September 27


Traffic Corner Tuesday, brought to you by Spack Enterprise and its family of transportation-related companies, completed its latest webinar just over a week ago. Traffic Corner Tuesday is our monthly webinar presenting research, case studies, and interesting discussions on our favorite topic – transportation. For those who missed it, here’s a summary of that webinar along with a link to the webinar if you want to watch it (it’s short, about a half hour). You can also sign up for our next Traffic Corner Tuesday here.

The statutory speed limit on residential roads in Minnesota is 30 mph. Our state is currently debating the merits of a 25 mph statutory speed limit for these roads. A key concern is the survivability for pedestrians and bicyclists involved in car crashes. At 25 mph, the chance of being killed if struck by a vehicle is about 12 percent. At 30 mph, that chance increases to 20 percent. That along is justification for a change for some people.

The MUTCD (Manual of Uniform Traffic Control Devices) provides minimal guidance, simply establishing two methods for setting speed limits. The first method is statutorily. A state may pass a law setting the limit for a particular class or type of road. The second method is as an altered speed zone. Essentially, an engineering study, typically using the 85th percentile speed, establishes a recommended limit which is then posted. Residential roads are generally set by statute.

We decided to complete some basic research by working with Tony Kutzke, City of Woodbury Minnesota and Reid Wronski, City of River Falls Wisconsin. As stated, Minnesota is a 30-mph residential speed limit state. Wisconsin, our next door neighbor, is a 25-mph residential speed limit state. Our basic conclusions:

  • Minnesota sites recorded lower speeds, suggesting posted or residential speed limits do not impact how fast people drive
  • Minnesota and Wisconsin speeds increase with roadway width
  • Minnesota had a greater rate of increase, better correlation between higher speeds and wider roads
  • Closer results with typical residential roadway widths of 32-feet or less, both states had similar 85th percentile speeds

There are multiple caveats with this study, but the above bullet points provide some interesting information for the speed limit debate. You can watch and listen to our discussion, including lots of questions after the main presentation, here.

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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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