1.1 Speed Limits

Effectiveness: Proven*

Use: High

Cost: Low

Time: Short

*When enforced and obeyed

Speed limits are only one part of the system that attempts to control driving speeds. Without broad public acceptance and active enforcement they have little effect. With public acceptance and enforcement, lower speed limits can reduce travel speeds and casualties.

Speed limits are set both by legislation and by administrative action. General speed limits apply to all roads in a class, such as rural interstates or local streets. They are set by State, municipal, or even at times by Federal law based on tradeoffs between safety, travel efficiency, and community concerns, taking into account the design characteristics of each road class. GHSA (2005) and IIHS (2005a) summarize each State's maximum speed limits and NHTSA (2003b) provides each State's complete speed limit laws. Speed zones apply to road segments where the general speed limit is thought to be inappropriate. Speed limits in these zones usually are set by administrative action based on the road segment's free-flowing travel speeds, crash experience, road and land use conditions, and other factors. Speed limits in many speed zones are set near the 85 th percentile travel speed: the speed at or below which 85 percent of vehicles travel in good weather, with no congestion (TRB, 1998, p. 2).

The effects of maximum speed limits on speeds, crashes, and casualties have been studied extensively over the past 30 years. In 1974 the 55 mph National Maximum Speed Limit (NMSL) was enacted to conserve fuel. Travel decreased, speeds decreased on roads where the speed limit was lowered to 55 mph, and total traffic fatalities decreased by 9,100 from 1973. The slower and more uniform speeds due to the 55 mph limit are judged to have saved between 3,000 and 5,000 lives in 1974 (TRB, 1984, p. 2). As fuel became plentiful again, travel increased and compliance with the 55 mph limit decreased markedly (TRB, 1984, p. 5). In 1987 Congress allowed States to raise speed limits to 65 mph on rural interstate highways. States that raised their limits generally saw increases of about 4 mph in average speeds and 85 th percentile speeds and statistically significant increases in traffic fatalities on these roads (TRB, 1998, p. 5). In 1995, Congress repealed the NMSL and returned full authority to set speed limits back to the States. Again, increased speed limits produced modest increases in both average and 85 th percentile speeds and increases in traffic fatalities (TRB, 1998, p. 6).

Few studies have examined the effects of speed limit changes on lower-speed roads. Those that did found little effect on driving speeds or crash rates when speed limits were raised to near the 85 th percentile travel speed or lowered to near the 35 th percentile speed, either on rural roads or on urban and suburban arterials. These results illustrate that changing the speed limit by itself may not affect driving speeds (TRB, 1998, p. 6).

Use: A speed limit is in effect on all road segments in all States.

Effectiveness: Lower maximum speed limits definitely reduce crashes and casualties when the limits are obeyed. The same holds true on any road: if a lower speed limit is obeyed, then crashes and casualties will drop. But lower speed limits by themselves may not reduce travel speeds.

Costs: The immediate costs of changing speed limits are for new signage and for publicizing the new limit. Enforcing the new limit may involve substantial costs.

Time to implement: Speed limit changes can be implemented quickly, as soon as signage is in place and the new limits are publicized.

Other issues:

• Public acceptance, roadway characteristics, enforcement, and publicity: Speed limit changes cannot by themselves reduce speeding, whether defined either as average travel speed or as the proportion of drivers traveling substantially faster than the average speed. Speed limits can reduce speeding if most drivers believe that the limits are reasonable and if the threat of enforcement is great enough to affect the few drivers who would not comply voluntarily. It is generally very difficult to enforce and obtain general compliance with a lower speed limit on a roadway designed for higher speeds (TRB, 1998, p 12). Thus, speed limits must be considered as part of a system including broad public acceptance, roadway characteristics, active enforcement, and publicity (TRB, 1998, p. 133).
  
• Rational speed limits: Speed limits on many road segments are frequently not obeyed, and average travel speeds on these segments substantially exceed the speed limit. One strategy that has been proposed to increase overall safety is to increase the speed limit to a generally accepted level on selected road segments and at the same time aggressively publicize and enforce this speed limit. The strategy's goal is to increase the public's overall acceptance of speed limits while reducing the number of people driving at speeds considerably higher than the limit. NHTSA began a demonstration of this strategy in 2003. Preliminary data as of December 2004 suggest that the increased speed limits have had little effect on travel speeds. Final demonstration reports, which will include data on speeds, crashes, and casualties, will be available in 2005.