The 85th Percentile Rule Is Killing Us

Photo: Anthony Quintano/Flickr
Photo: Anthony Quintano/Flickr

Traffic deaths in the U.S. are mounting, reaching more than 40,000 last year, and, according to a recent draft report by the National Transportation Safety Board, speed is the overlooked factor.

The NTSB reported that speeding accounts for about 10,000 deaths a year — as many as drunk driving. One of the agency’s key recommendation was to change the way streets are designed by reforming the “85th percentile rule,” a laissez faire approach that seeks to accommodate motorist behavior instead of engineering streets for safety.

It’s an argument that Randy LoBasso at the Bicycle Coalition of Greater Philadelphia has been making for a long time. Now the NTSB report is vindicating advocates’ critique of the 85th percentile rule, he writes:

The 85th Percentile idea, based on the 1964 “Solomon Curve” says speed limits should be set at what 85 percent of drivers think is healthy. It was created back when the highway system was still young, cars didn’t approach speeds as quickly as they do today, and we didn’t have the sort of statistics and research on traffic dangers we do today.

I have long trashed the 85th Percentile speed approach as outdated and never meant for cities. That hasn’t stopped some — who feel motor vehicle users should be able to drive as fast as they want — from lashing out at the Bicycle Coalition’s rational attempts to curb speed and make streets safer for everyone.

Among [the NTSB’s] specific recommendations: “Revise traditional speed-setting standards to balance 85 percentile approaches with safe systems approach that better incorporates crash history, safety of pedestrians, bicyclists.”

“In general, there is not strong evidence that the 85th percentile speed within a given traffic flow equates to the speed with the lowest crash involvement rate,” the NTSB says. “Alternative approaches and expert systems for setting speed limits are available, which incorporate factors such as crash history and the presence of vulnerable road users such as pedestrians.”

More recommended reading today: Pricetags shares some insight from transportation economist Todd Litman about the under-appreciated transportation costs of buying a house in the suburbs. Greater Greater Washington considers the potential drawbacks of diverting cut-through car traffic away from residents streets. And the Raleigh Connoisseur reports that a major increase in bus service goes into effect this week in Wake County, following a November vote to increase the local sales tax half a cent.

  • LinuxGuy

    Speeding causes 1.6% of crashes and was shown to be so per crash data. Running stop signs is clearly unsafe, but you want it both ways.

  • LinuxGuy

    Wrong again, as money gets diverted to other stuff and is merely put back. Roads promote economic development, so that part is also wrong. Run off has nothing to do with anything. Do you pay to breathe air?

    It is not up to you to tell people how to travel.

  • Derek Hofmann

    Actually, the tax revenue per acre of downtown areas is vastly greater than the suberbs, so it’s the poor subsidizing the rich. https://www.strongtowns.org/journal/2017/1/10/poor-neighborhoods-make-the-best-investment

  • LinuxGuy

    The poor are the ones getting all the government benefits, right? Many people make poor choices and refuse to work too.

  • Derek Hofmann

    “The poor are the ones getting all the government benefits, right?”

    Nope! City governments redistribute wealth from poor areas to middle class areas.

    “Many people make poor choices and refuse to work too.”

    Not only do poor choices cause poverty, poverty also causes poor choices: https://www.citylab.com/life/2013/08/how-poverty-taxes-brain/6716/

  • Derek Hofmann
  • Michael

    Those “magic tricks” are for things like leaking underground gas tanks and spilled oil from motor boats. The biggest chunk (still less than 20% of funds and much less than the recent rounds of bail outs) got to pay for bus stops and stations from bus systems that use the roads and highways. Because not every dollar can go to support incessant driving to walmart.

  • LinuxGuy

    IIHS has been called out for inaccurate data:

    http://thenewspaper.com/news/38/3801.asp

  • LinuxGuy

    Nobody makes people smoke, have 5 kids with 5 guys you were not married to, do dope, etc.

  • Derek Hofmann

    I couldn’t find any mention of IIHS on that web page.

  • Derek Hofmann

    I think you need to read the CityLab link again.

  • LinuxGuy

    I am not sending millions of links. IIHS was called out on WTSP-TV in Florida a few times and in various places. Their members sell car insurance, so they love to raise rates.

  • LinuxGuy

    No. Gas tax money has been spent on hiking trails, planting flowers, remodeling homes on arterial streets, etc. Lots of this goes on. You must not like Walmart. Why not? All the jobs were sent out of the US, so people cannot afford to shop elsewhere.

  • Robert Atallo

    The 85% rule is problematic, but its not the only criterion for setting speed limits. Road geometry, sight distance, the presence or absence of medians and median cuts, driveway accesses, protected turn lanes, etc. And in the end, most speed limits are set by a city council, county commission, or whoever. Bottom line: the AASHTO manual needs to be rethought, this time with input from others besides just civil engineers