14 votes

US traffic control device standards get long-awaited update

Posted by scroll_lock
Tags: usa, roads, safety.road, manual on uniform traffic control devices, urban planning, author.kea wilson, source.streetsblog
https://usa.streetsblog.org/2023/12/19/feds-advocates-talk-about-whats-in-the-new-mutcd-and-what-isnt

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Title
Feds, Advocates Talk About What's In The New MUTCD (And What Isn't)! - Streetsblog USA
Published
Dec 19 2023
Word count
1252 words

3 comments

  1. [3]
    scroll_lock
    Link
    The Manual on Uniform Traffic Control Devices (MUTCD), a guidebook for roadway engineers, has been updated for the first time since 2009 to address the many safety issues that result from its...

    The Manual on Uniform Traffic Control Devices (MUTCD), a guidebook for roadway engineers, has been updated for the first time since 2009 to address the many safety issues that result from its recommendations. (Yes, their website really looks like that.) It is not law, but is nonetheless the Bible that civil engineers use to design a baseline of a street.

    You can read about the changes on the Federal Register. Here is a PDF version of the MUTCD 11th Edition and a list of previous editions. It's over a thousand pages long though, so I understand if the answer is no.

    There are plenty of useful updates in there, but it's not a transformative change by any means. I think the most important thing here is that the Bipartisan Infrastructure Law (signed by Joe Biden in 2021) now requires the manual to be updated every four years, rather than... never.

    On the positive side, the new Manual clarifies that roads should be designed with the understanding that humans make mistakes, rather than "reasonable and prudent" drivers motoring amidst "alert and attentive" pedestrians who universally understand local laws. The text also has doubled the guidance on allowable bike signs, signals and markings and eased the process to install certain types of walking and transit infrastructure, like red pavement markings on bus lanes and pedestrian-activated signal beacons at crosswalks.

    Advocates expressed mixed emotions, though, about the Manual's updates to the 85th percentile method of setting speed limits [...] even when the speeds the formula spits out are far above the 20-mile-per-hour threshold at which crashes become far more lethal to people who walk. [...] The new Manual, by contrast, emphasizes more strongly that the 85th percentile "rule," as its colloquially known among engineers, has never been a federal law at all, and that it's just one of several factors roadway designers should consider when setting limits [...] Some advocates say, though, that the Administration is naive to believe that engineers will consider factors other than 85th percentile speeds — especially without clear guidelines telling them exactly how to set safer limits instead.

    "Unfortunately, I find that [the FHWA] fails to recognize its own authority and power," said Beth Osborne, director of Transportation for America. "They may not mean for something they publish to have such wide-reaching effects, but it does. ... Transportation officials treat the MUTCD as cover for liability; [they think] as long as you adhere to it, you’re covered. And courts treat it that way, too."

    Under the Bipartisan Infrastructure Law, the Manual is required to undergo an update every four years, which Osborne says could give the agency the opportunity to integrate guidelines from other countries with lower roadway death rates, as well as giving newer, Vision Zero-minded voices a seat at the drafting table.

    Title from Smart Cities Dive, whose article is more neutral but very short and not as useful as the one from Streetsblog.

    8 votes
    1. [2]
      updawg
      Link Parent
      So the speed limit is supposed to be set at a speed that only 15% of cars exceed? Pretty much my whole life (not counting years overseas) has been on roads where I would guess 85% of cars exceed...

      So the speed limit is supposed to be set at a speed that only 15% of cars exceed? Pretty much my whole life (not counting years overseas) has been on roads where I would guess 85% of cars exceed the speed limit, if anything. Is the 85th percentile rule then applied to, perhaps, the speed limit plus 5 mph in those instances?

      3 votes
      1. scroll_lock
        (edited )
        Link Parent
        The 85th percentile rule is a guideline used to set speed limits on new and existing roads, but is not a law. It is based on a prescriptive and generalized assumption of safety. Before speed...

        The 85th percentile rule is a guideline used to set speed limits on new and existing roads, but is not a law. It is based on a prescriptive and generalized assumption of safety.

        Before speed limits are officially posted on a new road*, engineers observe what speed drivers "naturally" go at (drivers intuitively determine how fast to go based on the characteristics and "feeling" of the road). Engineers then set the speed limit at the 85th percentile of drivers: in the tests, they determine the particular speed at or below (key word "below") which 85% of drivers are naturally traveling, without guidance from a posted speed limit. That particular speed is then designated as the new speed limit.

        This means that 85% of drivers, in tests without a posted speed limit, would naturally choose to drive at or below the speed that the engineers set as the maximum. Many of them would choose to drive significantly below the 85th percentile. Only 15% would exceed it. In other words, the "average natural speed" is virtually always lower than the speed limit derived from this formula ("average encouraged speed").

        From the Federal Highway Administration:

        The 85th percentile speed is the speed at or below which 85 percent of the drivers travel on a road segment. Motorists traveling above the 85th percentile speed are considered to be exceeding the safe and reasonable speed for road and traffic conditions. The 85th percentile speed should be taken from speed data collected during a 24-hour weekday period. Typically the data are collected with commercially available roadside units which sort and present the results in text as well as graphical format.

        But psychologically, the speed they choose to drive at changes when there is a posted speed limit higher than what they would otherwise do. Sort of like the quantum cat in the box: when you make your observation known to drivers, your data becomes useless at describing real-world conditions.

        This guideline is extremely dangerous anywhere other than a high-speed, fully separated highway because it artificially encourages people to drive faster than is actually safe in all conditions. As we all know, most people like to hover within 5 mph of the speed limit, no matter where they are. That includes arterial roads and local streets. Once the posted speed limit is higher than what a majority of people would naturally do, they feel empowered to drive even faster (up to the posted speed limit). That kills people.

        Strong Towns has a good article on why this guideline is problematic, though they aren't in favor of throwing it out altogether. They just want to use actual safety data instead of using pseudoscientific traffic safety theories to set speed limits on an extremely diverse set of roads and streets. For instance, banning cars from traveling more than 20 mph anywhere that pedestrians are realistically present is a far safer choice than literally guessing what the "safest" speed is using a completely arbitrary benchmark from a single 24-hour test.

        The key insight from Marohn's article is this:

        1. We know what speeds are safe to drive at, and it is not based on this formula. It is based on the blood of 40 thousand people who die every year in car collisions. Above about 20 mph, being hit by a car has a significant risk of death or serious injury. Above about 35 mph, the probability of death is over 50%.
        2. The 85% "rule" (formula) is useful only as an observation and should not be used to set speed limits, at least not on local streets and arterial roads. On highways, I think it is still suspect, but there are no pedestrians in that context so the only victims are drivers encased in protective metal boxes.
        3. If drivers are found using this formula to be exceeding real safe speed limits (see #1), the road should be redesigned to psychologically or physically discourage them from traveling that fast.

        * Engineers also use this on existing roadways with speed limits that they have previously set using this metric. I think this is actually even more unsafe in many cases because it can have the effect of further increasing speed limits. I suspect, but do not know, that this is a contributing factor in the gradually increasing speed limits of roads around the country in the last century.

        8 votes