Data

The calculator relies on data collected by the Pennsylvania Department of Transportation (PennDOT) and publicly available at: https://www.penndot.pa.gov/ProjectAndPrograms/Planning/TrafficInformation/Pages/default.aspx. We use the files listed under “Highway Statistics” for each year in the calculator. These data are in pdf files and were transferred to an Excel file for further processing.

The data provide estimates of road mileage and annual VMT by county for seven functional classifications of roads; there is no distinction made between urban and rural roads in the data. Functional classifications define the type of service provided by a road, for example a freeway, turnpike, 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.

We combined some of the road classifications to minimize those instances in which there were zero road miles. A shortcoming of Pennsylvania's data is that we do not have lane mileage data, only road mileage (center-line miles). To approximate lane-miles we used FHWA Highway Statistic from 2018 to calculate the ratio of lane-miles to road-miles for each functional classification (ratios were basically unchanged from year to year). 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 average state-level one-directional measures of the number of lanes for each functional classification.

Road classifications available in the data are listed in the table and those that were combined are shown. We combined interstates with freeways/expressways into one category. Major and minor collectors were combined into one category as well. There are 67 counties in Pennsylvania and we combined these based on metropolitan statistical areas (MSAs). Some counties are not included as they are not part of an MSA. For the Philadelphia metroplitan area we only include the five Pennsylvania counties (Philadelphia, Montgomery, Chester, Delaware, and Bucks) omitting that part of the MSA in New Jersey. The combinations 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 492.1 grams/mile. Data on total transportation emissions was from the 2023 Pennsylvania Greenhouse Gas Inventory Report for petroleum (Table 2 in report). This was divided by total VMT in 2022.

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.