Does Lowering Speed Limits Save Lives?

New York’s mayor signed into law lowering the speed limit in the city from 30 mph to 25 mph, assuming that this would result in fewer car crashes and pedestrian casualties. The change took place on November 7th.

The city experienced 291 traffic fatalities—passengers or pedestrians—in 2013. The mayor was quoted in New York Magazine, “When drivers are driving below 25 miles an hour, it gives them much more time to avoid crashes, gives drivers and pedestrians more time to see each other, greatly intensifies the opportunity to save lives.”

The 5 mph difference between 30 mph and 25 mph leads to less severe injuries too.

At first glance, it seems like a good idea. However, the research on this subject does not support this premise.

A study performed for the Federal Highway Administration and published nearly 20 years ago came to some counterintuitive conclusions.

It turns out that when before-and-after data were examined following speed limit changes, the differences in average speeds were less than 2 mph and did not correlate with the amount that the speed limit was changed.

This is especially true when speed limit changes were within 5 mph of the 85th percentile of existing traffic speeds. The report stated that “lowering the speed limit does not mean traffic will slow down.”

Another important conclusion was “Changing posted speed limits alone, without additional enforcement, educational programs, or other engineering measures, has only a minor effect on driver behavior.” So far, there has been no apparent increase in speed limit enforcement by the NYPD.

In fact, the other day a New York Post reporter risked her life while attempting to maintain a speed of 25 mph on city streets. She found that no one was observing the new limit.

Most of the fatalities, especially those involving pedestrians, seem to be caused by drivers who are going well over the limit anyway.

The speed limit reduction reminds me of another misguided law—the lowering of the legal limit of blood alcohol concentration from 0.1% to 0.08%, which is the law in all states. A paper in American University’s journal Public Purpose reviewed the subject in depth and found that while legal limits of 0.1 decreased auto crash fatalities, the reduction to 0.08 had no effect.

Justin Peters wrote in Slate “According to the NTSB’s own report, the yearly rate of alcohol-impaired driving fatalities has remained relatively stable since 1995, despite the big federal push to move from 0.10 to 0.08.”

US News also pointed out that the National Highway Traffic Safety Administration has found that in fatal drunk driving accidents, “92.5 percent of the time the driver’s BAC was 0.10 or higher.”

A recent study from Sweden found that of 188 drunk drivers killed in road crashes, their  average BAC was 0.172, well above anyone’s legal limit.

New York City’s new speed limit undoubtedly lets the politicians and maybe the public feel that they are doing something about traffic fatalities, but it is not likely to have any real impact.

What’s your opinion?

Skeptical Scalpel is a retired surgeon and was a surgical department chairman and residency program director for many years. He is board-certified in general surgery and critical care and has re-certified in both several times. He blogs at SkepticalScalpel.blogspot.com and tweets as @SkepticScalpel. His blog averages over 1400 page views per day, and he has over 10,700 followers on Twitter.