April 5, 2022

The Antiquated 85th Percentile & Why It Is Killing American Drivers

The quickest way to find out a transportation engineer or planner has not kept current with work and research is to hear them talk about the “85th percentile” as a factor in setting up road speed for drivers and assessing the safety of roads.

 

If you hear that antique phrase popular in the middle of the 20th century, ask if they know about Vision Zero or the Safe Systems Approach. Pioneered in Europe this refers to a road system designed so that no serious injuries or fatalities occur because of driver traffic or road use. Sweden has been a leader,  embraced the Vision Zero or Safe Systems Approach for twenty-five years.

The basic tenet of Vision Zero is that no life or person’s health can be expended for the benefits of road use, and is opposite of the 20th century model that places a cost benefit analysis on roads, a monetary value on the risk to fatalities and serious injury.

Back to the 85th percentile. In that outdated model a decision is made of how much expenditure will be made on roads to decreasing mortality and morbidity risk.

The “85th percentile” refers to the speed at or below which 85 percent of motorists will drive on a road based upon good weather and unimpeded by other driver traffic. It is simply a measure of what drivers “feel” is a safe road speed, with no science behind that judgement.

Sadly that is how the driver speed limits on highways, roads and streets has been established in the 20h century, where vehicle drivers were perceived as the major users of roads and their comfort and convenience assumed paramount.

As Tarek Sayed, a transportation professor at University of British Columbia points out in this Globe and Mail article, by Jason Tchir “relying on the 85th percentile to set speed limits is “outdated, because the majority of drivers often drive faster than the road’s design speed”. A study undertaken in the province in 2013 suggested that speeds could be 10 kilometers an hour faster on some highways. Under the Liberal provincial government road speeds were increased on over 1,000 kilometers of highways . Within two years deaths and serious injuries increased by 11 percent.

The use of the 85th percentile in the United States on highways has resulted in increasing mortalities and serious injuries. In 2021 in the United States  12 percent more road deaths occurred in the first ten months compared to the previous year.  That was 31,720 people killed on American roads in those ten months, equivalent to the population of Orillia Ontario.

One hundred thousand people have died on American roads in one decade, with one quarter of those road deaths attributed to driver road speed. The US Federal government is now looking at moving towards a Vision Zero or Safe Systems Approach.

The YouTube video below describes how the 85th percentile rule resulted in larger streets and highways devoid of trees, which in turn has made traffic faster, meaning that the road surfaces were then reassessed for higher speeds. The video describes the  five  billion dollar (US) National Road Safety Strategy being undertaken to move towards zero road deaths, as well as how the 85th percentile philosophy dominated road design and speed in a very last century approach to road use.

 

 

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  1. The 85th percentile is a lazy metric that reduces engineers’ burden of having to make decisions and do their jobs. It stinks but it’s at least understandable.

    What really gets me is the intentional over-engineering of roads to accommodate willful recklessness. For 100 years, traditional road design thinking has gone, ‘we know some people are going to speed no matter what, so whatever the road is designed for, let’s add design contingency to make it “safer” for them to do so’. Hence our roads meant for local traffic on which it’s perfectly suitable to go 60 km/h – 70 km/h. We can’t even begin to tally the century’s dead from this innocuous little decision.

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