Boston Metropolitan Planning Organization (MPO)
The regional group responsible for carrying out the Federally mandated regional transportation planning process. The Boston MPO has links to traffic count data and contacts for some other transportation related data for the Boston metropolitan region, as well as research links. Note: for any metropolitan region, the region’s MPO is a good place to start looking for transportation data at the regiona/urban level.
Bureau of Transportation Statistics – National Transportation Atlas Database
Download a variety of transportation infrastructure data in GIS format.
Federal Highway Administration (US) – GIS Data
A good list of transportation GIS data sources.
Transportation Research Bureau
Census Data for Transportation Planning.