May 8


What Difference Does a Sign Make?

By Mike Spack

May 8, 2018

speed limit

By Bryant Ficek, PE, PTOE

How much impact does the speed limit have on a local, residential road? That basic question has increasingly become a hot-button issue in Minnesota where these roads are usually 30 mph by state statute. Several cities and bicycle/pedestrian advocates are pushing to change the statute and lower the local speed limit to 25 mph. At 25 mph, the roads should be safer for bicyclists and pedestrians. If anyone is unfortunately struck by a car, the chances of survival greatly increase if the car is traveling at a slower speed. This increase in safety is a good reason to advocate for slower speeds. However, getting back to our initial question, we wondered whether simply changing the rule in the book or a posted sign will really impact speeds.

As we often do, we created a research project to attempt to answer this question. Luckily, our neighbor to the east, Wisconsin, has 25 mph as their base for local residential roads. With the similarities between our two states, it seemed natural that we could compare vehicle speeds on local roads with the two different speed limits.

To help us in the effort, we worked with the Cities of Woodbury Minnesota and River Falls Wisconsin to obtain speed data. Thanks to Tony Kutzke (Woodbury) and Reid Wronski (River Falls) for their help in providing the speed data and working with us on this research.

The goal for this quick review was simple – compare the speeds on the local roads for each city by roadway width. Our initial thinking and past experience suggested the width of the local road mattered more than the actual speed limit. In other words, drivers will travel to the condition and characteristics of the road more than what a posted sign tells them.

We are reviewing the speed data provided and found the following draft results:

As shown, the average 85th percentile speeds are relatively close between the two cities. In general, the speeds for both corridors increase as the road width increases. Although, the 25-mph roads have a slower rate of increase. Also, most local roads are typically narrower, roughly 32-feet wide or less, where the results between the two sets of data are similar.

So to rephrase our question – does the speed limit of the road matter? At this point, our answer is a decisive “maybe” due primarily to an admittedly small sample of data from only two cities. The small size could also mean the data is susceptible to factors like enforcement, surrounding geometry (horizontal or vertical curves), landscaping, and parking to name a few potential concerns.

We are continuing to review the data and will be providing a white paper on the results soon.


Bryant Ficke Bio

  • Hi Bryant,

    I would also index against the ADT or peak hour volumes of the local roads. The more congestion and/or conflict perceived on a local road, the slower people drive to negotiate that conflict. If you are comparing roadways with significantly higher/lower ADTs the results may be different.

    Other than that, this looks like good work! Let us know if we could help.


  • I think this is a great article. I would like to assist in your efforts if possible by placing counters up in comparable areas within our city where we have standard 30MPH, and petition areas of 25MPH to see if it has made a difference. If you aren’t needing the results in the next 2 weeks, I can try to get our guys out to place the counters for sake of checking results. We will consider road widths as well.

  • Hi Kyle – thanks for the comments. We will be in touch soon about the next steps on this research work. Thanks for reading.

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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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