The 1971 edition of the MUTCD mandated yellow as the standard colour of all center lines on all roads and highways in all contexts, and banned the use of white. The point was to establish a consistent colour code in pavement markings: from now on, yellow would always delineate opposing traffic flows and white would always delineate traffic flowing in the same direction. The changeover to the 1971 MUTCD standards took place between 1971 and 1975, with most repainting completed by the end of 1973, so for two years drivers still had to use the old and new. One odd exception was that white was still allowed as the left edge line color on the leftmost lane (closest to the median) on divided highways. The 1973 edition of the MUTCD fixed this by mandating yellow for left edge lines on divided highways.