International review confirms speed management is critical to road safety

By Chris McCahill

Speed reductions can lower crash risks significantly, confirms a new report by the International Transport Forum, an intergovernmental organization of 59 member countries including the U.S.

The research report, Speed and Crash Risk, looks at 11 case studies in 10 different countries around the world. Each case study focuses on speed changes due to changes in speed limits (increases or decreases) or automated speed enforcement. The U.S. case study looks at Charlotte’s 2004 speed camera program, called Safe Speed.

In every case, speed increases were associated with more crashes and more severe injuries, while speed decreases were associated with fewer crashes, injuries, and deaths.

The relationships, however, are not linear. The case studies generally confirm what is sometimes called the “power model,” which says that a 10 percent increase in mean speed is associated with a 20 percent increase in injury crashes, a 30 percent increase in severe crashes, and a 40 percent increase in fatal crashes (Figure 1).

Figure 1. The power model. Source: ITF and Nilsson, 2004.

The report offers a number of concrete recommendations for improving safety through speed management, described below:

  • Speed limits should be set based on the Safe System principles, which the World Research Institute recently identified as the most effective way to reduce traffic deaths. This approach accounts for “the built environment and how it affects travel choice and behavior,” according to WRI.
  • In urban areas with vulnerable users, speed limits should be around 20 mph and not more than 30 mph.
  • Speed limit increases should incorporate countermeasures to offset related safety issues.
  • Automated speed enforcement helps bring down the highest speeds and decreases the speed differentials among drivers. The report notes that the most effective cases incorporate section control, whereby average corridor speeds are monitored and fines are issued to violators, much like tolling. This method also improves traffic flow.
  • The most effective approaches are integrated and include communication, education, and enforcement programs that are maintained over time.

Chris McCahill is an Associate Researcher at SSTI.