Prior to the 1970s, some states allowed right turn on red. Most states on the eastern half of the U.S. did not allow it, though. Gas prices shot through the roof during the 1973 Oil Crisis and the 1979 Energy Crisis.
Due to the tough economy during that time and the high energy costs, people became much more conscious of their energy consumption. Myth Busters did an episode on gas and energy consumption.
They concluded that that by eliminating idling time of a car sitting at a red light and allowing right turns, fuel efficiency is increased exponentially. It is especially true for delivery trucks. Thus, the Right Turn on Red Policy was passed in the 1970s to help the economy and save gasoline.
They also encouraged car pools, public transportation, and riding bikes. Interestingly, UPS has taken the right turn on red policy a step farther. They actually encourage their drivers to take routes that allow them to make as many right turns as possible to save on gasoline.