August 12, 2020

Goodbye 85th Percentile~Hello Slower Road Speeds


Here’s the ugly secret in North America and we all know it is true~the number one priority in transportation policy is to let vehicles go fast.  As Beth Osborne, Director of Transportation for America observes “It has filtered into every level of implementation, down to the way we set speed limits. We raise the speed limit to suit the speeders, as long as there are enough of them and it doesn’t take that many”.

This is where the now antiquated 85th percentile system came from, which is defined as  “the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions past a monitored point.”  Think of that~instead of setting speed limits to what is safe,  decision makers based decisions on how fast drivers travelled  dependent on the visual “feel” of the road.  That’s exactly what we got in the 20th century, roads made for vehicle drivers with an increasing curve of road deaths despite enhanced vehicular safety systems.

Research conducted by the Insurance Institute for Highway  clearly shows that alarming increases in pedestrian fatalities needed to be arrested.  And their research suggests a direct, very simple, cost effective approach: “IIHS research demonstrates that lowering city speed limits curbs the most dangerous speeding and can make the roads safer for everyone who drives, walks, or bikes.”

When the National Transportation Safety Board (NTSB) recommended a “complete overhaul” of how speed is managed in municipalities, NACTO (the National Association of City Transportation Officials) responded. Municipalities base much of their road standards on the NACTO manual. NACTO  threw  out the 85th percentile system and is now embracing  the “safe systems approach” which like Vision Zero is based upon  no road deaths by any type of road user.

NACTO  recognized that their policies  often hindered setting road speeds that promoted  universally safe mobility, and they have issued a new framework to set safer speed limits on municipal streets.

Their document City Limits has a three pronged approach to safe streets. Firstly default speed limits for every street  with the suggestion of 25 mph (35 km/h) for major roads, and 20 mph (30 km/h) for minor streets; Secondly designating “slow zones” in areas that require slower road speeds; and thirdly setting corridor speed limits on higher volume streets using a “safe speed study” which looks at “conflict density and activity level” to set contextually acceptable speeds.

You can download NACTO’s handbook “City Limits” here.

This is a more sophisticated approach that goes beyond the past 85th percentile speed setting standard to take in account other road users, the potential for other street activities, and embraces the potential for conflict density and activity on the street.

And there are direct ways to measure the effectiveness too, by looking at mortality and serious injury rates on roads and highways.

Currently 35,000 people a year die on roads in the United States. In Canada in 2018, the number of  road deaths  was 1,922; up 3.6% from 2017 (1,856). That is the same number as the whole population of Tofino, British Columbia.

We must do better.


Images: Nacto






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