How Road Capacity Expansion Generates Traffic


Urban traffic congestion tends to maintain a self-limiting equilibrium: traffic grows until congestion delays cause travellers to forego some potential peak-period vehicle trips (indicated by the curve becoming horizontal). If road capacity is expanded,traffic increases until it reaches a new equilibrium. The additional peak-period vehicle traffic that results from roadway capacity expansion is called “generated traffic.”

The portion that consists of absolute increases in vehicle travel (as opposed to shifts in time and route) is called “induced travel.”

Generated and induces vehicle travel have the following implications for congestion evaluation (Handy 2015; Litman 2001):

· Traffic congestion seldom becomes as severe as predicted by extrapolating past trends. As congestion increases it discourages further
peak-period trips, maintaining equilibrium.

· Roadway expansion provides less long-term congestion reduction benefits than predicted if generated traffic is ignored.

· Induced vehicle travel increases various external costs including downstream congestion, parking costs, accident risk, and pollution
emissions, reducing net benefits.

· Induced vehicle travel directly benefits the people who increase their vehicle travel, but these benefits tend to be modest because the
additional travel consists of marginal-value vehicle mileage that users are most willing to forego if their costs increase.

Source: Smart Congestion Relief, Comprehensive Evaluation Of Traffic Congestion Costs and Congestion Reduction Strategies, 23 March 2016, Todd Litman,
Victoria Transport Policy Institute

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