Injury Law Blog

How are speed limits set?

After a motor vehicle accident occurs, one of the first things police investigate is whether any of the vehicles involved were speeding. As we all know, speeding is a factor in many motor vehicle accidents. However, have you ever wondered how speed limits are set?

You might be surprised to know that drivers play a big part in establishing speed limits, and that there usually isn’t much emphasis on pedestrian or bicyclist safety. Essentially, speed limits are based on the speed drivers feel safe driving on a particular street or highway.

According to research from the Federal Highway Administration, traffic engineers generally study the average speed drivers travel down a road and set the speed limit around the 85th percentile, meaning that 85 percent of drivers are within the speed limit and 15 percent are speeding.

In some cases, but not all, traffic engineers also take into account like daily traffic patterns, geographical factors such as mountainous terrain or proximity to schools or parks.

The old 85th percentile rule can be traced all the way back to a 1964 study that was put together for the U.S. Department of Commerce. It ultimately found that the chances of an accident are increased dramatically when a vehicle travels much slower or much faster than the rest of traffic.

However, there is plenty of doubt over whether the 85th percentile rule still makes sense today. For example, the study that it is based on didn’t take bicyclists or pedestrians into account. That means speeds that are considered safe for vehicles might still be very dangerous for pedestrians and bicyclists.

For these reasons, pedestrian and bicycle safety advocates are arguing that it might be time to revisit how speed limits are set, and this time include how safe pedestrians and bicyclists would fare in accidents involving vehicles traveling at different speeds.

Fivethirtyeight.com, “Why The Rules Of The Road Aren’t Enough To Prevent People From Dying,” Anna Maria Barry-Jester, January 15, 2015