the appropriate regulation.) Thus, what was formerly a quasi-official project became an official one. States are allowed to supplement the MUTCD but must remain in "substantial conformance" with the national MUTCD and adopt changes within two years after they are adopted by FHWA. The 1971 edition of the MUTCD included several significant standards. The MUTCD imposed a consistent color code for road surface markings by requiring all center lines dividing opposing traffic on two-way roads to be always painted in yellow (instead of white, which was to always demarcate lanes moving in the same direction), and also required that all highway guide signs (not just those on Interstate Highways) contain white text on a green background. Another major change, inspired by the Vienna Convention on Road Signs and Signals of 1968, was that the 1971 MUTCD expressed a preference for a transition to adoption of symbols on signs in lieu of words "as rapidly as public acceptance and other considerations permit." During what was then expected to be a transition period, the MUTCD allowed state highway departments to use optional explanatory word plaques with symbol signs and to continue using the previous standard word message signs in certain cases. Robert Conner, the chief of the traffic control systems division of the Federal Highway Administration during the 1970s, believed that symbol signs were "usually more effective than words in situations where reaction time and comprehension are important." Conner was active in the Joint Committee and also represented the United States at international meetings on road traffic safety. Most of the repainting to the 1971 standard was done between 1971 and 1974, with a deadline of 1978 for the changeover of both the markings and signage. The U.S. adoption of several Vienna Convention-inspired symbol signs during the 1970s was a failure. For example, the lane drop symbol sign was criticized as baffling to U.S. drivers—who saw a "big milk bottle"—and therefore quite dangerous, since by definition it was supposed to be used in situations where drivers were about to run out of road and needed to merge into another lane immediately. American highway safety experts ridiculed it as the "Rain Ahead" sign. Many American motorists were bewildered by the Vienna Convention's symbol sign with two children on it, requiring it to be supplemented with a "School Xing" plaque. The 1971 MUTCD's preference for a rapid transition to symbols over words quietly disappeared