Lowering Speed Limits on Residential Streets
Hi all,
I'm interning with the City of Bellevue WA's transportation department over the next year. I'm helping my team build a case for how/why to lower speed limits on residential non-arterial streets from 25mph to 20mph. The city has already enacted a pilot project in one neighborhood in the city. The question is whether to move forward with a citywide change of baseline speed limits or continue to go neighborhood by neighborhood. One the one hand strong town thinking would usually advocate for the street by street approach. On the other hand lowering speeds is broadly (if vaguely) popular in the city neighborhoods and I can see how having some streets at 20 and others at 25 while undergoing an incremental change could be confusing to communicate and educate and not very cost effective (thinking of all the additional signage needed to communicate transitions from 25 to 20 zones, and then changing them again later).
Our team has spoken with a few bigger cities who have done this already (Seattle, Portland, Cambridge MA) but would be interested in folks general opinions, especially for more post-war suburban contexts (Bellevue is itself an odd layout, a historical suburban bedroom community adjacent to Seattle but with an increasingly dense downtown of giant towers catering to tens of thousands of incoming tech jobs but a classic 1950's suburban single family build across the street in many cases).
Open to any advice, previous experiences, public outreach knowledge, how to convince engineers to be ok moving away from 85th percentile etc.
*Our team is aware physical interventions on the street are superior to regulatory change. We acknowledge a speed limit change is more of a communication/cultural change that hopefully works in tandem with other interventions (albeit slowly) being pursued by the city.
Thank you!
Comments
5 comments
I feel like there are a couple of questions here. Let me see if I can simplify them:
For those who don't know what the 85th percentile speed is, here is an article we wrote about it. To quote from that article:
The answer to Question 1 is really dependent on how far the gap is between the design speed and the enforced speed limit. The greater that gap, the more dangerous it becomes because some percentage of people will always drive the enforced speed and some percentage will always drive the design speed (watch the video in that article) -- the greater the gap between those, the more dangerous it will be.
Going from 25 to 20 mph is not likely to create widespread problems, though it could create localized problems where a street is actually designed for 45+ mph speeds. I will also argue that going from 25 to 20 mph is not going to make things any safer (because most people drive using the cognitive System 2 and thus drive the speed that is comfortable), although I'd love to see some before and after 85th percentile speed studies to provide some data.
For Question 2, I don't think it is so much about getting the engineer to give up on the 85th percentile speed as it is getting them to understand it differently and share in the goal of lowering speed. Here's a flow chart we created to show the difference between the standard approach (which assumes a speed that can't change through design) and a Strong Towns approach (which recognizes that design impacts speed).
The 85th percentile speed is the correct way to measure this -- so we don't want engineers to give it up -- but we need engineers to design for a travel speed. If their design results in higher speeds than what you want, that's not an enforcement or education problem, it's a design problem.
So, summary: The idea of a blanket lowering of speeds isn't something I'd spend a lot of time and energy on -- it's not likely to help or harm overall, but if it builds momentum than okay -- but I'd instead be obsessed with places where the gap between the desired travel speed and the actual travel speed are greatest. That is where I'd make design changes using the 85th percentile speed to measure how successful you have been.
Robbie,
This is a great question. I've asked a couple colleagues to weigh in and -- I hope you're okay with this -- I'm going to post a link to your question in the Strong Towns Facebook Group.
John Pattison
Content Manager
Strong Towns
That sounds great. Thanks John!
Not sure if you saw, Robbie Cunningham Adams, but Chuck talked about your question on today's episode of the Strong Towns podcast: https://www.strongtowns.org/journal/2021/8/30/chuck-marohn-answers-your-questions
Was not expecting to hear my name when listening to the podcast this morning! Thank you to Chuck and everyone again for taking the time to address my question. We really appreciated the insight and input. I shared the podcast with my team at Bellevue Neighborhood Traffic Safety Services this morning (hoping to get them set up with the confessions book ASAP as well), and our project and town will be stronger for it.
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