Passions run high over speed limits, but data is hard to come by

State and local officials who adjust speed limits up or down on certain roads might not get the results they’re looking for. Even worse, they might not even know it.

That’s one lesson that William Horrey, the technical director of the AAA Foundation for Traffic Safety, said transportation officials should take away from a recent study the group did on the effects of raising or lowering speed limits on different kinds of roads.

“Really think about the reasons for implementing the change, and then be sure to measure along those lines,” he said. “If the goal is to decrease travel time and increase throughput, measure that. But absolutely include safety as part of that component. Because if you’re talking about incremental savings for people’s time, but at great expense in fatalities and injuries, [your] perspective might change on that issue.”

The number of speeding-related deaths increased nationally since the start of the pandemic, and was a factor in 29% of traffic deaths in 2021.

The growing number of traffic fatalities, especially among pedestrians, has prompted city officials in places like Los Angeles; Columbus, Ohio; and Washington, D.C., to lower speed limits on their streets.

Often, though, the opposite move is happening in state capitols. At least nine states considered raising speed limits during legislative sessions this year, according to Advocates for Highway and Auto Safety. Proponents of the higher limits argue that big differences in the speed of vehicles can also create dangerous situations. Lowering the speed limit will only slow down some drivers, they say, which makes it harder for both fast and slow drivers to navigate safely.

“The movement in statehouses to raise speed limits is happening across the country,” said Jennifer Ryan, director of state relations for AAA in a statement. “But the benefits are overrated, and the risks are understated. Increasing speed limits does not always yield the positive results envisioned by traffic planners.”

A variety of factors other than speed limits can influence drivers’ behavior, Horrey explained in an interview. Those include road design, congestion, signal timing, enforcement measures and driver education efforts. Even the social norms in different regions can influence how drivers react to new speed limits.

The AAA Foundation found that raising or lowering speed limits by 5 mph had little impact on travel times.

“Even if there were a few statistically meaningful differences [in travel time], the actual magnitude of them was fairly small,” Horrey said. “That deflates the argument that travel time is really significantly impacted by these changes. So if that’s really one of the reasons for the change, maybe people can rethink that, especially when we want them to prioritize safety as much as possible.”

The seemingly straightforward task of gathering data on what happened when speed limits change showed the limits of the data that states collect on the safety of their roads, he said.

“It's difficult to gather the data. It can be expensive. But I think it's important especially when states want to kind of reflect on the safety of their roads,” Horrey said. If states gather information systematically, in formats that can be used with other datasets, it can make a big difference in safety. “When you come to like a milestone, like changing the speed limit, or some other change, it'll make it a lot easier to be able to do before-and-after comparisons.”

The drivers group gathered data on 12 different sites where speed limits changed, six where the limits were raised and six where they were lowered.

The researchers found that driver behavior varied. On three sections of interstate highways where speed limits went up, the average speed of vehicles did, in fact, increase. The number of fatalities and injuries increased on two of those sites, but decreased on the third.

Travel time and traffic volume remained the same on all three roads after the speed limit increased.

Travel times decreased, though, at three sites where officials raised speed limits on arterial roads (higher-speed local roads). The safety ramifications were less clear, in part because of limited crash data, the researchers said. One site lacked sufficient data to draw conclusions. The number of deaths increased at one site and decreased at another, but the changes were not significant.

To study the effect of lowering speed limits, the AAA Foundation studied more local roads. Again, the safety effects were mixed. But, for the five sites where sufficient data was available, travel times decreased slightly.

“Despite the varying results found in the current study in relation to the impact of changes in posted speed limits, the AAA Foundation’s past studies, along with other literature, suggest that practitioners should apply integrated or holistic approaches when setting or changing posted speed limits,” the researchers wrote.

Horrey said policymakers should have a broader view of the impacts of the changes they make.

“Especially in urban settings, you have to think beyond the throughput, because you’re also looking at the safety of other road users as well, such as bicyclists and pedestrians,” he said. “When you look at the net impact of speed decreases and the survivability of crashes, that speaks to the arguments for lowering speed limits as well. All of a sudden, crashes that might not have been survivable in one setting might be survivable in another.”

The AAA recommends that changes in speed limits should be based on several factors, including the type of road, land use and historical crash data. The group also supports automated speed enforcement under certain circumstances.