehicle may have difficulty determining two-lane conflicts from limited suitable driving speeds. opportunities for passing Appropriate use of speed zones and absence of access con- and warning signs and advisory trol; poor safety record for speeds recommended this road class generally (continued on next page) 197
Table 6-2 (continued) Available Information for Motorist Speed Choice Enforcement Practicality Road Class Effects of Externalities Fair--may be difficult for drivers Coverage is a problem because of Urban High--high potential for to anticipate conditions and extensive mileage residential vehicle conflicts at inter- determine appropriate driving streets secting streets and drive- speeds, but speeds, in general, ways; many vulnerable road are sufficiently low that drivers users (e.g., pedestrians, may have time to react to unan- bicyclists) share the road ticipated situations Fair to good--roads are mostly Low volume of traffic suggests Low--very little traffic; roads Rural minimal enforcement used by those familiar with sur- are mostly used by local res- unpaved idents roads roundings, although conditions can change rapidly depending on weather and amount of road maintenance Adequate coverage can be a prob- Poor--appropriate driving speeds High--opportunities for con- Special lem in areas with large numbers may not be apparent and speed flict with vulnerable road zones of school zones; difficulty of adaptation nearing the work or users (e.g., children on foot (school apprehending violators can be a school zone can be a problem or on bicycles, workers) and work problem in work zones depend- zones) ing on traffic volume
199 Guidance on Setting and Enforcing Speed Limits More specifically, legislators should obtain information on four key technical factors to guide their decisions: · Design speed, that is, the design speed of a major portion of the road, not of its most critical design features; · Vehicle operating speed, measured as a range of 85th percentile speeds taken from spot-speed surveys of free-flowing vehicles at rep- resentative locations along the highway; · Safety experience, that is, crash frequencies and outcomes; and · Enforcement experience, that is, existing speed tolerance and level of enforcement. To the extent possible, a range of data should be provided for each measure at various locations along the highway. Locations with con- ditions that differ considerably from these ranges may be suitable for speed zones or advisory speed warnings. Examples of such locations are where crash rates are high or geometric features are particularly restrictive. As the following discussion illustrates, the weight given to the four factors, particularly those related to speed, differs by road class. Obtaining representative data on vehicle operating speeds and design speeds becomes more difficult on lower functional road classes because of greater variability of conditions and driving speeds on these roads. Rural Interstate Highways Roadway and traffic conditions are more uniform on rural Interstates than on any other road class. Hence, they provide a useful benchmark for comparison with lower functional road classes. Decision makers should obtain information on the four factors mentioned previously but should give greater weight to information on vehicle operating speeds and safety in determining a maximum speed limit on rural Interstates. The risks imposed on others are likely to be small, and information enabling drivers to select an appropriate speed is gener- ally good on this road class. The risk of conflict is relatively low on many rural Interstates because traffic is generally free flowing and access is limited. Vulnerable road users (e.g., pedestrians, bicyclists) typically are not present. Rural Interstate highways have the lowest
MANAGING SPEED 200 numbers of fatal crashes, fatalities, and injuries and the lowest injury rate of any road class; they have the lowest fatality rate of any rural road class (Table 6-1). Restricted geometry is generally not a prob- lem (with the exception of some rural Interstates that pass through mountainous terrain) because rural Interstates are usually constructed to the highest geometric and roadside design standards. Thus, design speeds provide little additional information to decision makers about appropriate speed limit levels for this road class; most drivers can anticipate conditions and select an appropriate driving speed. An appropriate maximum speed limit should be established, how- ever, because of the link between crash severity and high travel speeds and evidence of increased probability of crash involvement for drivers who travel at speeds well above the average speed of traffic.6 The higher fatality rate on rural than on urban Interstates--a product of higher speeds and more severe crashes--reinforces the need to limit speeds at the high end of the speed distribution (Table 6-1). The maximum speed limit, however, should be set at a level that the police can reasonably enforce and the courts adjudicate. Selective, targeted enforcement on certain highway sections can have a positive though short-lived effect, but maintaining high levels of enforcement is dif- ficult on long stretches of rural Interstate highways. Moreover, inten- sive enforcement is not a desirable use of scarce resources because of the good safety record on rural Interstates compared with other rural road classes. States are also urged to review their policies for requir- ing slower vehicles to keep to the right on these highways as one method of reducing dispersion in driving speeds.7 6 As discussed in Chapter 2, according to some studies, crash involvement rates are lowest in the interval between the average speed and the 85th percentile speed of the speed distribution, above which point they rise sharply. The same sources would also support establishment of minimum speed limits because crash involvement rates also rise sharply below average speeds--at about the 15th percentile. These results were found on Interstate highways and on rural nonlimited-access highways. 7 According to NHTSA (1998), the following states reserve the left lane for passing: Arkansas, Connecticut, Hawaii, Idaho, Illinois, Indiana, Kentucky, Maine, Massachusetts, Michigan, Mississippi, Missouri, Nevada, New Jersey, Ohio, Oregon,
201 Guidance on Setting and Enforcing Speed Limits Despite the relatively low fatality rate of rural Interstates, decision makers are still faced with a trade-off between the level of safety and increased mobility (improved travel time) when choosing a speed limit. The committee expects that legislators grappling with the issue of which maximum limit to choose would want to do so with knowledge of changes in safety among alternative maximum limits. At the time of this writing, the most relevant information on changes in safety comes from studies of the effects of changes in the NMSL in 1987 and the repeal of the NMSL in 1995, which led to increased speed limits in most states.8 As is often the case with studies of real-world safety effects, neither the results nor the conclusions drawn by different researchers are totally consistent across all states or all studies. As would be expected because of the many factors affecting fatal- ities on rural Interstate highways, not all Interstate sections where the maximum speed limit was raised demonstrated the same effects. However, taken as a group, the studies that examined effects of the 1987 changes on Interstate sections alone indicated a relatively con- sistent pattern of overall increases in fatalities and fatal crashes. In short, raising the speed limit from 55 to 65 mph (89 to 105 km/h) or higher had significant adverse effects on safety on the roads where the speed limit was increased. A similar result was found on highways on which speed limits were raised after 1995. However, the latter findings should be considered preliminary because they are generally based on 1 year of data or less. In a more limited number of studies, primarily of the 1987 change, researchers have attempted to examine "system effects" of raising the speed limit, noting that decreases in safety on the Interstates them- selves might be offset by improvements in safety on other roads, per- haps through such mechanisms as diversion of traffic to the safer Interstates and better deployment of enforcement resources to the less safe (non-Interstate) roads. One study concluded that the overall effect of the changes was neutral (using counties within one state as Rhode Island, Tennessee, Utah, Virginia, and Washington. States vary widely, however, concerning the types of roads and vehicles for which the restriction applies. 8 Chapter 3 provides a more detailed discussion of these studies.
MANAGING SPEED 202 the system), a second concluded that increasing speed limits had a positive safety effect on the system (using entire states as the system), and one other found no offsetting effects but rather some evidence of spillovers in the form of higher fatalities on non-Interstate rural highways--although a much smaller increase than on the rural Interstates. Subsequent authors have raised the issue of how best to define the system that should be studied. The committee found that combining such opposing findings into a consensus view of the effects of increasing limits on rural Interstates was difficult. There is agreement that speed limit increases that result in increased driving speeds--and the preponderance of the evidence suggests that increased speeds will occur--will likely result in higher fatalities and fatal crashes on Interstate sections where the limit is raised, as would be expected from studies relating speed to crash severity. There is also committee agreement that, to the extent system changes have occurred, the total safety effect could be different because most current estimates of effects are based on Interstate seg- ments alone. Additional research on such possible system effects is clearly needed. Urban Interstate Highways Maximum speed limits on urban Interstate highways are often set, appropriately, at a lower level than on rural Interstates. The potential for vehicle conflicts is greater on urban Interstates because of higher traffic volumes and more interchanges, but it is still expected to be smaller than on many other road classes. In fact, the fatality rate is lower on urban than on rural Interstates, although the numbers of fatal crashes, fatalities, and injuries and the injury rate are higher (Table 6-1). Roadway geometry may be more restricted on urban Interstates, and closer interchange spacing increases the potential for disparities in speed and vehicle conflicts; hence the desirability of lower speeds to reduce speed dispersion. Maximum speed limits, however, should be set in a speed range that the police are able to enforce. Although the concentration of urban Interstate mileage and traffic volume makes enforcement easier, it can be difficult for the police to apprehend speeders in traffic. Photo radar can be a useful supplement to conventional police enforcement on urban freeways.
203 Guidance on Setting and Enforcing Speed Limits When conditions become congested during peak periods, drivers may have difficulty anticipating appropriate speeds, particularly as they approach traffic bottlenecks or incidents.9 Variable speed limit systems, which indicate appropriate maximum and minimum speeds on the basis of actual traffic volume, speed, and density on specific highway sections, are well suited to address such conditions. These systems have been deployed on several high-speed European motor- ways, often in combination with automated speed enforcement. Variable speed limit systems are under development for limited application on U.S. highways, particularly on highway sections affected by adverse weather. Once their effectiveness is better under- stood, broader application of variable speed limit and related auto- mated enforcement systems, particularly on urban Interstate highways, should be explored. Because the systems are new and unlikely to be well understood, care should be undertaken to explain the basis for setting and varying the speed limits to the courts, law enforcement officials, and motorists. Speed thresholds for enforce- ment should be set high at first to give drivers time to adjust to the system and to gain their support. Rural Multilane and High-Speed Two-Lane Highways10 Highways in this road class range from multilane, divided highways with some access control to two-lane, undivided highways with at- grade intersections and restricted roadway geometry. Maximum speed limits should be lower than on rural Interstate highways to reflect lower design standards generally on this road class. The poten- tial for vehicle conflicts is greater on many of these highways than on rural Interstates because vehicles can enter and exit at intersections at speeds considerably lower than the average speed of traffic. Also, many of these roads do not have median barriers, thus increasing the 9 Drivers may have particular difficulty anticipating "shock waves"--the traffic slow- downs that build up in response to a traffic incident or bottleneck that may be created by large numbers of entering or exiting vehicles. The slowdown occurs upstream from the incident or bottleneck; the magnitude of the effect depends on the traffic volume and the duration of the incident. 10 The committee defines "high speed" as at least 55 mph (89 km/h).
MANAGING SPEED 204 chance of head-on collisions. Roadside hazards, such as trees and utility poles, are often present on these roads. Fatal crashes and fatal- ities are more than three times higher--and the fatality rate is approximately twice as high--on this road class as on rural Interstates (Table 6-1).11 Design speed should be an important con- sideration in setting appropriate speed limits on these highways. The cues available to motorists to select an appropriate driving speed vary depending on roadway geometry and the amount of access control. Because these highways carry a substantial amount of traffic (Table 6-1) and their function is to accommodate relatively high-speed through travel (AASHTO 1994, 11), speed zones should be used where speed limits should be appropriately lowered rather than imposing lower speed limits throughout the system. Limited enforce- ment coverage also supports this strategy. Urban and Suburban Multilane Roads This road class probably encompasses the greatest variation in road- side conditions, thus making it difficult to specify a suitable sys- temwide speed limit. Maximum speed limits should be set somewhat lower than for rural multilane highways, and extensive use of speed zones is recommended. The risk of vehicle conflicts is greater on urban roads because of more roadside activity, access points, and cross traffic. Bicyclists and pedestrians--the most vulnerable road users-- are apt to be more common on these roads. Drivers appear to make some accommodation to these differences; fatal crashes, fatalities, and the fatality rate are considerably lower on these roads than on their rural counterparts, although injury levels and the injury rate are nearly two times higher (Table 6-1). Roadway geometrics may not differ greatly from those on rural multilane highways, but motorists are apt to have greater difficulty determining appropriate driving speeds in areas that are heavily developed. Speed adaptation can also be a problem, particularly lowering speeds appropriately as drivers travel from rural to suburban and urban areas. Enforcement is easier from a coverage perspective; there are fewer miles of urban than rural 11 Injuries are more than four times higher and the injury rate is more than twice as high (Table 6-1).
205 Guidance on Setting and Enforcing Speed Limits multilane roads (Table 6-1). Also, there is often room to pull over speed limit violators. These roads may be candidates for photo radar enforcement at high-crash or highly hazardous locations. Rural Lower-Speed Two-Lane Roads12 The potential for vehicle conflicts is great on this road class because of the absence of access control and limited opportunities for passing. Fatal crashes, fatalities, and the fatality rate are among the highest for all road classes; injuries and the injury rate are among the highest for all rural road classes (Table 6-1). The roads are not designed to the highest standards but rather to accommodate topography and expected traffic (AASHTO 1994, 460). Thus, motorists may some- times have difficulty determining appropriate driving speeds for con- ditions. Design speed should be a key factor in establishing suitable speed limits on these roads. Appropriate use of speed zones, warning signs, and advisory speeds is recommended. Adequate coverage of extensive road mileage poses a problem for enforcement. Thus, speed limits must be reasonable for conditions and set at levels that are largely self-enforcing. Fortunately, there is some evidence to suggest that drivers do restrict their speeds on roads with lower design speeds.13 Urban Residential Streets The potential for inconsistent application of speed limits is high on this road class. Neither vehicle operating speeds nor design speeds are likely to provide useful input for determining appropriate speed limits on residential streets; safety experience and enforcement prac- ticality should be given higher priority. Neighborhood pressures may result in setting speed limits very low, but often they are not enforced and compliance is poor, even by neighborhood residents. Speed lim- its based on vehicle operating speeds, however, may be inappropriate because there is a high potential for vehicle conflicts, and drivers are 12The committee defines "lower-speed" as less than 55 mph (89 km/h). 13Agent et al. (1997) reported that the only roadway type in Kentucky where the 85th percentile speed was less than 5 mph (8 km/h) higher than the posted speed limit was two-lane rural roads without full-width shoulders (p. 11).
MANAGING SPEED 206 not always aware of the danger they pose to bicyclists and pedestri- ans with whom they share the road. The fatality rate on residential streets is the highest for all urban road classes; the injury rate is the highest for any road class (Table 6-1). Design speed also has limited practical significance for determining speed limits on residential streets; the frequency of intersections, presence of stop signs, and amount of roadside activity (e.g., parking, driveways) have a greater effect on actual vehicle speeds (AASHTO 1994, 429). The risks imposed on other road users is sufficient reason for limiting driving speeds on residential streets. However, speed limits should not be set below enforceable levels.14 Even when there is a commitment to enforcement, there are practical limits because of the extensive mileage of residential streets. Alternative measures, such as traffic calming and other highway design techniques, should be considered to achieve desired driving speeds. Rural Unpaved Roads This is a road class for which posted speed limits are generally inap- propriate. The basic law that drivers should adopt a reasonable and prudent speed should govern. Risks of vehicle conflict are very low on these roads; most are used by residents who are familiar with the roads and their condition. Roadway geometry varies, and roadway conditions can change rapidly depending on weather, season, and amount of road maintenance, so that establishing an appropriate speed limit is difficult even for favorable conditions. Finally, enforce- ment is minimal on roads with such low traffic volumes. Special Zones At least two situations--school zones and work zones--warrant spe- cial handling in establishing speed limits. Risks to others are likely to be great in both because of the presence of vulnerable road users-- children in school zones and workers in work zones. Drivers are 14Another option is to use prima facie speed limits more widely, which would provide greater enforcement flexibility. This alternative would require legislative changes in the two-thirds of the states that currently have only absolute speed limits.
207 Guidance on Setting and Enforcing Speed Limits unlikely to anticipate appropriate driving speeds for negotiating these zones. Typically, they represent an exception to normal driving speeds, and adequate speed adaptation is a problem as drivers approach the zones. Moreover, work zones often have narrow lanes and restricted alignments at detours and lane shifts that require speed reduction. School zones are prime candidates for variable speed limit systems because lower driving speeds are generally required only for certain hours of the day. Changeable conditions in work zones as well as limited hours of operation also make them amenable to variable speed limits. Photo radar enforcement may be appropriate in those circumstances where patrolling large numbers of school zones or apprehending speeding violators in high traffic volumes in work zones proves difficult. Setting Speed Limits in Speed Zones Speed zones are established for highway sections where legislated limits for that road class do not fit specific road or traffic conditions. Determination of appropriate speed limits in speed zones should be made on the basis of an engineering study. Speed zones should be reviewed periodically--with greater frequency where conditions are changing rapidly (e.g., developing suburban areas)--to determine whether changed conditions warrant an adjustment in the speed limit or in the boundaries of the zone itself. California, for example, has established a 5-year review cycle; police will use radar enforce- ment in speed zones only if an engineering and traffic survey has been conducted and reviewed within the past 5 years to set an appro- priate speed limit. Traffic engineers or technicians under their super- vision should conduct the engineering study. Consultation with law enforcement officials is advised where this is not already accepted practice so that the proposed speed limit is enforceable. Elected offi- cials and citizen groups may also become involved when community concerns have been expressed about driving speeds. In addition to speed data, engineering studies can provide road-specific historical data on crashes and information about hazards (e.g., pedestrian crossings, intersecting streets with restricted sight distance) not read- ily apparent to motorists. These data help the engineer in determin-
MANAGING SPEED 208 ing an appropriate speed limit. However, unless the decision has the support of enforcement officials and the general public, speed zoning may not result in desired driving speeds, particularly if the speed limit is set below the 85th percentile speed. On roads where enforcement is infrequent and low speeds are desirable, such as residential streets, alternatives like traffic calming should be investigated. The most common factor considered in setting speed limits in speed zones is the 85th percentile speed, although frequently the limit is adjusted from this value on the basis of such factors as crash experience, roadside development, roadway geometry, and parking and pedestrian levels. Speeds are measured from spot-speed surveys of free-flowing vehicles taken at representative locations in the pro- posed speed zone. The speed limit typically is set near the speed at or below which 85 percent of motorists are driving. The advantages of setting the speed limit near the 85th percentile speed are that (a) police are enabled to focus their enforcement efforts on the most dangerous speed outliers, and (b) the 85th percentile speed is gener- ally at the upper bound of a speed range within which crash involve- ment rates are lowest, at least 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. The potential safety benefits may not be realized on roads with a wide range of speeds. Basing the speed limit on a measure of unconstrained vehicle operating speeds is not appropriate on urban roads with a mix of road users, including bicyclists and pedestrians, and with high traffic volumes and levels of roadside activity. An expert-system approach, either formal or infor- mal, could be developed to establish speed limits in speed zones.15 The expert-system approach deserves consideration because it pro- vides a systematic and consistent method of examining and weighing factors other than vehicle operating speeds in determining an appro- priate speed limit. 15Details of Australia's expert system for setting speed limits are provided in Chapter 3.
209 Guidance on Setting and Enforcing Speed Limits Differential Speed Limits The committee remains neutral on the desirability of differential speed limits for passenger cars and heavy trucks that have been established in some states on some road classes. It did not find com- pelling evidence16 to support more widespread application of differ- ential speed limits. Neither did it find strong evidence that differential speed limits should be eliminated where they are in use. More research on and evaluation of the effects of differential speed limits on driving speeds and safety outcomes are needed in the states that have adopted them. The committee found little evidence to suggest that motorists decrease driving speeds at night when lower nighttime speed limits are in effect. However, it did not find compelling evidence to suggest that nighttime speed limits be eliminated in states that have adopted them. CAN DRIVERS BE INDUCED TO OBEY SPEED LIMITS THROUGH ENFORCEMENT? Most experts agree that enforcement is critical to achieving compli- ance with speed limits. Simply posting a speed limit sign will not achieve desired driving speeds. Even if most motorists believe that the speed limits are reasonable and they comply within a small toler- ance, enforcement is still necessary to ensure the conformity of driv- ers who will obey laws only if they perceive a credible threat of detection and punishment for noncompliance. The problem with traditional enforcement methods is their short-lived effect in deterring speeding or other unwanted behavior. Maintaining the deterrence effect requires a level of enforcement that is difficult to sustain because of limited resources provided for speed enforcement and competing enforcement priorities. Policy makers can affect the level of enforcement through resource allocation, but enforcement is expensive. Thus, the police should deploy enforce- 16 A review of many of the key studies concerning differential speed limits can be found in Chapter 3.
MANAGING SPEED 210 ment efforts strategically on those roads and at times when speed- related incidents are most common or where road conditions are most hazardous. (The infrequent nature of crashes, however, makes targeting difficult.) There is some evidence17 that planned patrols at varying time intervals and locations can extend the time- and distance-halo effects of enforcement at particular locations, but only after an initial period of continuous patrolling. In addition, the patrols must be visible and sufficiently frequent to convince drivers of a credible threat of detection for noncompliance. Police can boost the longevity of the deterrence effect by combining enforcement initia- tives with high-profile public information campaigns to increase driver awareness that speed limits will be enforced. Publicity must be followed up by actual enforcement if the approach is to successfully deter speeding. Moreover, making permanent behavior changes requires a long-term sustained effort. Automated enforcement--for example, photo radar--can be used to complement traditional enforcement methods, particularly where roadway geometry or traffic volume makes traditional methods dif- ficult or unsafe. Photo radar has been shown to be efficient and effective where it has been used for speed control, particularly on high-volume, major arterials where compliance with speed limits is often poor. Photo radar enforcement could also be coupled with vari- able speed limit systems for use on urban Interstates where high traf- fic volumes make it difficult to apprehend speeding drivers. Photo radar is controversial. Legal issues, such as privacy and owner (versus driver) liability for speeding infractions, must be resolved. Successful introduction of automated enforcement also requires funding, public education, and careful deployment (i.e., on roads that are especially hazardous and at high-crash locations where speeding is a contribut- ing factor or where traditional enforcement methods are hazardous) to ensure essential public support. Other alternatives to traditional enforcement may be required in some circumstances to achieve desired driving speeds. Traffic calm- ing has successfully reduced speeding on many residential streets, but 17 Chapter 4 provides a more detailed discussion of studies on different enforcement strategies.
211 Guidance on Setting and Enforcing Speed Limits it is difficult to determine net areawide effects on safety because of the difficulty of accurately measuring traffic diversion and the small size of "before" and "after" crash data that limits statistical analysis.18 Redesigning roads to achieve greater congruity between driver per- ceptions of appropriate travel speeds and cues provided by the road itself (narrowing lanes, etc.) has promise. The approach should result in more consistent vehicle operating speeds, but additional study of the relationship between operating speeds and roadway geometric elements is required. In view of the size of the U.S. road network and the cost and pace of road rehabilitation, road redesign is a long-term strategy. 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. HOW CAN THE JUDICIAL SYSTEM ASSIST IN ACHIEVING DESIRED DRIVING SPEEDS? Actions by law enforcement officials and the justice system can undermine the effectiveness of speed limits in achieving desired driv- ing speeds. The police can choose not to enforce speed limits where they think the limits are unreasonable. Traffic court judges throw out speeding violations or reduce fines in cases when they believe the speed limits are inappropriate or the fines too harsh. Thus it is important that the police and traffic court judges perceive that speed limits are reasonable and enforceable. When setting speed limits, care should be exercised that primary consideration is given to safety, not revenue enhancement. Driver perception of entrapment from speed limits set unreasonably low to generate income erodes the credibility of traffic regulation. Where the courts have broad discretion in assessing penalties for speeding violations, inconsistent treatment of violations can lead to a public perception that speed limit laws are arbitrary and capricious. Development of sentencing guidelines and training for judges who 18 Chapter 5 provides a lengthier discussion of traffic calming and road redesign.
MANAGING SPEED 212 handle speeding violations can help ensure consistent treatment of violators. Licensing point demerit systems have already gone a long way toward reducing inconsistencies in penalty assessments by pro- viding a uniform system of graduated penalties for various traffic vio- lations. The deterrence effect of sanctions for speeding violations is often limited by lengthy backlogs of cases in courts where traffic violations are often perceived as minor infractions and not serious crimes. Automated enforcement has the potential to relieve court backlogs through the use of administrative adjudication procedures for many speeding infractions. Such procedures can reduce the number of court hearings as well as the cost of processing speeding violations. WHAT POTENTIAL DOES TECHNOLOGY OFFER TO IMPROVE METHODS OF DETERMINING AND ENFORCING SPEED LIMITS? Technology can advance the state of the art, but it does not hold the complete solution. Many applicable technologies already exist or are currently being enhanced as computing capability has grown. The difficulty in using them often lies less with the technologies them- selves--although cost is an important consideration--than with political and legal hurdles to be overcome in deploying them widely. The most promising technologies for near-term adoption are dis- cussed here. The technology to support variable speed limits and improve traf- fic flow efficiency is available, but more experimentation and evalu- ation 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 limits opportunities for their deployment to urban Interstates and freeways with large traffic vol- umes or to selected segments of major roads where weather (e.g., fog, visibility) is a frequent problem. Automated enforcement, particularly photo radar, can provide an effective complement to traditional enforcement methods, particu- larly where police patrol vehicles cannot be deployed effectively or
213 Guidance on Setting and Enforcing Speed Limits safely. Successful introduction of automated enforcement may require adoption of legal changes; strong public support is essential for its success. Intelligent Transportation System technologies that support more efficient and safer travel are being developed and demonstrated on U.S. vehicles and highways. New techniques for communicating information to drivers about appropriate driving speeds are under development, and advances in vehicle control technologies have the potential to automate some speed-related driving functions. Some technologies, such as "smart" cruise control, are close to commercial- ization. Other vehicle-related technologies, such as frontal-collision and lane-departure avoidance systems, are still in the research and development phase. Key concerns include reliability, liability, driver control, and acceptance. The technologies represent important inno- vations that require watching and evaluation. CONCLUDING COMMENT The issue of appropriate driving speeds and safety will persist as long as there are individual drivers making choices about driving speeds. Most states have recently raised speed limits on many major high- ways following repeal of the NMSL. Close monitoring of effects, particularly changes in driving speeds and safety outcomes, is desir- able; vigilant enforcement is needed; and redoubled efforts should be taken to mitigate adverse safety outcomes by such continuing initia- tives as increased safety belt use and reductions in driving while intoxicated--measures with large and proven safety benefits. The speed-safety problem may become more acute with increased numbers of older drivers, who may not themselves speed but who have reduced capacity to handle high speeds. Congestion--which is unlikely to abate in the near term--contributes to the problem by increasing driver frustration and encouraging unsafe driving behav- iors, such as speeding to avoid red lights or high-speed weaving on crowded Interstates and freeways. Speed limits are one of the oldest and most widely used methods of controlling driving speeds, but speeds limits alone are not effective in all situations. Technology can help establish limits that are more
MANAGING SPEED 214 sensitive to actual changes in conditions and thus provide drivers with better information. It can help outfit the vehicles and highways of the future with speed monitoring and control devices. Finally, technology can help improve the efficiency and effectiveness of enforcement. But the efficacy of speed limits will continue to depend largely on driver perception of the reasonableness of the limits and the willingness of the police and the courts to enforce the limits and punish violations. Where they are not present and limiting speed is desirable, alternative measures to managing driving speeds will have to be sought. REFERENCES ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials FHWA Federal Highway Administration NHTSA National Highway Traffic Safety Administration TRB Transportation Research Board AASHTO. 1994. A Policy on Geometric Design of Highways and Streets. Washington, D.C. Agent, K.R., J.G. Pigman, and J.M. Weber. 1997. Evaluation of Speed Limits in Kentucky. KTC-97-6. Kentucky Transportation Center, Kentucky Transportation Cabinet, and Federal Highway Administration, April, 90 pp. Bloomquist, G. 1984. The 55 mph Limit and Gasoline Consumption. Resources and Energy, Vol. 6, March, pp. 2139. FHWA. 1997. Highway Statistics 1996. U.S. Department of Transportation, Washington, D.C. NHTSA. 1998. Life in the Fast Lane. DOT-HS-808-670. U.S. Department of Transportation, Feb. TRB. 1984. Special Report 204: 55: A Decade of Experience. National Research Council, Washington, D.C., 262 pp.