Level of service was a child of the Interstate Highway era. The LOS concept was introduced in the 1965 Highway Capacity Manual, at the very moment in American history when concrete ribbons were being tied across the country, and quickly accepted as the standard measure of roadway performance. LOS is expressed as a letter grade, A through F, based on how much delay vehicles experience; a slow intersection scores worse on LOS than one where traffic zips through. Planners and traffic engineers use the metric as a barometer of congestion all over the United States.
In California, LOS has an especially high-profile. As the primary arbiter of traffic impacts under CEQA[CA Environmental Quality Act]—adopted in 1970 by Governor Ronald Reagan—the metric not only determines the fate of many transportation and development projects, but has the awkward role of promoting car use within a law designed to protect the environment. "We have one section of CEQA saying we've got to reduce greenhouse gas emissions," says transportation consultant Jeffrey Tumlin of Nelson\Nygaard, "and another section of CEQA saying we need to accommodate unlimited driving."