National Research Council (U.S.). Highway Research Board, National Research Council (U.S.). Highway Research Board. Meeting, & National Research Council (U.S.). Highway Research Board. Special Committee on International Cooperative Activities. (1969). International transportation topics: 4 reports ; sign uniformity, closing the Darien gap, transportation in Europe, and transportation in Great Britain. Highway Research Board.

National Research Council (U.S.). Highway Research Board, National Research Council (U.S.). Highway Research Board. Meeting, and National Research Council (U.S.). Highway Research Board. Special Committee on International Cooperative Activities. International Transportation Topics: 4 Reports ; Sign Uniformity, Closing the Darien Gap, Transportation in Europe, and Transportation in Great Britain. Washington: Highway Research Board, 1969.

National Research Council (U.S.). Highway Research Board, et al. International Transportation Topics: 4 Reports ; Sign Uniformity, Closing the Darien Gap, Transportation in Europe, and Transportation in Great Britain. Highway Research Board, 1969.