Definitions

Vehicle-miles of travel or VMT: This is a measure of the number of miles traveled by every motor-vehicle on a network of roads. For example, if one vehicle travels 100 miles on the network, this represents 100 vehicle-miles of travel; if 10 vehicles travel 10 miles each, this would also be 100 vehicle-miles of travel. In the calculator, we use VMT for nine different types of roads and sum this up across all the mileage driven in a county for an entire year. Thus, the VMT represents the total vehicle-mileage for an entire year.

Lane-miles: This is a measure of the capacity of a road. It is calculated based on the total length of a road and how many lanes it has. In the calculator, we use data provided by NJDOT for road miles (i.e., the total length of roads) and multiply this by the number of lanes for each road category (based on estimates provided by NJDOT). The more lanes a road has, the more capacity it has, or ability to carry more vehicles per hour.

Road categories or functional classifications: Roads are defined by how they are used and how they have been designed. These are known as “functional classifications”, defining the function of each road category. For example, freeways and turnpikes serve high-speed traffic on roads with controlled-access (i.e., off and on-ramps) and no traffic signals or cross-streets. Arterials also serve relatively high speed traffic, normally have more than one lane in each direction, but have traffic signals, cross-streets, and often have parallel sidewalks and may have bicycle lanes. Collector roads are smaller in scale, with lower speed limits, and may have one or two lanes of traffic in each direction. These tend to serve or “collect” traffic that is headed towards arterials or freeways. Local roads are low speed roads, most with one-lane in each direction that serve local destinations (e.g. residential areas). There is overlap between these various road categories, but we use the data provided by NJDOT that is based on each road category.

Elasticities: In economics, an elasticity of demand represents how the demand for a product varies with changes in the price of the product. As the price increases, one purchases less of it. We use elasticities to calculate how the price of travel, which is primarily the travel time, changes the amount of travel (VMT). The elasticities we use are based on estimates that have been derived from a variety of studies. When demand is “inelastic” this means it does not respond as much to a price change as when it is “elastic”. Induced travel assumes an elastic response to travel time changes, while if there was no response, the demand would be “inelastic”.

Implicit Elasticity: The calculator includes a “back-calculator” option. This can be used if there is a forecast of how much VMT will be increased due to a given road project. The elasticities used in the calculator are based on empirical estimates, but these may not always apply for every specific project. The back-calculator allows one to calculate an “implicit elasticity”, that is, the elasticity implied by the forecast increase in VMT for a specific road expansion project. This allows one to determine whether the forecast is reasonable given the local context of the project and the elasticity one might expect to be associated with the road expansion.