At the risk of providing an oversimplified answer, we'd have to say that would be the Compendex database, searchable through VDOT's subscription to Engineering Village.
Part of the reason for this answer is the quantity of citations in Compendex (20 million), part is due to the broad engineering span in Compendex, which goes way beyond Civil Engineering (190 engineering disciplines in all) and the fact that the scope extends beyond the United States (which is one of 77 countries indexed). Quality of research and quality of indexing are also huge factors. It is very current, with 30,000-35,000 new citations added every week, and the quality of the research is high, since it is focused primarily on peer-reviewed sources. For these reasons, Compendex provides perhaps the most holistic and global view of peer reviewed and indexed engineering publications available. Check out this Quick Start Guide to Compendex.
However, one sometimes a strength is also a weakness.
In the case of Compendex, it covers ALL engineering disciplines, which may be more than is needed. Many of the amazing citations it contains are more relevant to nuclear engineering, electrical engineering or some other sub-discipline that is not very relevant to a typical state DOT.
So for DOT-specific engineering questions, we sometimes recommend (even ahead of Compendex) the wonderful transportation research database TRID, which contains 1.3 million records of transportation-related research citations published worldwide, including extensive research from or for state DOTs, including a lot of "Grey Literature." What is Grey Literature, you ask?
According to Wikipedia, Grey Literature is "materials and research produced by organizations outside of the traditional commercial or academic publishing and distribution channels."
That's a great start. To that I would add this: "Gray literature is not a pejorative, but it serves a useful purpose and can be of very high quality. By nature Grey Literature is hard to discover and hard to access. It is less likely to be collected, preserved, indexed or accessible than its mainstream scholarly counterparts. Examples of Grey Literature include annual reports, technical reports, working papers, and all manner of municipal, state and federal government documents. Organizations that produce grey literature include government agencies, non-governmental organizations, academic centers, private companies and consultants."
-Ken Winter, VDOT Research Library
