"Since the beginning of the 112th Congress, convened in January 2011, the House and Senate have observed a ban on earmarks, formally known as congressionally directed spending. The ban has led to changes in the way transportation funding decisions are made. This report explains what earmarks are and discusses their use in surface transportation finance. It then discusses how federal transportation funding is distributed with a ban in place and how Members of Congress might influence the distribution."
CRS Report for Congress, R41554
Congressional Research Service: https://crsreports.congress.gov/