Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Executive Summary
Executive Summary
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Several are monitoring the effects of changes in speed limits on driving speeds and safety outcomes. In this study current practice in setting speed limits on all roads-not just major highways -- is reviewed, and guidance to state and local governments on appropriate methods of setting speed limits and related enforcement strategies is provided.
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However, state and most local governments have the authority to change the limits by establishing speed zones for highway sections where statutory limits do not fit specific road or traffic conditions, and to determine alternative maximum speed limits in these zones. Legislated speed limits are established by state legislatures, city councils, or Congress on the basis of judgments about appropriate trade-offs among public safety, community concerns, and travel efficiency.
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For example, the risks imposed on others by individual driver speed choices are likely to be relatively small on rural Interstate highways where free-flowing traffic creates fewer opportunities for conflict with other road users. Moreover, under normal conditions, drivers typically have adequate information to determine appropriate driving speeds because these highways are usually built to the highest design standards, access is limited, and roadside activity is minimal.
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The primary purpose of speed limits is to enhance safety by reducing the risks imposed by drivers' speed choices. Speed limits enhance safety in at least two ways.
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Thus, the posted speed limit should inform motorists of maximum driving speeds under favorable conditions that decision makers consider reasonable and safe for a road class or highway section. EFFECTIVENESS OF SPEED LIMITS The principal objective of speed limits is improved safety, but simply posting a speed limit does not guarantee the desired change in driving speeds or a reduction in crashes or crash severity.
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In contrast to the extensive analysis of speed and safety changes on Interstate highways, few studies have examined the effects of changing speed limits on lower-speed, nonlimited-access highways. Those that were identified found little effect on driving speeds or crash rates when speed limits were raised to near the 85th percentile speed or lowered to near the 35th percentile speed in selected speed zones on rural roads and on urban and suburban arterials.
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Automated enforcement, particularly photo radar, has been shown to be efficient and effective where it has been used for speed control, particularly on high-volume arterials. Photo radar could also be coupled with variable speed limit systems on urban Interstate highways where high traffic volumes can make traditional enforcement methods hazardous.
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From page 8... ...
The committee attempted to be as specific as possible, but the relevant studies and the data on which the guidance is based fall short of providing sufficient support for quantifying with much precision the effects of changes in speed limits on driving speeds and safety. General Framework for Establishing Speed Limits The current general approach -- legislated speed limits and administratively established speed zones -- is sound.
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From page 9... ...
Methods of Setting Speed Limits Legislated Speed Limits The strong link between speed and crash severity supports the need for setting maximum speed limits on high-speed roads (e.g., Interstate highways, freeways, high-speed rural multi- or two-lane roads) to place an upper bound on travel speeds to reduce crash probability and severity.
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For example, on many rural Interstate highways, vehicle operating speeds should be a major factor in setting speed limits. Design speeds provide little additional information because restrictive design features are not generally present on these highways; typically drivers can anticipate conditions and determine appropriate driving speeds.
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More research and evaluation of the effects of differential speed limits on driving speeds and safety outcomes are needed in states that have adopted them. Motorists do not appear to decrease speed at night when lower nighttime speed limits are in effect.
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Once their effectiveness is more clearly established, broader application of variable speed limit systems should be considered on urban Interstate highways in the United States because they are well suited to addressing temporal changes in traffic volumes, speed, and density on these highways. Enforcement and Other Speed Management Strategies Policy makers can affect the level of enforcement through resource allocation, but they must recognize that if drivers believe that a speed limit is unreasonable, enforcement will be difficult and expensive.
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Speed limits alone will not be effective in all situations. Keeping driving speeds at desired levels in urban areas poses a particular challenge.
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From page 14... ...
The issue of appropriate driving speeds, however, will persist as long as there are individual drivers making choices about risk and time efficiency. Ultimately, decisions about appropriate speed limits depend on judgments about society's tolerance for risk, valuation of time, and willingness to police itself.
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