Application: Accessibility in Benefit-Cost Analysis

Under typical evaluation of the costs and benefits provided by a transportation network investment, planned changes in speed and reduction in delay of vehicles is monetized, typically by assigning a value to time lost in congestion, or to the travel time reductions forecast to be provided by the project. There are two fundamental objections to this approach. First, the travel time or congestion reductions are often estimated for pieces of the network (roads, intersections) without respect to the origin and destination of the potential users. In other words, a travel time differences along a segment are expanded and applied on average to the region, whether a person would be likely to use that segment regularly or not. Second, even when origins and destinations are included in the analysis, such as the with output of a travel demand forecast model, the value of time multipliers treat seconds and minutes of travel time savings at a constant value of time. These may multiply out to become a large average benefit across many vehicles and many days to across a regional daily travel average (indeed they have to, in order to balance the costs). But the benefit to individuals may be trivial, especially if they are already well connected to their destinations. 

Accessibility, especially its connection to land value, provides a tractable way to measure the full benefit of projects. By forecasting the change in access from a transportation project, especially where that change is going to manifest, the expected land value increase can be modeled. This can be a heuristic multiplier, the same way that travel time savings are monetized (e.g., 1000 newly reachable jobs from an area raise land values by $50/acre). Alternatively, models such as the ones explored here can be used to estimate elasticities (a 10% increase in job access leads to a 3.5% increase in land value for a residential parcel, for instance). These values will differ across contexts and regions, but that is not different than current practice of valuing time through speed increases. 

Finally, using accessibility to analyze potential benefit has one significant advantage over other measurements of benefit. By mapping where changes in access will occur as the outcome of a project, agencies can project who is likely to experience those changes. These analyses are straightforward as accessibility benefits are mapped to residential blocks, which are well-connected to census data and other descriptors of the beneficiaries. For instance, will travel time savings from highway expansion accrue to low-income edge communities, or high-income areas? Similar analysis can be constructed for commercial benefit, given the origins and destinations of interest to freight can be specified. Benefits of transportation projects are not spread evenly across a town, county, or region, and using accessibility metrics makes it easy to demonstrate how the actions of DOTs will manifest to the residents of the agency’s jurisdiction.