Posted by Jonathan Maus (Publisher/Editor) on April 15th, 2020 at 2:29 pm
On May 1st, traffic engineers in the state of Oregon will no longer rely on an outdated and dangerous method for setting speed limits. Thanks to new rules adopted by the Oregon Transportation Commission last month, the process for for designating speeds has changed dramatically and now goes way beyond the traditional 85 percentile method.
The 85th percentile rule has dominated U.S. traffic engineering since it was championed in the 1960s. It says limits should set at the speed which 85% of drivers are currently driving at or under. What could possibly go wrong? Since it’s a universal phenomenon that people drive faster than what’s safe, this methodology is very biased toward higher speeds and it’s a big factor in America’s rising traffic death toll.
Thankfully it has fallen out of favor and Oregon has taken a big step toward modernizing its approach. The new rule (which applies to all public roads except interstate freeways), adds many new factors into the decision-making process. When setting speeds, engineers will now have statutory guidance to consider things like: dense urban contexts, presence of foot and bicycle traffic, demographics of road users, crash rate, public input, and other new factors.
The adopted rule also introduces a fiftieth percentile rule along with new functional street classifications such as “urban mix”, “urban core”, and “suburban fringe” that allow engineers to further tune their analysis. It has also codified a list of recommend speeds for those new classifications (see graphic below).
In short, the new rule gives city traffic engineers powerful new options when making a request to ODOT for a different speed limit.
This change comes as the result of growing national awareness of the failings of the 85 percentile rule and amid an alarming rise in traffic fatalities where speed was a major factor.
The City of Portland was instrumental in lobbying for this change. Portland Bureau of Transportation hasn’t used the 85th percentile rule for many years and has spent years urging ODOT to change its speed setting methodology.
Reached for comment about the new rule, ODOT Deputy Director Travis Brouwer said they are in the process of filing them with the Secretary of State with a requested effective date of May 1st. “That slight lag in implementation from OTC approval will give us a chance to get guidance out to investigators and traffic engineers about the new process,” Brouwer said.
It can’t happen soon enough.
— Jonathan Maus: (503) 706-8804, @jonathan_maus on Twitter and firstname.lastname@example.org
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