Let's be Honest About Highway Expansions
The Yolo 80 Corridor Improvements Project between Davis and Sacramento has been underway for months. The current phase of the project involves the construction of modern concrete median barriers along with a widening of the paved shoulders for safety purposes. Assuming legal actions do not interfere, the second phase of the project will use that extra pavement to add a fourth lane in each direction. The new “managed” lanes will, according to official reports, provide an incentive for carpooling or taking the bus, and will reduce travel times for all drivers. Proponents promise that the project will “ease congestion.”
The project will undoubtedly ease congestion in the short term, especially in comparison to the nightmare drivers face during construction. But will it ease congestion in the longer term? The answer to that question depends on the degree to which the additional lanes encourage more driving, a phenomenon known as “induced travel. ”This phenomenon is explained by simple economic principles: reducing the cost of a desirable good is likely to increase consumption of that good. In this case, the good is driving, and the cost includes the time cost of travel. If it takes less time to drive between Davis and Sacramento, some people (like me) might choose a restaurant in Sacramento rather than Davis, decide to visit the superb Crocker Museum more than once a year, or switch from bus or train to driving when they go. Bay Area residents, who use this route to get to the wonders of Lake Tahoe, might enjoy a few more ski weekends if the Friday afternoon traffic jam in Davis dissipates. Many people will adjust their behavior in small and large ways in response to the freeway improvements.
These behavioral adjustments, as a substantial and rigorous body of research now convincingly shows, lead to an increase in the total amount of driving that, after a period of 5 years or so, is equal in terms of percentage change to the increase in freeway capacity. In economic terms, the “elasticity” of driving with respect to freeway capacity is around 1: a 1 percent increase in capacity is associated with about a 1 percent increase in driving. The net result is that congestion is likely to be just as bad in 5 years, all else equal as it was before the state spent hundreds of millions of dollars on the widening and drivers endured years of added delays – and greater risks – during construction. The amount of driving is not a direct measure of congestion, which is a function of the ratio between the number of vehicles and roadway capacity at a given time, but more driving generally means more congestion in terms of both worse congestion and longer periods of congestion.
Which isn’t to say that a widening project doesn’t have benefits: people are driving more because that increase in driving makes them better off in some way. In other words, the true benefit of the project is that it enables more driving, not that it eases congestion. But that increase in driving that benefits individual drivers comes with significant societal costs. For one thing, it adds to the wear and tear on our roadways, which adds to the public cost of maintaining those roadways. For another, it produces what the economists call “negative externalities,” that is, costs that my driving imposes on others for which I do not compensate them, namely increases in air pollution, noise pollution, vibrations, crashes, land consumption—and also congestion. More driving means more costs, and these costs offset—in part or in whole—whatever benefits the widening project brings.
So why are we still widening freeways? Especially in California, where the state has adopted targets for reducing driving as a part of its effort to reduce greenhouse gas emissions? The answer is complicated, as documented by Dr. Amy Lee who interviewed more than 50 state, regional, and local officials about this question for her 2023 dissertation. The short version of the answer is a combination of strong economic interests, the need to respond to public demands for solutions, and short-sighted decision making, among other factors. In a word: politics.
While California has not (yet) imposed a ban on freeway widenings, it has pushed for better accounting of the induced travel effect in the environmental impact review process under the California Environmental Quality Act. The California Department of Transportation (Caltrans) must assess the impact of projects that expand highway capacity on the total amount of driving, measured as vehicle miles traveled (VMT). They must then commit to actions that would “mitigate,” at least in part, the increase in VMT in some way, for example, by improving transit service to encourage a switch from driving. If this all sounds a bit circular—or illogical—that’s because it is: the only way to fully mitigate the VMT induced by the expansion is to not do the expansion. Partial mitigations are complicated, in that the expansion reduces the effectiveness of the mitigation: with less congestion, why would drivers switch to transit? The transit improvement would be more effective without the expansion, but the funding for the improvement may only be available because of the expansion.
The California requirements to estimate induced VMT—also now adopted in Minnesota and Colorado—have focused attention on the tools used to forecast the future performance of the roadway system. These travel demand forecasting models (or TDMs, as they are known) were developed decades ago to be used in a “predict and provide” planning process. The TDM predicted congestion levels twenty or thirty years in the future assuming no expansion of the system, and the state provided additional capacity in those parts of the system where predicted congestion levels were deemed unacceptable. While TDMs have become more sophisticated over time, experts now recognize that these models do not fully capture the induced travel effect. That recognition has led to the development of elasticity-based tools that draw on that robust body of evidence mentioned earlier and provide a second way of estimating the increase in VMT that an expansion project will induce.
The availability of competing tools that produce sometimes drastically different estimates has provoked much debate and disagreement. The professional community may never come to a consensus about the best methods for producing the most accurate estimates of induced VMT. What the community does need to agree on is that accurate estimates are important. If our forecasts do not fully capture the induced VMT effect, then our analyses are over-stating the benefits of highway expansion projects in terms of congestion relief, and they are under-stating the negative impacts of those projects in terms of increased emissions, noise, vibration, injuries, fatalities, etc. If these expensive projects are sold as “easing congestion,” the public has right to know how temporary this relief will be. More accurately accounting for induced VMT might not change decisions made about projects, but we have a moral obligation to be as honest as possible with the public about what they are getting for their investment.
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For more information about the methods used by agencies in California to estimate induced travel for highway projects, please see the following reports:
- Review of the Travel Demand Model Benchmarking Method Used to Estimate Induced VMT for the I-680 Express Lane Project
- Review of LA Metro’s Proposed Induced VMT Estimation Methods
- Review of the Rural Induced Demand Study
Susan Handy is a distinguished professor of environmental science and policy at UC Davis and the author of Shifting Gears: Toward a New Way of Thinking About Transportation, published by MIT Press.