In 1962, Anthony Downs put forward the fundamental law of peak hour congestion on urban commuter expressways: peak-hour traffic congestion rises to meet maximum capacity. Larger bus and train networks and more cycle-ways are never the solution. Just as more highways leads to more congestion because more people drive to work, with fewer people driving to work if they fit enough to ride on the new cycle-ways, other people will start driving to work, instead of taking public transport (Downs 2005).
Downs’ law of peak hour congestion or “triple convergence” means that road will be as congested as before after any new investment in capacity because fewer people are taking public transport or postponing their trips to outside the peak hour times. More commercial driving in peak hours by trucks and delivery vans is an obvious response to more road capacity (Downs 2004). Increases in road capacity do not reduce congestion because of a triple convergence of new users from buses, trains and off-peak.
If an expressway’s capacity were doubled overnight, the next day’s traffic would flow rapidly because the same number of drivers would have twice the road space. Word will soon spread that this highway is less congested. Drivers who used that road before and after the peak hour to avoid congestion will shift into the peak hours. Other drivers using alternative routes will shift to this more convenient expressway. Bus and train passengers will start driving on the upgraded road in the peak periods. In a short time, this triple convergence of bus, train and car users onto the improved or new road in the peak hours will make that road as congested as it was before its expansion. Duranton and Turner (2011) found that vehicle kilometers travelled increases proportionately to increase road space for interstate highways and slightly less rapidly for other roads. The increased vehicle miles travelled is a mix of more driving by current residents, more commercial traffic and some migration from other types of roads.
Increases in bus or train capacity had the same triple convergence effect at peak times as new roads. Duranton and Turner (2011) found that increased provision of public transport does not relieve road congestion. The road space freed up by the motorists who switched to the additional buses or trains is filled by other motorists and commercial transport who previously planned to travel outside of the peak hours. Downs considers that peak-hour congestion is inherent to how modern societies operate:
…congestion exists because societies organize economies so most people will work during the same hours each day. This makes interaction among firms and agencies possible, thereby increasing society’s productivity, and raising overall efficiency. But it also requires most workers and students to travel to and from their places of activity at the same times. This overloads ground transportation systems during the morning and evening peaks, and often longer.
No large metropolitan areas have enough infrastructure to transport everyone who wants to move during peak hours simultaneously; nor do they have enough resources to build it. Hence some travelers must wait until others have moved. That waiting constitutes traffic congestion. (Downs 2006).
Road capacity expansions and extensions to bus and train networks do not reduce congestion. The added space means the extra traffic can be handled with still tolerable congestion. Judiciously selected road investments are still value for money because the additional road space allows more commuters to travel by the fastest, most convenient, most flexible method of urban travel, which is in a car.