Changeable message signs are being used increasingly to reduce congestion and prevent accidents in both rural and urban locations. Because of the rapid development of improved technology, there is a need for up-to-date information on the characteristics, applications, and effectiveness of changeable message signs. This report of the Transportation Research Board includes information on requirements for design, installation, operation, and message effectiveness. It also contains a review of previous and current research, and future development needs are identified.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3495