Thanks to Tim Byrne, Office of Scientific and Technical Information, who sent this information:
The United States Department of Transportation (DOT) has joined Science.gov. This means that when you launch a search at the Science.gov gateway, your single query can find science information from 14 government agencies, including the DOT National Transportation Library (NTL) Integrated Search and more than 30 DOT websites. Major subject areas of highway and transportation engineering, statistics, planning,policies, and research are included in the Science.gov basic search.
The DOT Web site includes links to a number of specialized libraries, such as coast guard, pipeline safety, and transportation law collections.
Try a search on pipeline safety in Science.gov, then refine the results with the additional keyword alaska, for an example of finding DOT results within a broader search.