A new study finds that raising posted speed limits may do little to save time and increase traffic flow but could lead to more crashes, injuries, and deaths.
The AAA Foundation’s said in a statement its research results varied across all 12 roadway sites examined. All had new posted speed limits — six raised and six lowered — and included various road types.
Raising posted speed limits was associated with increased crashes on two interstate highways, according to the AAA Foundation's study. The study also found that lowering posted speed limits led to fewer crashes in many cases examined, according to AAA. But the likelihood of speed limit violations increased after lowering posted speed limits. AAA said this suggests the need for better public awareness education tied to these changes.
The foundation study found:
- Raising posted speed limits was associated with increased crash frequencies and rates for two of the three interstate highways examined.
- Lowering posted speed limits was associated with decreased crash frequencies and rates for one of the two principal arterials examined.
- Changes in travel times were small in response to both raised and lowered speed limits.
AAA said it urges transportation officials to apply a “holistic” approach when setting or changing posted speed limits and prioritize safety over speed and capacity.
“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,” said Dr. David Yang, president and executive director of the AAA Foundation. “However, it is critical to consider the safety implications when local transportation authorities contemplate making changes with posted speed limits.”
AAA said it recommends changes in posted speed limits should consider a range of factors, including but not limited to the type of road, surrounding land use, and historical crash data. AAA also said it supports automated speed enforcement, but programs must be carefully implemented to maintain community support, prioritize equity and consistently drive improved safety.
“The movement in statehouses to raise speed limits is happening across the country in at least eight states this year,” said Jennifer Ryan, director of state relations for AAA. “But the benefits are overrated, and the risks are understated. Increasing speed limits does not always yield the positive results envisioned by traffic planners.”
AAA said this study is the third phase of the its foundation's research examining the effect of posted speed limit changes on safety. In the Foundation’s first study, traffic engineers were asked how posted speed limits are set and what factors they consider in changing them. In the second phase, crash testing revealed that small speed increases have severe and potentially deadly effects on crash outcomes.
