November 22


The Traffic Diffuser

By Mike Spack

November 22, 2016

intersections, Pedestrians, Roundabout, traffic diffuser, vehicles

Rick Harrison at Rick Harrison Site Design Studio has an interesting video on what he calls a traffic diffuser. The video is only a couple of minutes long and its worth the time. Rick rightly points out that traffic engineers don’t do a good job of designing for the mixing of pedestrians with vehicles. Historically, intersections are designed to maximize vehicle throughput while controlling the pedestrians. Washington County in Minnesota put pedestrian tunnels around its multi-lane roundabout in the City of Cottage Grove to remove the pedestrian/vehicle conflict, which is great for vehicular circulation but pedestrians don’t like going into tunnels.

Rick’s concepts will work in neighborhoods where the traffic volumes are relatively low. To move vehicles at busy junctions we’ll still need traffic signals or roundabouts to allow access to/from the minor street. Although his incorporation of the Michigan Left helps from a capacity standpoint.

One minor issue – Rick picks on traffic circles, but doesn’t acknowledge the difference between a traffic circle (usually stop sign controlled with no splitter islands) and a modern roundabout (yield sign controlled, with splitter islands so pedestrians cross half the road at a time, and the roundabout is designed to force all vehicles to go 15 to 20 mph). In my book, there’s no good place for a traffic circle.

Rick is an innovative suburban neighborhood designer (see Rick Harrison Site Design Studio) and his new design software looks intriguing (and inexpensive compared to other CAD systems).  I appreciate that as a designer he is thinking holistically about the functionality of the neighborhood.  Too often, traffic circulation and access are an after thought in neighborhood design.

  • I think you’re almost too generous to the video. If I hadn’t seen the website too I could have binned it in the “crackpot” file. First example, if the “traffic circle” is that busy that people had so much trouble navigating it, it wouldn’t be a small local street circle/RBT as shown. The third example makes no sense at all, you could easily add the michigan left features to help the outbound path of the group of houses and still maintain the roundabout for an efficient continuous flow intersection.
    The wide medians of his diffusers are nice for landscaping, but besides your minor street access point the short throat in between the two directions is a setup for queuing issues once it gets any significant traffic at all, and makes signalization problematic as miniature diamond interchanges. At intersections the two directions need to either tighten up to become one intersection, or widen out enough that it is a decent distance in between. Or just put in the dang modern roundabout the way you should have in the first place 🙂
    The thing that really helps peds is the culture of the area and the details of the street designs which keep auto speeds down. The video even falls into the usual autocentric pattern of describing the pedestrian as waiting for gaps and yielding to the autos.
    Anyways, as a consultant here in TX I enjoy your blog, as I think our businesses are pretty similar.

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    Mike Spack

    My mission is to help traffic engineers, transportation planners, and other transportation professionals improve our world.

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