Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Chapter 3: Managing Speeds: Speed Limits
Chapter 3: Managing Speeds: Speed Limits
Pages 77-138
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From page 77... ...
Speed is directly related to injury severity in a crash, reflecting the laws of physics. The link between speed and the probability of being in a crash is weaker, reflecting the fact that motor vehicle crashes are complex events that can seldom be attributed to a single factor.
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From page 78... ...
The chapter ends with a discussion of the implications of these findings for speed limit policies. REGULATING SPEED -- A THEORETICAL JUSTIFICATION Drivers continually make choices about appropriate driving speeds, making their own assessment concerning the amount of risk they are willing to bear.
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From page 79... ...
(Speed limits, of course, are not the only possible regulatory response.) The externalities -- particularly the risks to others -- may be relatively small on lightly traveled Interstate highways but quite large on streets adjacent to schools or in highly congested areas.
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From page 80... ...
The justification for imposing speed limits, however, still leaves open the question of how the limits should be set, the topic of the following section. METHODS OF SETTING SPEED LIMITS Brief History of Speed Regulation The idea of regulating the speed of motor vehicle travel has a long history.
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From page 81... ...
and introduced the concept of speed zones, wherein state agencies would determine alternative maximum speed limits at particular highway locations on the basis of engineering and traffic investigations (UVC 1967, 437, 446) .3 With the rise of the traffic safety movement during the 1930s, the National Safety Council organized a Committee on Speed Regulation in 1936 to study the speed problem.
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From page 82... ...
. Speed limits in speed zones, on the other hand, are established by administrative action and are intended to be determined on the basis of an engineering study (ITE 1992, 347)
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From page 83... ...
By establishing an upper bound on speeds, the objective is to reduce both the probability and severity of crashes. Speed limits also have a coordinating function -- to reduce dispersion in driving speeds (Lave 1985)
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From page 84... ...
The maximum speed limit should be related to the actual risk characteristics of the road (e.g., curvature, lane width) if drivers are to perceive the speed limit as credible and if adequate levels of voluntary compliance are to be achieved (Fildes and Lee 1993, 22)
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From page 85... ...
In this section, the major methods of determining speed limits are described. The section begins with a description of the methods most appropriate for setting statutory or legislated speed limits and continues with a discussion of the methods appropriate for setting speed limits in speed zones (Table 3-1)
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From page 86... ...
to any road class ficult to quantify because socially optimal minimize social key variables speed limits are typically highway transport lower than what individ costs ual drivers would select Engineering Speed zones Not necessarily a safe May not be as appro- Well-established Helps establish a reason study speed for all road priate for urban methodology for able target of out-of method with classes; it depends, roads, particularly determining 85th compliance drivers for speed limits for example, on the residential streets, percentile speed enforcement set near the dispersion of speeds with greater mix of 85th per- between the slowest road users and func centile speed and fastest drivers tions than major arterials and freeways
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From page 87... ...
Expert Speed zones Helps identify many Probably most useful Complex system to System has been used to system factors, in addition and appropriate for develop, requir- target photo enforce based to vehicle operating roads in urban ing knowledge- ment (i.e., where recom approach speeds, that may areas where speed able experts and mended program limit is affect safety limits based solely computer capa- substantially below driv on 85th percentile bility ers' preferred operating speed may be inap- speeds) propriate Variable speed Freeways Not fully demon- Because of expense, Limited experience Systems are often com limits strated -- some indi- most appropriate in United bined with photo radar cation that more for highest-class States -- new tech- enforcement uniform speeds roads with large nologies are being reduce crashes traffic volumes introduced
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From page 88... ...
The trade-offs between safety, travel time, and other costs implicit in setting speed limits involve value judgments that are often best resolved by the political process. On the other hand, legislated speed limits can be arbitrary.
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From page 89... ...
However, it can also be used for setting speed limits in speed zones by adjusting optimal speeds to reflect the specific physical and environmental features of a given highway segment. Marcellis (1962)
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From page 90... ...
Successful implementation of the approach depends on driver compliance and perception that the speed limit is reasonable and on the level of enforcement activity. Engineering Study Method The most common method for determining speed limits in a speed zone sets the limit on the basis of an engineering study.
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From page 91... ...
advocated determining critical or maximum safe speeds by observing the operating speeds at or below which 80 or 90 percent of drivers travel under normal weather and daylight conditions. Although the method was recommended for establishing 8 At least two additional measures of speed dispersion are available for calculating operating speed as a basis for setting speed limits.
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From page 92... ...
. appropriate speed limits in speed zones, it was also advocated as a way of determining proper values for general speed limits, particularly on rural highways.9 A 1960 survey of the member departments of the American Association of State Highway Officials found that the majority set speed limits in speed zones primarily on the basis of the 85th percentile speed, although a few departments used the 90th percentile speed; such factors as design speed, geometric characteristics, crash experience, traffic volumes, and development were secondary considerations (Sub-Committee on Speed Zoning 1969 in Joscelyn and Elston 1970, 99)
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From page 93... ...
whose strengths and weaknesses were discussed in the preceding chapter. The studies found that crash involvement rates on certain road classes were lowest for vehicles traveling in a speed range whose upper bound was about one standard deviation above average traffic speeds, or approximately at the 85th percentile speed.10 Thus, the 85th percentile speed not only represents the upper bound of the preferred driving speed of most drivers, but, according to some studies, for some roads it also corresponds to the upper bound of a speed range where crash involvement rates are lowest.11 Setting the speed limit near the 85th percentile speed has great appeal from a behavioral and an enforcement perspective.
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From page 94... ...
Monitoring data collected on speeds, including the 85th percentile speed, following the relaxation of speed limits on some rural Interstates in 1987 showed an increase in 85th percentile speeds both for states that raised and states that retained the 55-mph (89-km/h) speed limit (Godwin 1992, 4)
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From page 95... ...
Lowering travel speeds, particularly on residential streets, may require other speed management strategies. Expert SystemBased Approach Several states in Australia have developed an expert systembased approach to setting speed limits in speed zones.
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From page 96... ...
, and (e) 85th per 15 Similar to the United States, Australia uses general speed limits supplemented by speed zones where the general speed limits are not considered suitable for the particular road and traffic conditions.
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From page 97... ...
The computer-based advisor17 is used primarily to assist regional road authorities to determine appropriate speed limits in speed 17System developers have moved away from calling the program an expert system, because it does not "learn" from its previous experience. Rather, the current system
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From page 98... ...
. The system appears to be most useful on roads where the 85th percentile speed is seen as an inappropriate basis for setting speed limits.
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From page 99... ...
. 20 Variable speed limits and other speed management approaches are reviewed in detail in Appendix D in the section entitled "Automated Speed Management." 21 The highway-related part of this program, whose primary objective is more effi cient use of the existing roads, is focused on equipping both vehicles and highways with electronic controls to provide the driver with more real-time information on traffic conditions, among other objectives.
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From page 100... ...
22 have introduced variable speed limits on a pilot basis on major motorways. Their primary purpose is to improve traffic flow in congested conditions by equalizing speeds in all lanes.23 Preliminary results indicate that, when the variable speed limits are in effect, traffic speeds are 22 Finland has also installed and is in the process of monitoring the effectiveness of a system of variable speed limit signs and message boards on a 9-mi (14-km)
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From page 101... ...
Department of Transportation is developing a variable speed limit system that reflects actual traffic speeds and weather conditions on a section of Interstate highway that is frequently subject to adverse weather. Deployment of the system will be accompanied by a monitoring effort to assess effects on driving speeds and crash experience.
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From page 102... ...
Similar to school zone limits, however, work zone speed limits alone26 have not proved very effective in reducing vehicle speeds 25 Data collected on free-flow average and 85th percentile speeds at six sites with speed limits ranging from 25 to 55 mph (40 to 89 km/h) found a 0- to 3-mph (5-km/h)
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From page 103... ...
In this section, what is known about each of these factors and their effect on establishing suitable speed limits to encourage appropriate driving speeds is discussed. Roadway Functional Class and Geometric Characteristics Roadway functional class and geometric design features are among the characteristics with the greatest effect on driving speeds and the determination of appropriate speed limits.
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From page 104... ...
28 In fact, policy guides for setting speed limits in speed zones require that speed studies be conducted during off-peak traffic hours during the day in fair weather conditions when traffic speeds are unimpeded.
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From page 105... ...
However, peak-period traffic congestion, more entering and exiting traffic, and generally more frequent interchanges suggest the need for somewhat lower speed limits than on rural freeways. Variable speed limits would be appropriate for this road class to manage temporal changes in traffic demands and speeds.
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From page 106... ...
Thus, speed limits must meet the objectives of this broader group of road users. Speed limits that give priority to travel efficiency will not be valid for all urban streets.
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From page 107... ...
roads representing a range of different conditions and speed lim 30 This approach is discussed in more detail in Chapter 5. 31 As Ullman and Dudek point out, lower speed limits were not accompanied by any increased enforcement or public notification, which could have changed driver compliance levels (p.
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From page 108... ...
on qualifying sections of rural Interstate highways.35 States that raised speed limits were faced with the decision of whether to set the limits uniformly for all vehicles. Several studies reported that average truck speeds were lower in states with differential speed limits (Baum et al.
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From page 109... ...
. 38 For example, Harkey and Mera collected detailed speed data on matched pairs of highways with and without differential speed limits, but the crash data were drawn from a single broad road category -- mainline rural Interstates.
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From page 110... ...
Ideally, speed distribution data must be closely linked with crash data to understand both whether and how actual speed changes -- as opposed to changes in speed limits -- have affected crash probability and outcomes. Even when speed data are good, isolating the effect of the speed change from all the other factors that affect traffic safety (e.g., changes in traffic volume, alcohol use)
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From page 111... ...
The spillover effect refers to the adjustments resulting from the diversion effects. For example, if increased speed limits on rural Interstates have diverted traffic from roads with lower speed limits, then the remaining traffic on these lower-speed roads may be able to travel faster.
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From page 112... ...
For the most part, it concentrated on studies that examined at least 2 years of postchange experience. Effect of Speed Limit Changes on Driver Speeds Most studies that examined the effect of speed limit changes on speed distributions provided information on several key speed measures, including average traffic speeds, 85th percentile speeds, and speed dispersion (typically defined in these studies as the difference between 85th percentile speeds and average traffic speeds)
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From page 113... ...
other 55-mph roads 55-mph states: 18 per cent increase in speed on rural Interstates; 37 percent increase in speed on other 55-mph roads (continued on next page)
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From page 114... ...
NHTSA 1989 Thirty-eight 65- 3.0-mph increase in aver- 65-mph states: 35 percent increase in fatalities between mph states; ten age speeds in 65-mph 1986 and 1988; 18 percent increase in fatality rates on 55-mph statesb states rural Interstates Before/after 3.5-mph increase in 85th 55-mph states: 9 percent increase in fatalities between 1986 (19861988) percentile speeds in 65- and 1988; 0 percent increase in fatality rates on rural Regression trend mph states Interstates analysis 0.7-mph increase in esti (19751988)
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From page 115... ...
deviation (85th per centile minus average speed) in 65-mph states Lave and Elias Thirty-eight 65- No speed data 65-mph states: 3 to 5 percent reduction in statewide fatality 1994 mph states; eight rates on average, controlling for the effects of long-term 55-mph states trends, exposure, safety belt laws, and economic factors Before/after analy sis, 1986 versus 1988 Regression analysis, 19761990, using and extending Garber and Graham's data (continued on next page)
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From page 116... ...
See Appendix C for discussion of methodology and more detailed discussion MANAGING SPEED of study results. Equivalences between miles per hour mentioned in the table and kilometers per hour are as follows: mph km/h 0.7 1.1 3.0 4.8 3.4 5.5 3.5 5.6 4.1 6.6 55 89 65 105 a Speed analysis based on nine 65-mph states and seven 55-mph states.
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From page 117... ...
The studies manifested two additional limitations. First, those that used comparison groups to examine the speed effects in states that raised the speed limit [65-mph (105-km/h)
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From page 118... ...
are located in the eastern United States, where population density, levels of congestion, and traffic volumes are different from those of the states that raised speed limits. Second, to the extent that the studies examined the effects of raising the speed limit on speed distributions on other roads (i.e., the spillover effects on 55-mph highways in 65mph states and on speed distributions on Interstate and non-Interstate highways in 55-mph states)
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From page 119... ...
. Similarly, estimates by NHTSA of increased fatalities for states that raised rural Interstate speed limits in 1987 dropped by one-third after travel increases were taken into account (NHTSA 1989, 1)
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From page 120... ...
Garber and Graham's analysis had found evidence that spillover effects from higher rural Interstate speed limits on rural non-Interstate highways offset traffic diversion effects in most states. They reported a net median 5 percent increase in rural non-Interstate fatalities in those states in which speed limits had been raised (Garber and Graham 1989, 18)
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From page 121... ...
The net effect across all counties showed no safety decrement. The author concluded that overall safety on California roads had not been compromised by the speed limit increase, but negative effects were experienced in the counties with rural Interstate highways where speed limits had been raised (McCarthy 1994, 362363)
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From page 122... ...
However, the data they provide are provocative and worth a brief discussion here. Effect of Speed Limit Changes on Driver Speeds Most studies of speed limit changes in individual states tracked data on changes in speed, including average speeds and 85th percentile speeds, before and after the new speed limits came into effect.51 A few studies (Retting and Greene 1997; Pezoldt et al.
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From page 123... ...
. Following the speed limit change, the range in driving speeds widened initially, and average and 85th percentile speeds reportedly increased (Montana Department of Transportation 52 Retting and Greene (1997)
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From page 124... ...
. Higher average speeds and 85th percentile speeds are clearly associated with greater crash severity.
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From page 125... ...
However, if much less than 100 percent of Interstate mileage was affected by increased speed limits, then the baseline number of fatalities used in the denominator for computing the percentage change would be too large, and the percentage change would be too small. Also, the current analysis did not include any spillover effects on non-Interstate roads (i.e., higher speeds and crashes on these roads)
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From page 126... ...
Speed limit increases on rural Interstates were associated with a statistically significant 11 percent increase in occupant fatalities; no statistically significant changes were found for speed limit increases on urban Interstates and freeways (Farmer et al.
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From page 127... ...
On two rural Interstate highways where speed limits were raised from 65 to 75 mph (105 to 121 km/h) , speeds [i.e., average speeds, 85th percentile speeds, and the percentage of drivers exceeding 80 mph (129 km/h)
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From page 128... ...
Changes in driving speeds and crash experience at these sites were compared with closely matched comparison sites where speed limits remained constant.63 The study found that changing posted speed limits had little effect on driving speeds. Specifically, a review of before and after speed data at the selected sites revealed that differences in average speeds, standard deviations of speeds, and 85th percentile speeds were generally less than 2 mph (3 km/h)
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From page 129... ...
. The study findings, however, cannot be generalized to all nonlimitedaccess roads because the site selection process was not random.66 The lack of observed changes in driver behavior raises the concern that, if the planned speed limit changes simply legalized existing behavior, the results could be significantly biased in favor of the finding that the speed limit changes had little effect on driver behavior and thus offer little insight into the independent effect of a change in speed limits on the distribution of driving speeds.67 Nevertheless, Parker's results were confirmed in another recent study of speed limit changes for a range of road types, mainly nonlimited-access state highways (Agent et al.
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From page 130... ...
This section focuses on a more limited number of recent studies of speed limit changes on highspeed roads. In contrast to the United States, where most studies have evaluated the speed and safety effects of raising speed limits on limited-access highways, international studies have primarily examined the effects of reductions in speed limits.
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From page 131... ...
Numerous methods are available for setting speed limits, ranging from legislated limits on broad road classes, to limits in speed zones determined on the basis of an engineering study, to limits established by local ordinance on residential streets. Whatever method is used, speed limits reflect implicit trade-offs among road user safety, travel efficiency, and practicality of enforcement.
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From page 132... ...
U.S. experience with raising speed limits on qualified sections of rural Interstate highways in 1987 suggests that higher speed limits resulted in higher average and 85th percentile speeds and modest increases in speed dispersion.
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From page 133... ...
The available studies of speed limit changes in speed zones on nonlimited-access highways in the United States found little change in speeds or crashes when speed limits were raised to near the 85th percentile speed or lowered to a limit well below the 85th percentile. Although methodological problems limit the generalization of study results, the findings suggest the difficulty of changing driver behavior merely by changing the sign.
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From page 134... ...
1997. Effect of 1996 Speed Limit Changes on Motor Vehicle Occupant Fatalities.
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From page 135... ...
1970. Maximum Speed Limits -- Volume II, The Development of Speed Limits: A Review of the Literature.
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From page 136... ...
A Comment on Lave and Elias's Question "Did the 65 mph Speed Limit Save Lives? " Insurance Institute for Highway Safety, Arlington, Va., 5 pp.
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From page 137... ...
1997. Traffic Speeds Following Repeal of the National Maximum Speed Limit.
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From page 138... ...
1971. Accidents, Speed Deviation and Speed Limits.
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