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.
Download a variety of transportation infrastructure data in GIS format.
The BTS has numerous tabular data sets, mostly by state, regarding all aspects of transportation in the US.
A good list of transportation GIS data sources.
Census Data for Transportation Planning.