"Traffic" and "traffic congestion" were not even distinct categories of thought in city administration until about 1915, when motor vehicles made them so. . . . "Traffic" was simply not yet a topic of city administration. Articles about "traffic congestion" before 1915 were more likely to be about crowding on streetcars or inadequate harbor facilities than about the crowding of streets with vehicles and pedestrians. Around 1915, however, the term "trafic congestion" was transformed. Thenceforward it nearly always meant the crowding of streets with motor vehicles.
Peter D. Norton, Fighting Traffic: The Dawn of the Motor Age in the American City (Cambridge, MA: MIT Press, 2008), p. 49