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About the Transportation Accessibility Model (TRAM)
The technical evaluation of capital improvement project proposals sometimes considers criteria related to the number of beneficiaries that reside within a certain proximate distance to the project. These zones of benefit can be measured in GIS with simple buffers around the project. A more accurate measure, especially for transportation projects, would take into consideration actual driving time via the road system.
To implement this measure, the County uses a GIS-based modeling system that can depict areas that are within various driving distances of selected points of interest. This model is called TRAM, and is a customized application that runs as an extension to the ArcGIS desktop GIS system.
The TRAM model operates off of a database that depicts the roadway system. Roadway features appearing in the roadway database are coded with information about prevailing driving speeds. In Bernalillo County, driving times on roadway segments throughout the network are available from the metropolitan planning organization (MPO) based on posted speeds or based on congested speeds for the AM and PM periods. The MPO, in Albuquerque the Mid-Region Council of Governments, periodically surveys driving times on regional highways and maintains such a database for the region. TRAM users can also estimate speeds on highways through other means. For example, estimates of speeds in future years can be obtained from the region’s travel demand forecasting model, if so needed.
The TRAM model generates travel time contours around locations of interest for whatever time intervals are desired – for example, 5 minutes, 10 minutes, etc. These contours indicate the areas of the County that are reachable from these points of interest, via the roadway network, for the time periods of interest. The model generates these contours as true GIS polygon features, which themselves can then be superimposed over sources of demographic information (e.g., census population or employment) in order to provide a statistical summary of the number of people, households, jobs, and other relevant socio-economic indicators are captured within the influence area around a project.