Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Chapter 1: Introduction
Chapter 1: Introduction
Pages 15-35
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of 55 mph (89 km/h) , which had been in effect since 1974 following the oil shortages experienced in the preceding year.1 The National Highway System Designation Act of 1995 returned to the states the prerogative of setting speed limits on major highways.
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70 mph Figure 1-1 Maximum speed limits on Interstate highways as of June 10, 1998 (data from National Highway Traffic Safety Administration and Federal Highway Administration regional offices, state legislatures, and other sources)
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Basic Law j,k (60) Montana 75 l Nebraska 75 60 Basic Law j Nevada 75 70 (55)
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. NMSL = National Maximum Speed Limit.
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at night. The Texas Transportation Commission approved speed limits lower than the state maximum of 70 mph on about half of the state's farm-to-market system.
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More specifically, the interdisciplinary committee of experts convened to conduct the study has, in response to the charge, · Reviewed research to establish what is known about various methods of setting speed limits; the role of speed in safety; the role of road design and function in setting and enforcing speed limits; the effectiveness of speed limits, particularly with regard to safety; enforcement of and compliance with speed limits; and social benefits and costs of speed limits (i.e., trade-offs among safety, travel efficiency, and other factors that affect driver speed choices) ; · Considered the effects of new and emerging technologies for speed management and speed enforcement and expected changes in the highway environment (e.g., growing numbers of older drivers)
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-- setting speed limits in the United States has been a responsibility of state and local governments. Every state has a basic speed statute, which requires drivers to operate their vehicles at a speed that is reasonable and prudent for existing conditions and hazards.4 State statutes authorize maximum speed limits that may vary by highway type (e.g., Interstate highways)
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In setting speed limits, decision makers attempt to strike an appropriate societal balance between travel time and risk for a road class or specific highway section. Thus, the posted legal limit informs motorists of maximum driving speeds that decision makers consider reasonable and safe for a road class or highway section under favorable conditions.
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Drivers, neighborhood residents, traffic engineers, law enforcement officials, and legislators may differ as to what constitutes a reasonable balance between risk and travel efficiency. For example, local governments frequently receive requests to lower speed limits from neighborhood residents who seek to reduce speeding on local streets.
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The relationship among speed limits, driver speed choice, and safety on a given road is complex. Setting appropriate speed limits and related enforcement strategies is the first step in a chain of events that may affect crash probability and crash severity (Figure 1-2)
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. Speed choices made by individual drivers determine the aggregate distribution of traffic speeds on a particular road section, which in turn affects both the probability and severity of crashes.7 Finally, over time the safety record of a particular road may influence drivers' speeds and, in the long term, could result in a change in the speed limit or in enforcement strategies.
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. Where possible in this study, the relationships between speed limits, speed, and safety are analyzed by road type, and the methods of setting speed limits for different road classes are discussed.
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Interstate highways and freeways are a type of principal arterial highway characterized by multiple lanes with traffic separated by direction, controlled access (i.e., limited access) , and grade separation (rather than intersections)
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Drivers often underestimate the risks of traveling at high speeds. Younger drivers, for example, frequently say "I can handle the speed; my reflexes are good." In addition, drivers often misjudge the speed 8 Estimates of driver involvement in motor vehicle crashes provided by the National Safety Council -- 18,900,000 driver involvements in 1996 -- were used for this calculation (National Safety Council 1997, 78)
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The more drivers perceive speed limits to be credible and reasonable for conditions, and enforced, the more likely they will be obeyed. FACTORS AFFECTING DETERMINATION OF APPROPRIATE SPEED LIMITS Those responsible for determining appropriate speed limits -- state and local legislators and traffic engineers, often with input from law enforcement officials and community groups -- must define limits that are appropriate for different road classes and users.
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12 Concerns about liability when posted speed limits based on vehicle operating speeds exceed the design speed of a highway segment, however, may lead to a redefinition of terms to bring about greater consistency among posted speed limits, operating speeds, and design speeds (Fitzpatrick et al.
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Thus, travel efficiency is not the primary consideration in neighborhood travel, and speed limits often correspond to speeds at the lower end of the speed distribution.13 These distinctions may not be as clear on other road classes. For example, travel efficiency is still an important goal on urban Interstate highways, but in many metropolitan areas congestion limits travel speeds for several hours of the day.
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However, the potential for conflict at intersections and driveways, and between motor vehicles and farm equipment on the road, often requires setting speed limits lower than if travel efficiency were the primary objective. Similarly, the exposure of pedestrians and cross-street traffic to through traffic operating at high speeds on arterials in rapidly developing suburban areas may also warrant lowering speed limits on these roads.
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Highways and speed limits must accommodate a broad spectrum of drivers. Minimum speed limits have been established on some high-speed roads that may deter slow drivers as well as vehicles that cannot maintain adequate speed levels.
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Finally, the committee's guidance on appropriate strategies for both setting and enforcing speed limits is provided in Chapter 6. REFERENCES ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials FHWA Federal Highway Administration ITE Institute of Transportation Engineers NHTSA National Highway Traffic Safety Administration TRB Transportation Research Board AASHTO.
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Federal Highway Administration, June.
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