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85th percentile important to setting speed limits

March 10, 2012
By DAVE WERNER ( , Franklin County Traffic Safety Board

If you are a frequent reader of these articles on traffic law and traffic safety, you have seen numerous references to the "85th percentile speed." The consensus of traffic engineers throughout the country indicates that the appropriate value for a speed limit will almost always be that indicated by the 85th percentile speed (to the nearest 5 mph).

The 85th percentile speed is that speed at which or below that 85 percentage of vehicles are traveling on a given highway. Traffic engineers sample a minimum of 200 free-flowing vehicles, when motorists are able to travel at a speed that is reasonable and prudent for conditions in which they can freely maneuver their vehicles and are not subject to the influence of other motorists who may be simultaneously sharing the highway at a given time. During periods when traffic is heavy or congested, vehicles may not be free flowing; as such, they actually may not be able to travel even at the posted speed limit and a true 85th percentile speed cannot be realistically obtained. If 200 vehicles is not obtained within two hours, and if traffic engineers are satisfied that fewer vehicles provides a realistic sample, less than 200 vehicles may be used.

In the majority of cases, the 85th percentile speed is the appropriate speed limit. It assumes that motorists properly adjust their speed while encountering different roadside development and highway geometrics and the adjustment is reflected in the 85th percentile speed.

The reality of speed limit setting is that everyone wants a speed zone in front of their house but not in front of anyone else's. Also, without training in recognizing vehicle speeds, the perception is that vehicles are traveling faster than they actually are. Thus, citizens are constantly complaining about excessive speeds and often request slower speed limits from their elected officials.

However well-intentioned this is, the reality is that it doesn't matter what the posted speed limit is. Motorists will drive at what they perceive to be a safe and reasonable speed. Thus, if drivers perceive an unrealistic low speed limit, they will not obey it absent heavy enforcement. Proof of this is evident from a 2007 study of traffic speeds on Finney Blvd. (State Route 30) in Malone near the Gateway Motel, where the legal speed limit at that time was 30 mph. The 85th percentile speed was 44 mph. It is interesting to note that of the 200 vehicles measured, only four were traveling at or below the legal 30 mph speed limit. Also, the fastest vehicle was clocked at 60 mph. Because of this study, the posted legal speed limit has been raised to 40 mph which is more realistic and more closely conforms to the 85th percentile speed.

Setting appropriate speed limits is a difficult and not often popular duty not popular because the residents and businesses along any street or road generally want speed limits to be set lower than what they should be. The twin goals of setting speed limits are to provide appropriate speed limits on all highways but also to prevent the setting of unrealistic speed limits to satisfy political pressures. Accordingly, this is the reason that an engineering study is always required, and the 85th percentile speed always plays an important role.


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