MnDOT SPOT SPEED STUDY

Traffic Data Inc., along with Stonebrooke Engineering, is assisting MnDOT with the “Evaluation of Certain Trunk Highway Speed Limits” project, also known as the “Spot Speed Study.” This study will determine if the speed limit should be raised from 55 mph to 60 mph on any two lane highways in Minnesota.

For this past year, and the next few years, we will be out documenting the current conditions on Minnesota’s 7,000 miles of two-lane, two-way highways. Examining the existing characteristics will determine if these roads are good candidates to raise the speed limit to 60 mph.

How does one cover 7,000 miles? To start with, Traffic Data Inc. sub-divided the various corridors into hundreds of small sections, ranging from under one mile to about 25 miles depending upon where the speed limits change for cities and towns, where major intersections are located, and where a major curve may be located. Each of these sections then goes through a review of data in the field and at the Traffic Data Inc.

Office work includes using MnDOT’s video log system, which is a compilation of photographs every few feet on every MnDOT road. We make notes on each road section such as:

• lane and shoulder materials
• the presence of rumble strips
• locations of steep vertical grades
• number of accesses
• regulatory or advisory speed reductions
• intersection controls
• major developments (traffic generators) adjacent to the road
• a roadside assessment of the potential hazards in the clear zones

Once the office prep has been completed, a TDI field technician travels to each section to be evaluated.  They take photographs of anything of interest, note significant corridor grades, and record any changes from what was reviewed from the video log. The field technician will also complete measurements at one spot along each section, known as the “typical section.” Measurements that are taken include roadside inslopes, lane widths, and shoulder widths. This Typical Section is assumed to be representative of the entire section.

Also at this typical section, the technician will gather speed data of vehicles on the road. Depending on the average daily traffic, they will either set up a tube counter to collect a day of speed data or collect a radar sampling of vehicles.

All of this data is then compiled into a report for each section that, based on certain requirements, states whether or not that road section is a good candidate for a speed limit increase. Based on the locations TDI has studied so far, it is clear there is a large mix of roads that should have a speed limit of 60 mph and roads that should not. With use of the data from the study, MnDOT can make sound decisions for the safe improvement of travel times throughout Minnesota.