The 1961 edition of the MUTCD mandated the use of yellow for the two specific exceptions where yellow had been previously recommended but not required, and concurrently banned white in those contexts. The drafters of the 1961 MUTCD gave three reasons for the change: "(1) It contrasts with the normal white center or lane lines and thus gives emphasis to the hazard; (2) Yellow has been accepted as a symbolic warning color in signs and signals; and (3) It is consistent with the standard for no-passing-zone markings approved by the American Association of State Highway Officials and is in use in more than two-thirds of the States for barrier lines."