Data

The calculator relies on data collected by the New Jersey Department of Transportation (NJDOT) and publicly available at: https://www.nj.gov/transportation/refdata/roadway/vmt.shtm. We use the files listed under “VMT by Functional Classification Distributed by County” 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 in urban areas and an additional seven for rural areas. 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 (for example, many of the more urbanized counties had no roads classified as rural or had minimal miles of roads under that category). A shortcoming of New Jersey’s data is that we do not have lane mileage data, only road mileage (center-line miles). Based on a pdf file provided by NJDOT, we applied conversion factors to the road mile data as 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. For urban roads we combined interstates with freeways/expressways as these both largely have identical road designs, including being largely controlled access highways. For urban roads we combined interstates with two additional road types encompassing freeways and principal arterials. For both urban and rural collector roads we combined major and minor collectors. In total we have five urban road categories and four rural road categories in the calculator. This was done for all 21 counties in New Jersey.

Our first year of data is 2017 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 on-road transportation CO2 emissions per mile for the entire state which is 483.2 grams/mile. Data on total state on-road transportation emissions was taken from the 2024 New Jersey Priority Climate Action Plan and is an average of light-duty and heavy-duty vehicle emissions.

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 NJDOT 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.

Elasticity estimates used in the calculator
Urban areas Factor to convert to lane miles Elasticity range
Interstates Interstate/Freeways/Expressways 3 0.7 - 1.0
Freeways/Expressways
Principal arterials Principal arterials 2.5 0.7 - 1.0
Minor arterials Minor arterials 1 0.5 - 0.7
Major collectors Major and Minor collectors 1 0.5 - 0.7
Minor collectors
Local Local 1 0.3 - 0.5
Rural areas
Interstates Interstate/Principal arterial/freeways 3 0.7 - 1.0
Principal arterials/freeways
Principal arterial - other
Minor arterials Minor arterials 1 0.5 - 0.7
Major collectors Major and Minor collectors 1 0.5 - 0.7
Minor collectors
Local Local 1 0.3 - 0.5

Calculations

The elasticities shown above are used to determine how a change in lane miles will lead to a change in annual VMT. The formula for estimating the induced VMT is:

Induced VMT = ε ∙ NewLM LM ∙ VMT

Where:

The back-calculation simply reverses this equation but requires the user to input both “new lane miles” and the “forecasted annual increase in VMT”. The output is an implicit elasticity, that is, the elasticity that is implied by the forecast increase in VMT.

The formula is as follows:

ε ¯ = VMT forecast VMT ∙ LM NewLM

Where: