Much of the literature that you will find on transportation is grey literature. The term grey literature refers to literature that is not commercially published. Rather, it is published by non-profits, research agencies, governments, etc. TRID, the main database for transportation, includes many citations to grey literature, but it can be helpful to supplement your searches by looking at institutional and agency web sites, and by searching databases containing citations to working papers.
Watch this two-minute video for a brief introduction to grey literature from Western University.