Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Chapter 6: Guidance on Setting and Enforcing Speed Limits
Chapter 6: Guidance on Setting and Enforcing Speed Limits
Pages 186-214
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From page 186... ...
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.
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From page 187... ...
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)
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From page 188... ...
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.
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From page 189... ...
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.
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From page 190... ...
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 arterials. Evidence of increased crash probability from traveling below the average speed is found primarily on Interstate highways near interchanges where traffic slows to merge or exit and on rural roads where vehicles slow at intersections or when negotiating turns.
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From page 191... ...
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 determine 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.
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From page 192... ...
. Where statutory limits do not fit specific road or traffic conditions, speed zones may be established administratively, and speed limits for that highway section 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)
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From page 193... ...
Thus they are provided here, although there is not always a one-to-one correspondence between the AASHTO and the committee road classification systems. Guidance on Setting Legislated Speed Limits by Specific Road Class In determining appropriate speed limits by road class, decision makers 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.
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From page 194... ...
2,324.5 Rural Interstate Rural Interstate 32,818 2,388 2,850 1.23 91,200 39 (9.4)
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From page 195... ...
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.
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From page 196... ...
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
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From page 197... ...
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)
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From page 198... ...
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 conSpecial 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)
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From page 199... ...
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 generally good on this road class.
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From page 200... ...
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.
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From page 201... ...
As would be expected because of the many factors affecting fatalities 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 consistent pattern of overall increases in fatalities and fatal crashes.
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From page 202... ...
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.
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From page 203... ...
Maximum speed limits should be lower than on rural Interstate highways to reflect lower design standards generally on this road class. The potential 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.
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From page 204... ...
Fatal crashes and fatalities 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 consideration in setting appropriate speed limits on these highways.
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From page 205... ...
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 practicality should be given higher priority.
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From page 206... ...
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.
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From page 207... ...
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.
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From page 208... ...
An expert-system approach, either formal or informal, could be developed to establish speed limits in speed zones.15 The expert-system approach deserves consideration because it provides a systematic and consistent method of examining and weighing factors other than vehicle operating speeds in determining an appropriate speed limit. 15Details of Australia's expert system for setting speed limits are provided in Chapter 3.
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From page 209... ...
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 tolerance, enforcement is still necessary to ensure the conformity of drivers who will obey laws only if they perceive a credible threat of detection and punishment for noncompliance.
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From page 210... ...
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 variable speed limit systems for use on urban Interstates where high traffic volumes make it difficult to apprehend speeding drivers.
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From page 211... ...
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 driving speeds.
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From page 212... ...
The current high cost of variable speed limit systems limits opportunities for their deployment to urban Interstates and freeways with large traffic volumes or to selected segments of major roads where weather (e.g., fog, visibility) is a frequent problem.
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From page 213... ...
Most states have recently raised speed limits on many major highways following repeal of the NMSL. Close monitoring of effects, particularly changes in driving speeds and safety outcomes, is desirable; vigilant enforcement is needed; and redoubled efforts should be taken to mitigate adverse safety outcomes by such continuing initiatives as increased safety belt use and reductions in driving while intoxicated -- measures with large and proven safety benefits.
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From page 214... ...
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.
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