85th Percentile Speed

Q: What is the 85th percentile speed?

ANSWER: If the speeds of all motorists are ranked from slowest to fastest, the "85th percentile speed" separates the slower 85% from the fastest 15%.

(Similarly, the average speed separates the fastest 50% and slowest 50% of motorists in a normal speed distribution. In other words, the average speed is typically the "50th percentile speed").

Q: Why the 85th percentile? Why not the 99th, or 75th or 30th percentile?

ANSWER: Two reasons.

1. Most motorists travel at about the same speeds, so setting the speed limit at the 85th percentile legalizes the vast majority of motorists. About 70% of motorists travel in a 10-mph grouping (called the "pace"), which generally covers all but the fastest 15% and slowest 15%. If the speed limit were set at the average speed, only 50% of motorists would be legalized. Setting the speed limit about 5-mph higher (at the 85th percentile) legalizes the vast majority of motorists. Raising the speed limit another 5-mph wouldn't legalize that many more drivers, because the fastest 15% slowest 15% tend to more widely dispersed in traffic speeds.
2. Long standing research suggests that motorists far outside the normal traffic flow have higher accident rates. A speed limit at about the 85th percentile ("about" because speed limits are posted in 5-mph increments) legalizes consensus of most motorists, and after a reasonable enforcement tolerance, focuses law enforcement on motorists far outside the normal flow.

Q: Why the 85th percentile? What about other factors? For example, the number of driveways, or traffic volume, or pedestrain counts, or the distance of sidewalks from the road, or design speed, or roadside residents' opinions?

ANSWER: Again, two reasons.

1. Most motorists evaluate the existing road conditions, and adjust their travel speeds accordingly. If a speed limit is set at the 85th percentile of free-flowing traffic under favorable conditions, all relevant factors are accounted for in the motorists' speeds. Past research shows motorists adjust their speeds based on lane width, access points, etc. ("Free-flowing" traffic means traffic is light enough that only normal key factors are reflected in motorists' choices, not bad weather conditions, or congestion, or rush hour.) Just as we presume most citizens are reasonable and prudent when voting, most citizen-motorists should be presumed reasonable and prudent when driving.
2. There is no "objective" mathematical formula for adjusting the speed limit based on other factors. So it becomes a simple policy choice: do we base speed limits on
   • consensus opinion of citizen-motorists, or
   • opinion of a minority?
   Allow the minority to set policy for the majority, and you simply get corruption and abuse under the banner of safety.

Q: Wouldn't a speed limit at the 85th percentile impact safety?

ANSWER: Why would legalizing the actions of prudent people cause more accidents? Most speed limits are arbitrary, recognized as such by the actions of motorists, so recent research shows changing the speed limit doesn't significantly affect speed or accidents {link to Federal Highway Administration}. Interestingly, accident rates decreased insignificantly in this study when the speed limit was raised.

Q: Don't travel speed increase with a higher speed limit?

ANSWER: Not significantly. The key word is significantly. I personally observed slightly higher travel speeds, for example, after the speed limit was raised from 35-mph to 50-mph on Ohio Route 835 in Beavercreek average speeds increase about 2-mph, but this was primarily the slowest, most law-abiding drivers catching up with the majority. The largest increase I've observed was about 4-mph (between raising the speed limit on I-675 from 45-mph to 65-mph), but my samples were taken about 3 years apart (data provided in my recent testimony on truck speed limits).

Recent Federal Highway Administration research (using undetectable sensors rather than radar; and samples of thousands of speeds, rather than my 100 or 200) shows changing the speed limit doesn't significantly affect speed or accidents {link to Federal Highway Administration}. And one must remember that speeds have always increased historically. The speed creep was observed under the 55-mph limit, and will probably continue indefinitely, except for events like World War II and the 1973-1974 oil-price-crisis.

Q: Won't higher speed limits cause more speed-related accidents?

ANSWER: "Speed-related" is police shorthand for "excessive speed for conditions." Conditions have little to do with posted speed limits, rather conditions like curves and bad weather, although the Government would like you to believe otherwise.

Back to the Ohio NMA rootpage.

June 04, 1997