Recently I posted about the increased speed limits on a handful of Texas roads, which drew the ire of my most prolific commenter, Sanjay. In his comments, he challenged my assertion that the 85th percentile is regarded as the safest speed. He claimed his instinct told him that my sources (a 25 year police veteran) was “wrong” or had been fed incorrect information. A quick web-search of “85th percentile driving” will yield a number of results, that seem to unanimously approve of my position.

Now, with all due respect to Sanjay, I doubt very seriously that among his eclectic studies he’s had even a modicum of training that would qualify as even remotely comparable to the ongoing defensive & aggressive driving training, as well as the forensic training and information that would typically be taught to the Detectives/Sergeants for a wealthy suburban police department, coupled with 25 years of experience on the road in various jurisdictions. He is challenging my appeal to authority based solely on his own very questionable authority. The proper recourse is to challenge the legitimacy of my authority, not to invoke some bogus “instinct” that I (and my source) are incorrect.

Enough of that, let’s make with the info:

Michigan Department of Transportation:

Speed studies are taken during times that represent normal free-flow traffic. Since modified speed limits are the maximum allowable speeds, roadway conditions must be close to ideal. The primary basis for establishing a proper, realistic speed limit is the nationally recognized method of using the 85th percentile speed. This is the speed at or below which 85% of the traffic moves. Studies have shown that changing the posted speed limit does not significantly change the 85th percentile speed. The driving environment, including other traffic on the roadway, roadway conditions, pedestrian traffic, etc., is a significant factor which influences the prevailing speed . . . Use of the 85th percentile speed acknowledges that 15% of the drivers are traveling above a speed that is reasonable and proper. It is to this 15% that enforcement action is directed. Studies have shown that these are the drivers who cause many of the crashes and have the worst driving records…. Studies have been conducted over the years to relate crashes to speed. Findings show that a driver’s risk of being involved in a crash is lowest at 5 to 10 mph above the average speed of traffic. This is coincidentally usually the 85th percentile speed. Based on those studies, the lowest risk of being involved in a crash occurs at approximately the 85th percentile speed.“

Similar results are found in various States’ Department of Transportation websites, and a host of other sites, including the notorious Wikipedia, which discusses the 85th percentile rule from Wikipedia in its article about the Speed Limit:

“In the United States, traffic engineers may rely on the 85th percentile rule[1] to establish speed limits. The idea is that the speed limit should be set to the speed that separates the bottom 85% of vehicle speeds from the top 15%…Most U.S. jurisdications report using the 85th percentile speed as the basis for their speed limits, so the 85th percentile speed and speed limits should be closely matched. However, a review of available speed studies demonstrates that the posted speed limit is almost always set well below the 85th percentile speed by as much as 8 to 12 mph (13 to 19 km/h). Some reasons for this include: [* Political or bureaucratic resistance to higher limits -and- * Statutes that restrict jurisdictions from posting limits higher than an arbitrary number.”

Is that sufficient to put this to rest?