The Directory of Transportation Data Sources, published by the Bureau of Transportation Statistics (the Mecca of free transportation data), is the best collection of database descriptions anywhere. (And it's also free.) It includes a comprehensive listing of data collected by dozens of federal agencies and who to call for information. It also gives you that crucial what-is-this-darn-database-called edge when filing your FOIAs. Call BTS at (202) 366-DATA or order from the Web site at http://www.bts.gov.
Keep in mind that nearly everything collected federally comes from a local agency first - so updated and complete information might be just around the corner instead of in D.C. Cultivate your state police department for freeway accident data, your city police department for pedestrian hit-and-runs, your county transportation department for road condition and traffic information, your state Department of Natural Resources for boating or snowmobile accidents.
Heather Newman can be reached by e-mail at newman@freepress.com