A new study by the AAA Foundation for Traffic Safety finds that raising posted speed limits would likely do little to save time and increase traffic flow and could lead to more crashes, injuries, and deaths.

States are allowed to set their own speed limits, apart from federal approval.

The AAA Foundation’s research is based on 12 speed limit changes — six raised and six lowered— and included various road types.

Raising posted speed limits was associated with increased crashes on two of the three interstate highways AAA included in the study while lowering posted speed limits led to fewer crashes in many of the cases that were researched. The likelihood of speeding tickets going up after posted speed limits are lowered suggests there’s a need for better public awareness education tied to the changes, AAA said.

“AAA urges transportation officials to apply a ‘holistic’ approach when setting or changing posted speed limits and prioritize safety over speed and capacity.

AAA Foundation President and Executive Director David Yang added, “Our study analyzed before-and-after data on a dozen roadways that raised or lowered posted speed limits and found no one-size-fits-all answer regarding the impact of these changes. However, it is critical to consider the safety implications when local transportation authorities contemplate making changes with posted speed limits.”

According to the National Highway Traffic Safety Administration (NHTSA), there were more than 42,000 traffic deaths in 2021 and again in 2022 — the highest in 16 years. NHTSA reports that speeding was a factor in nearly 29% of the fatalities in 2021 and 27% in 2022.

AAA recommends that when speed limit changes are considered a range of factors should be looked at, but not limited to, the type of road, surrounding land use, and historical crash data.

“AAA supports automated speed enforcement but programs must be carefully implemented to maintain community support, prioritize equity and consistently drive improved safety,” AAA said.

AAA Director of State Relations Jennifer Ryan added, “The movement in statehouses to raise speed limits is happening across the country in at least eight states this year. But the benefits are overrated, and the risks are understated. Increasing speed limits does not always yield the positive results envisioned by traffic planners.”

In June, NHTSA shared that traffic fatalities had dropped 3.3% during Q1 2023, representing the fourth consecutive quarterly decline.

Its initial projections show that 9,330 people were killed on U.S. roadways during the first three months of 2023, fewer than the 9,645 people who died during the same period last year.

The projected decrease in deaths was despite more drivers hitting the road, NHTSA said.

The AAA Foundation’s study was the third phase of its research into the effect of posted speed limit changes on safety. In the first phase, traffic engineers were asked how posted speed limits are set and what factors they consider in changing them.

Some key findings include:

• • “98% of survey respondents consider the 85th percentile operating speed (the speed at or below which 85 percent of the drivers travel on a road segment) when raising or lowering posted speed limits;
  • “57% of respondents reported their agencies have a standard operating procedure/policy for setting speed limits;
  • “30% of respondents either had never heard about an expert system (i.e., USLIMITS or USLIMITS2) or had any understanding of the system;
  • “Nearly half of the respondents (49%) who had heard about the expert system reported that they never base a decision about changing a speed limit on it;
  • “The top reason for raising a speed limit is a change or changes in infrastructure, network, land use or road function (63%); and
  • “The top reason to lower speed is receiving requests from the public to improve safety (76%).

In the second phase, crash testing in collaboration with the Insurance Institute for Highway Safety (IIHS) revealed that small speed increases have severe and potentially deadly effects on crash outcomes.

Overall, as the crash speed increased, the additional occupant compartment deformations and higher crash energy resulted in higher peak injury measures recorded by dummy sensors over the entire body region.

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