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Lowering Speed Limits on Residential Streets

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5 comments

  • Official comment
    Charles Marohn

    I feel like there are a couple of questions here. Let me see if I can simplify them:

    1. Is a blanket lowering of speed limits a good thing to do?
    2. How do we get engineers to vary from the 85th percentile speed?

    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 85th percentile speed is the speed at or below which 85 percent of the drivers will operate with open roads and favorable conditions. The assumption underlying the 85th percentile speed is that most drivers will operate their vehicle at speeds they perceive to be safe. Speed limits set above or below the 85th percentile speed will create unsafe conditions due to speed differential as some driver adhere strictly to the law while others drive the naturally-induced speed.

    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.

  • Strong Towns Staff

    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

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  • Robbie Cunningham Adams

    That sounds great. Thanks John!

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  • Strong Towns Staff

    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

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  • Robbie Cunningham Adams

    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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