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Why are speed limits set the way they are?

On Behalf of | Jan 16, 2015 | Car Accidents

On occasion, as you drive through Chambersburg and head out on U.S. 30 to Gettysburg or up I-81 to Harrisburg, you may wonder how speed limits are determined. Interstates are designed to speed traffic across country, so high speeds make some sense, but in smaller towns and cities, the speed limit often seems arbitrary.

Apparently, because in many cases, it is. In something of “Which comes first, the chicken or the egg?” speed limits are determined by assumptions on how fast motorists will drive and feel safe on the road. They are then set at the 85th percentile of that speed. And this is used in turn to develop a “design speed” when other roads are built. 

This means that on many roads, you are likely to feel comfortable and safe driving above the speed limit. But study much of this was build on is over 50 years old, and what may be the greatest shortcoming of how speed limits are set, is that for the most part, they ignore pedestrians and people on bicycles.

Fivethrityeight.com notes that jaywalking laws were created to criminalize walking, and encourage car sales, by shifting the fault to reckless drivers and walkers.

The truth remains that many car accidents are caused by excessive speed. The physics of a car moving at 30 miles per hour striking a person are unforgiving. And traffic laws have been unforgiving when facing off cars against humans, promoting traffic flow at the expense of lives.

However, change can happen. Changes in the speed limit in New York City appear to working, and 2014 appears to have the lowest number of pedestrian deaths since 1910.  

Speed limits often are like wallpaper—always there, but little noticed, but sometimes things that are seen as normal should not. Speed limits do not exist in a vacuum, nor are they immutable. They can be changed and lives can be saved.

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


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