Why new roads do not solve traffic congestion – and how to do it
Traffic congestion is a persistent issue in urban environments, often leading to calls for increased roadway capacity. Conventional transportation planning assumes that reducing congestion increases economic productivity and social well-being by allowing more vehicle travel. However, emerging evidence suggests that beyond an optimal threshold, increased roadway capacity can lead to adverse effects, including induced demand and inefficient resource allocation. What are the mechanisms of traffic evaporation and its implications for urban transport planning? Here is a summary of the latest works from transportation researcher Todd Litman of the Victoria Transport Policy Institute in Canada.
The Concept of Traffic Evaporation
Traffic congestion maintains equilibrium: as congestion increases, travelers adjust their behavior by modifying their travel schedules, selecting alternative routes, or opting for different modes of transportation. For instance, an individual planning a cross-town errand may choose to postpone the trip or visit a local store instead of a distant but more affordable alternative. Conversely, if a roadway is expanded, additional peak-period trips are generated, leading to a recurrence of congestion. This phenomenon is detailed in the report ”Generated Traffic: Implications for Transport Planning” (Litman, 2001).
The Elasticity of Vehicle Travel
Many urban trips are of marginal value to users, making them elastic—subject to variation based on travel conditions, cost, and convenience. Traditional transportation economic models often assume that all trips are high value, assigning significant economic benefits to congestion reduction. However, research demonstrates that excessive roadway expansion and increased per capita vehicle travel correlate with reduced economic productivity. This counterintuitive effect is explored in ”The Mobility-Productivity Paradox” (Litman, 2021).
Economic Considerations in Roadway Pricing
From an economic perspective, an efficient road pricing system should reflect users’ willingness to pay for roadway capacity. However, in practice, most highway expansion projects would fail this test. Motorists generally support additional capacity when it is subsidized but are reluctant to pay for marginal costs directly. This issue is further examined in ”The Roadway Expansion Paradox” (Litman, 2022). Just as preventive healthcare is more cost-effective than treating illnesses, demand management strategies—such as congestion pricing and improved public transportation—are generally more efficient than expanding urban roadways.
Transportation Demand Management (TDM) and Alternative Mobility Solutions
The elasticity of vehicle travel, and thus the effectiveness of pricing policies, depends significantly on the availability of alternative transport options. In automobile-dependent areas, price sensitivity is lower due to the lack of viable alternatives. Conventional transportation modeling often underestimates the benefits of non-auto travel modes and TDM incentives, leading to a systematic overestimation of highway expansion benefits. A holistic approach to urban mobility should integrate multimodal transport solutions, including high-quality public transit, cycling infrastructure, and pedestrian-friendly urban designs.
Policy Implications and Conclusion
Public discourse on congestion frequently emphasizes its inconvenience, yet highway expansions tend to be economically inefficient and inequitable. These projects impose high costs on taxpayers while disproportionately benefiting a small subset of affluent suburban commuters. Moreover, the resulting congestion relief is often temporary, as induced demand restores pre-expansion traffic levels within a few years. Instead, policymakers should emphasize the broader benefits of alternative traffic management strategies, such as congestion pricing, investment in public transit, and urban design improvements. By shifting the focus from increasing vehicle throughput to optimizing urban mobility, cities can achieve more sustainable and equitable transportation outcomes.
References
- Litman, T. (2001). Generated Traffic: Implications for Transport Planning. Victoria Transport Policy Institute. Available at: https://www.vtpi.org/gentraf.pdf
- Litman, T. (2021). The Mobility-Productivity Paradox. Victoria Transport Policy Institute. Available at: https://vtpi.org/ITED_paradox.pdf
- Litman, T. (2022). The Roadway Expansion Paradox. Planetizen. Available at: The Roadway Expansion Paradox | Planetizen Blogs
