In 2014, the Legislature required MnDOT to study all of its existing 55 mph 2-lane rural trunk highways to determine whether it would be safe to increase some or all to 60 mph. Stonebrooke was selected by MnDOT to provide traffic investigations and safety studies on over 5,000 miles of trunk highways, to determine which areas can have speed limits safely increased from 55mph to 60mph. Our work includes spot speed sampling, field investigations, crash and other data analysis, and reporting. Spot speed studies are being performed for each of more than 350 study segments and measure point travel speeds of a representative sample of vehicles in the population. Field investigations are needed to assess the physical attributes of the roadway and surrounding environment, including travel lanes and shoulders, presence or lack of rumble strips, access density, vertical grades, roadside assessment, steepness of inslopes, adjacent development, traffic control along mainline, and advisory and regulatory speed reductions.
Analysis and reporting include analyzing speed and crash data, with summary reports provided to MnDOT. Primary calculations for speed sampling include 85th percentile and 10 mph pace. “Good” speed samples are considered to be those that have the 85th percentile in the upper range of the 10 mph pace, and roughly 2/3 of the sample included in the 10 mph pace. The 85th percentile is the speed at which 85% of the population travels at or below. This value is generally considered a good starting point for setting speed limits. The 10 mph pace is the 10 mph range of speeds that include the highest percentage of vehicles from the sample. Analysis of crash data includes calculating overall crash rates and Fatal + “A” severity crash rates for the study segment and comparing to average and critical crash rates.
Stonebrooke customized user-friendly reports that provide an at-a-glance look at crash rates, speed sample results, and field investigation findings. We started collecting data and creating reports for trunk highways across the state in 2014, and our work on this project will continue through 2019.