Triple Convergence

Traffic congestion during peak traffic hours is ubiquitously present in almost all metropolitan areas. Despite numerous supply-side interventions (e.g., [Page 1627]building additional expressway lanes, major arterial streets, and expressways), most metropolitan areas continue to remain congested during peak traffic hours. The principle of triple convergence, first posited by Anthony Downs, explains the persistence of congestion on limited-access expressways (also called freeways) during peak traffic hours.

According to the principle of triple convergence, traffic congestion on freeways during peak hours represents a rough state of equilibrium in which freeways do not offer any travel time advantage over alternative routes. Any freeway capacity enhancement is likely to relieve traffic congestion only temporarily. The freeway will eventually become congested again, and the system will arrive at another equilibrium.

The rationale behind the principle of triple convergence is straightforward. Individuals, as rational decision makers, try to optimize their travel costs. Travel cost has two components: time and money. Different travel modes impose different costs: public transit often costs less than driving in monetary terms, but public transit is usually slower than driving. Hence, depending on how much they value their time, individuals choose different travel modes (public transit versus personal vehicle).

If the time it takes to reach a destination by driving is similar to that of traveling by public transit, individuals may be indifferent to the time cost. Other factors such as monetary cost and/or convenience then become more important determinants of mode choice. By the same reasoning, drivers always prefer the fastest route because it imposes the lowest time cost, all else being equal.

Most automobile drivers choose to take the quickest route to their destination, one that is direct, has highest speed, and is least encumbered with obstacles such as traffic signals and stop signs. Except for trips of a very short distance, in most cases this route includes a freeway. Drivers converge on the freeway from various access points until the freeway is loaded with a much higher number of vehicles than it could carry at an optimum speed—that is, it becomes congested and does not offer any travel time advantage over the alternative routes.

It follows that the commuters could reach their destinations using alternative routes (such as surface streets) in as much time as it would take using the freeway. However, despite the congestion, many commuters continue to drive on the freeway out of habit.

Congestion on the freeway continues to increase to the extent that the freeway route actually becomes slower than the alternative routes. This prompts some drivers to switch to the alternative routes that are now faster than the congested freeway. Eventually, the surface street routes also slow down as a result of increased traffic volume. This process continues until none of the routes offer any advantage over the alternatives, and the transport network is in a state of rough equilibrium.

During the equilibrium, traffic moves at below-optimal speeds on any given freeway or major arterial street. Despite severe congestion, the freeways still carry more vehicles per hour than the surface streets, simply because they have a much larger capacity.

...