From the Document: "The Bureau of Transportation Statistics (BTS) presents 'State Transportation Statistics 2015,' a statistical profile of transportation in the 50 States and the District of Columbia. This is the 13th annual edition of 'State Transportation Statistics,' a companion document to 'National Transportation Statistics (NTS),' which is updated quarterly on the BTS website. [...] Included in this 'State Transportation Statistics 2015' report is a brief description of the data sources used and a glossary of terms. Also contained in this publication is a summary table that displays the approximate timing of future data releases and contact information for each State's department of transportation."
U.S. Bureau of Transportation Statistics: http://www.bts.gov/