October 18


I saw Joe Gustafson, PE, PTOE of Washington County give a great presentation at last week's NCITE meeting.  He talked about the County's experience with Minnesota's first two lane roundabout (at least the first roundabout with two lanes on all approaches). 

Joe shared his powerpoint (Download Roundabout 2-Year Study NCITE Oct2011) with me and also pointed me to an online edition of the presentation (this was a rehash of a presentation he originally gave at TRB).  If you work with roundabouts, I highly recommend you watch the 20 minute online edition of the presentation.  Per minute, it's the most informative roundabout training I've had (and I've sat through more than 60 hours of seminars and classes on roundabouts in the last 11 years).

A few highlights –

  • Compared to a nearby signalized intersection that is very comparable in traffic volumes and entering roads, the roundabout adds about 12.5 seconds of control delay to the average through movements in the p.m. peak hour while the signal adds 29.5 seconds of delay to comparable through movements.  The left turns operate even better – 18.1 seconds of control delay at the roundabout versus 43.7 seconds at the signal.  The roundabout is operating a lot better during rush hour than the signal.
  • No injury crashes, no wrong way crashes, sideswipe crashes are rare, crashes where cars run into the center island are rare.  However there have been 140 fender benders in the four years since it opened.  City of Woodbury police is keeping a separate, detailed record because most of the crashes don't even get reported to the Department of Public Safety. 
  • Fender bender rate is high.  There is a systematic failure of people coming into the roundabout to yield to the people in the inside lane who are legally exiting the roundabout (see picture).  Joe has a great explanation of the crashes in the online edition of his presentation.

Roundabout Crash Type

Lessons Learned –

  • Our industry needs to be consistent with our message – entry and exit maneuvers are through movements at a roundabout (not merges or turns). Roundabout crossingsjpg
  • Don't build more lanes than needed.
  • Don't refer to the "circle" as one road.  Think of it as four different intersections, each one made up of two crossing one-way streets (see diagram).
  • Roundabout signs – use standard turn arrow signs, but tweak by adding a dot representing the roundabout (don't try to use "fish-hook" and other confusing signs).
  • Use 8 inch stripes at exit lanes for better visibility.
  • Do a lot more human factors testing on methods for conveying that the entry into a roundabout is a straight movement (not a merge or right turn).

Joe – thanks for the great presentation and for sharing your materials with me!

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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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