Q. Glossaries of Transportation Terms
The Bureau of Transportation Statistics (BTS) Dictionary, which is the combined online version of Transportation Expressions and the Transportation Acronym Guide, lists official definitions of transportation-related terms: http://www.bts.gov/publications/transportation_expressions/
You may search by term or definition or browse alphabetically.
Many of these terms come from the Code of Federal Regulations, which you can access at http://www.gpo.gov/fdsys/browse/collectionCfr.action?collectionCode=CFR
For a list of selected transportation glossaries on the web, please see the attached file.
Note that many of these are non-DOT and non-government sites.
You may also want to explore the Transportation Research Thesaurus, a hierarchical thesaurus of transportation terminology, from the Transportation Research Board and the National Transportation Library: http://ntlsearch.bts.gov/tris/trt.do