Data

The calculator relies on data collected by the Minnesota Department of Transportation and publicly available at: http://www.dot.state.mn.us/roadway/data/index.html.

The data provide estimates of road mileage and annual VMT by county for seven functional classifications of roads. Functional classifications define the type of service provided by a road, for example an interstate, freeway, or expressway allows for higher speeds and will accommodate more vehicles per hour compared to an arterial road, which will have traffic signals and lower speeds. At the far extreme are local roads with low speed limits that serve final destinations, such as residential and local commercial areas.

The data available for road center-line miles is broken down by urban and rural areas, however, there is no corresponding breakdown for vehicle-miles of travel (VMT). Therefore, urban and rural data were combined by road category. Lane mileage data is available, but is not linked to VMT. To estimate lane-mileage the ratio of lane-miles to center-line miles for each road category is calculated for each metro area (using 2023 data). This provided us with a conversion factor to approximate the lane miles for each functional class. These are shown in the table below and represent averages by metro area.

Road classifications available in the data are listed in the table. There are 87 counties in Minnesota and we combined these based on metropolitan statistical areas (MSAs). Some MSAs include counties in Wisconsin and North Dakota; these were not included in the MSA totals. Most counties are not included as they are not part of an MSA. Fargo, Grand Forks, and LaCrosse-Onalaska MSAs are not included as they contain only one county in Minnesota. The combinations included are as follows:

Our first year of data is 2018 and we will update to the latest year available. Caution should be used when analyzing data and interpreting results for 2020 and 2021 due to changes in travel patterns due to the COVID pandemic.

We also include an estimate of CO2 emissions. This is estimated based on the average transportation CO2e emissions per mile for the entire state which is 484 grams/mile. Data on total transportation emissions was from the Minnesota Department of Transportation, Performance Measure Dasbboard for transportation emissions. We assumed an average of 40 million metric tons CO2 equivalent in total. This was divided by total VMT in 2023.

Elasticity assumptions in the calculator are shown in the table for each road classification. In general, the empirical evidence suggests that larger higher speed roads have a larger elasticity. We have included a range and the calculator output provides a range of estimates for induced VMT. Given the potential range of estimates and variation in how VMT may be affected, we have included a “back-calculator” which allows the estimation of an “implied elasticity” based on forecast growth in VMT associated with a capacity expansion. This can be thought of as the elasticity implied by any forecasts of travel done by the state or a local agency promoting the capacity expansion. The assumption often made, is that there is no change in VMT attributable to the expansion, despite the evidence for induced travel. The user of the calculator is thus able to assess whether estimates made in forecasts are reasonable.