Every year, MnDOT publishes a number of safety statistics. A lot of that data is available here. One of the products they publish annually are the “Green Sheets,” which they publish for both various intersection types and roadway section types. The chart below is adapted from the 2013 Section Green Sheet. It shows the statewide crash rates and severity rates associated with various roadway section types.
Crash rate is expressed in units “crashes per million vehicle miles” on a particular roadway segment. Severity rates are unitless. It’s calculated the same way as crash rates, but crashes of greater severity are given a greater weighting in the formula. In other words, high numbers in this chart are bad.
This data has some limitations. The simple reporting measure strips away some important information about traffic volumes, access density, surrounding land uses, parking, roadway width, etc. But I think it’s still pretty illuminating by showing, on average, what type of urban roadways have higher crash rates than other roadway types.
For me, the take home message in this data is just how problematic four-lane undivided roadways are. By a large margin, these types of roadways have higher average crash rates than any other configuration. It is also striking that the 3-lane roadways exhibit relatively low crash rates – substantially lower than any type of four or five lane roadway – lower even than two-lane roadways in some cases. In this data set, 4-lane undivided roadways exhibit crash rates and severity rates of 81% and 76% higher than 3-lane roadways, when junction-related crashes are included.
I have deep concerns about four-lane undivided roadways. They have had their time in the history of roadway design, but their time is up. We should work to eliminate them from our cities either by restriping as a 2-lane or 3-lane, or expanding to a 4-lane divided. Obviously, one of these options is cheaper and easier than the other, but there is a place for both.
My home isn’t far from a 4-lane undivided roadway (46th Street in south Minneapolis near I-35W), and it is easily my least favorite part of the neighborhood. It is directly adjacent to a grades 5-8 school, and it is terrifying to walk along or across with my children. I’m told Hennepin County may be looking at a mill & overlay on 46th Street sometime in the near future, and I’m hoping County staff will be amenable to restriping as a 3-lane. I will gladly endure any congestion that might result (though I think it is unlikely), in return for the improved safety conditions.
What conclusions should we draw from the data provided on the MnDOT Green Sheets? What are the limitations? Share your ideas in the comments below.