38-40, 42, 44-47 Apr

ITE JOURNAL / APRIL 2001 TRANSPORTATION PLANNERS often compare traffic to a fluid, assuming that a certain volume of vehicles must flow through the road system like water through a network of pipes. But in many situations urban traffic is more comparable to a gas, which expands to fill available space. Road improvements that reduce users’ travel costs tend to attract traffic from other routes, times and modes, and encourage longer and more frequent vehicle trips. This is called generated traffic, referring to increased vehicle traffic volumes at a particular time and location, including travel diverted from other times and routes. A portion of this additional traffic usually consists of induced travel, referring to increases in total vehicle trips and distance traveled, excluding diverted traffic.1 Induced travel is therefore a subcategory of generated traffic. This additional traffic reflects the economic “law of demand,” which states that consumption of a good usually increases as its price (consumers’ perceived costs) declines, all else being equal. Reducing congestion reduces the generalized cost of driving, thus encouraging more peak-period vehicle travel. To put this another way, most congested roads have latent travel demand: additional peak-period vehicle trips that will occur if congestion is relieved. Generated traffic has three different impacts to consider in transportation planning and project appraisal: 1. It reduces the congestion-reduction benefit that can result from increased road capacity. 2. It often increases external costs, such as downstream congestion, parking demand, uncompensated crash damages and environmental damages. This is particularly true of induced travel. 3. Since it consists of marginal value trips (vehicle travel consumers are most willing to shift or forego if their costs increase), consumer benefits from generated traffic tend to be modest. This is not to suggest that expanding roadway capacity provides no benefits, but generated traffic significantly affects the nature of these benefits. Ignoring generated traffic in transportation planning distorts transport decisions. This feature describes how generated traffic occurs, defines different types of generated traffic, discusses their impacts and describes ways to incorporate generated traffic into transport planning and modeling.