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Setting Speed Limits: The 85 Percentile

Video Tuesday

We’ve heard this spiel before…now it’s in a video. Thanks.

This process needs to be altered so that the comfort and safety of our driving-selves isn’t the only item considered in the setting of speeds. The 85th percentile has some logic to it, but it’s highway logic and not neighborhood and city logic. It has a built-in bias towards faster drivers. The quick is, the 85th percentile assumes that most drivers are reasonable, don’t want to crash and only concerned with the shortest amount of travel time. The latter of which we have seemed all to willing to reward.

What’s missing from the process is reminders are needed to help us be prudent drivers. Any street that runs through a city needs to be designed and managed in the priority of first and foremost the safety for all users before the setting of traffic speeds. This may will involve different approaches to designing roads than has been customary. Traverse City’s Division St. is a great example. Sure, an officer may argue that perhaps 40-mph is too slow according to the 85th percentile numbers, but the context of the street (family homes, city parkland, and businesses) needs to rise in level of importance in the consideration.

Of course, the National Motorist Association disagrees with all of this.


  1. October 4, 2011 at 10:16 am

    Thanks for posting this. It’s always been a pet peeve of mine. On the surface it seems reasonable and sound. The State Police might even say its market based. The bias however is clearly towards speed limits that go up. In one of our meetings on Division Street there was some grumbling from the public about getting the speed there “re-shot” by the State Police. The wise MDOT staffer raised an eyebrow and said something to the effect of, “Be careful what you as for. It is extremely rare for speed limits to be set downward.”

    Furthering this bias for speed, and my bigger pet peeve on the issue, is Michigan’s insistence that no public roads have posted speed limits below 25 MPH. I can think of a lot of streets in the community that could be served well with lower thresholds. And I’m not just talking about pedestrian/bike safety. To paraphrase F/LT Thad Peterson of the MSP, “If a speed limit is set artificially HIGH it can be unsafe.” Continued evidence of the bias can be found in their own video. The justification for upticks in speed limit are for vehicular safety only. Take a close look at the MSP speed limit video posted above. There isn’t a single pedestrian or bicycle or crosswalk to be found.

    I think we’d all be better off if our perspective on transportation choices changed a bit. When we travel in a vehicle, we don’t simply move on roads… we move through communities.

  2. October 4, 2011 at 12:01 pm

    State policy does allow for 15-mph zones through school zones and areas adjacent to parkland. It allows for it, doesn’t mean that it happens. Getting that the posted speed limit was attempted in the new South Campus entrance project to no avail.

  3. Raymond Minervini
    October 4, 2011 at 6:07 pm

    Yeah, the 85th percentile rule is seriously biased in favor of vehicular traffic and against neighborhoods. I am sure we can all think of prominent examples of speed zones that have no basis in common sense: like city allies where 25MPH is the lowest allowed by state law; or winding, blind driveway N Long Lake Rd west of Cedar Run is 55 MPH — en route to TC West HS. Cause and/or effect– there’s no safe route to walk or bike to that school, and young driver accidents along there are a regular occurrence.

  4. Matt
    October 5, 2011 at 12:21 pm

    Great we can lower the speed on GVP to 15 mph!!

  5. October 5, 2011 at 12:23 pm

    Ha, not without changing the context of it you won’t. It’s built for speed.

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