States Raise Speed Limits while Safety Debate Continues

On May 22, 2013, Illinois became the fourth state since March where the legislature has sent a bill to the governor’s desk either ordering or permitting a speed limit increase on some roads.  Governor Pat Quinn has been coy on the measure but the overwhelming support by the legislature would seem to make the initiative veto-proof. Ohio, Iowa and Maine have all passed similar measures since March and 34 states already have speed limits of 70 miles per hour or greater on some roads. With initiatives working through the North Carolina and Nevada legislatures and with bills being introduced in at least eight more states, it appears more of America’s roadways will permit higher speeds. States raise speeds on some toll roads, like America's fastest road, as an inovative funding mechanism and congestion measure. Higher speeds are sold to the consumer as a premium service.  Some question the timing of and rationale for these actions, coming as recently released preliminary traffic fatality analysis data for 2012 from the National Highway Traffic Safety Administration showed a 5.3 percent uptick in motor vehicle fatalities nationwide. But a review of the research on speed and safety isn’t as cut and dried as one might think.

The Great Debate

Safety advocates argue the speed limit increases could make the roadways more dangerous. The increased rate of speed could cause more accidents and could make accidents more deadly. Because of the physics inherent in traveling at higher speeds, most policy makers and researchers agree with the latter; however the former is the subject of rather intense debate.

Increasing the speed limits on some roads may not necessarily increase the frequency of accidents. Some research indicates that increases in the speed limit can actually decrease or maintain the rate of accidents. The key, researchers claim, is to reduce the variation between drivers’ speeds that can occur if speed limits are too low and many drivers speed anyway. Drivers naturally react to the built environment–the shape, width and slope of the road--and the traffic around the road before they react to the imposed speed limits on the road. Some drivers more readily obey speed limits than others, creating two groups of drivers--one driving the speed limit and one driving more closely what the built environment allows. This difference in speed can create conflict on the roads. Faster drivers must change lanes and often change speeds in the process of passing, creating an opportunity for accidents.

Why Speed Limits May Be Too Low

The Federal Highway Administration’s “Speed Concepts” guide explains that limits are safest at the 85th percentile of measured free flowing traffic. The crux of this approach is that too much variance in the speed of drivers on the same roadway is dangerous. The 85th percentile is the hypothetical sweet spot; it allows drivers to follow the natural flow of traffic while still banning speed-demons. This has long been the rule of thumb for establishing limits but beginning in the 1970s, Congress created a different standard.

The federal government imposed a national speed limit of 55 mph in 1974 in reaction to the 1973 oil crunch. Congress raised the limit to 65 in 1987 and then repealed the law in 1995 allowing states full authority to determine their interstate speed limits. Bureaucratic and legislative inertia caused some states to maintain limits lower than the 85th percentile.

In a 2001 survey, engineers were asked about roads with speeds not set at the 85th percentile. Engineers reported that 33 percent of the time the cause for the difference was political. In context, engineers reported that “accidents” accounted for the change only 13 percent of the time, while “roadway areas” made up 11 percent and “roadway geometry” only nine percent.

After 20 years of a national speed limit, the relationship between the built environment, traffic flow and the speed limit had been, if not severed, strained. If the original 85th percentile rule is correct, it follows that the speed may need to be higher on some of these roadways.

What Does the Research Say?

The empirical research on the effect of raising speed limits is mixed. Most research does not focus on the 85th percentile rule, instead often studying the effect of a particular speed limit change on particular roads. The urbanism blog, Copehagenize, explains that the 85th percentile rule probably goes back to 1964 research on a phenomenon called “The Solomon Curve.”

The Solomon's Curve

The Solomon Curve is a visual representation of the likelihood of being involved in a crash as one deviates from the average speed of traffic. Note that likelihood of crashing is higher when one is traveling substantially slower than traffic. However, David Levinson at cautions that “Solomon's curve is not gospel, Davis et al. (2006)  could not corroborate it, finding higher speeds associated with higher likelihood of certain crash types, but not lower Speeds.” While no one study is definitive, the validity behind very foundational assumptions of highway design and management are at question and require greater examination.  

There are two issues: speed variation and speed limits. We assume they are related but their relationship is not simple. Simply raising the speed limit may not result in a reduction of speed variance if drivers continue to drive faster than the new speed limit. Researchers found that when Texas raised the limit from 75 to 80 on parts of its system, the average speed increased. This is problematic if the variance remains constant for two reasons: first, because the crux of the argument for increased safety at higher speed limits lies in reduced variance and secondly, we now have a new 85th percentile higher than the last. Groups like the Insurance Institute for Highway Safety have taken issue with the “moving target” problem.

Recent research found that it is safest to raise speed limits when traffic is already free flowing -as on some sections of rural interstates. Policymakers, when considering raising limits, should take current traffic patterns into account when making these decisions. Additional research found that enforcement and public education are important factors in maintaining safety after a state raises limits. They found that 80 percent of the public wanted the change. This public support translated to faster traffic once the limit was raised but also into higher compliance with the speed limit. Speed variance did not increase and crash data indicated that the speed limit change had not been responsible for additional crashes. The authors assert that their data represents a victory for advocates of raising limits because the combination of public campaigning and enforcement resulted in an equally safe but faster highway.

  In considering speed limit increases, highway departments and policy makers should be cautious to balance speed and safety. A faster transportation network can move people and the goods they buy to their desired destination in less time, but consideration should always be given to roadway conditions and policing options when making decisions that are very much about life and death.