The first Highway Capacity Manuals—the guidebooks for how, where, and what types of roads get built in the United States—are refreshingly upfront about why we built our current road system: “to serve traffic.” In this respect, our national roadbuilding project has been a tremendous success. We have more roadway per capita and vehicle miles of travel than any other large industrialized country.

There is also a strong relationship between where we have built the most roadway and where we drive the most. Across metropolitan areas, each additional road mile per capita is associated with two-to-three thousand more daily miles of driving per capita. Metropolitan areas with a standard deviation more roadway per capita on average have almost twice as much driving per capita as those with a standard deviation less roadway. New road investments appear to do such a good job of serving traffic that empirical studies, like those conducted by my dissertation advisor Robert Cervero (UC Berkeley) and Wharton’s Gilles Duranton, find a one-to-one relationship between road investments and vehicle travel. New roads beget new traffic.

Alas, our national roadbuilding experiment’s record is less stellar by other measures. Metropolitan areas and counties with more roadway are no healthier, happier, or wealthier than those with less roadway. In fact, our poorest metropolitan areas tend to have the most roadway. Each ten percent increase in roadway per capita is associated with around a 4% decrease in GDP per capita. Places with more and bigger roads per person also have more traffic fatalities per person. While there is variation in happiness across places, people in metropolitan areas with a lot of roadway are no happier than those in metropolitan areas with only a little bit of roadway per person.

While roads are an essential part of urban and rural life, serving traffic was never a great justification for where, how many, or what types of roads to build. In an era of climate change, serving traffic is deeply problematic. Road-based vehicle travel accounts for around a quarter of all US CO₂ emissions and roughly four-fifths of emissions from the transportation sector.  New traffic begets new CO₂ emissions.

In addition to a climate change crisis, declining gas tax revenues and increasing road-maintenance obligations are creating a general funding crisis that could help lead to a paradigm shift in how we raise and spend money on transportation. I recommend three broad, ambitious, and somewhat aspirational shifts for public policy.

First, stop subsidizing new roads with public dollars. The economic and social benefits are questionable and frequently do not justify the expenditure. The economic and social costs of increased CO₂ emissions, by contrast, are apparent and substantial. Instead, new roadways should generally be financed through tolls. If tolls are insufficient to cover construction and operating costs, this suggests that the road is probably not worth as much to those using it as it costs others to pay for it. Financing these types of roadways would certainly help explain the inverse relationship between GDP and roadway across metropolitan areas.

Second, develop national, state, and regional systems for downgrading roads. Highways, arterials, and local roads need to be rebuilt periodically and often at great expense. With the exception of replacing the occasional damaged urban freeway with an urban arterial, reducing roadway capacity is rare. We need a mechanism for ratcheting back an overbuilt road system that, at the margin, probably does more harm than good in most US urban areas. Instead, our current financial and political models tend to support expansion, road widening, and rebuilding. We can ratchet up, but not back, regardless of population trends, the economy, or fiscal health.

These relationships are not causal, but they do suggest that road building has not produced anything like the promised economic benefits.

Third, stop treating the fuel tax like a user fee. Although there is a natural fiscal argument for paying for roads out of gas taxes, the external economic costs of driving, including things like congestion and pollution, are estimated at nearly twice the gasoline tax. With an EPA-estimated social cost of CO₂ of $45 per metric ton, road-based transportation created $7 billion dollars of CO₂-related harm, equivalent to around a fifth of the total annual revenue raised by federal gas taxes, in 2017. These funds should be used to offset and reduce harm not to build new roads that generate more traffic and create more harm.

Although there is an opportunity to reconsider how and where we fund roadways, I am not particularly optimistic. Our national dialogues tend to treat transportation investments as inherently good and productive. Rebuilding infrastructure is presented not just an economic imperative but a moral one. The media and politicians frequently refer to federal and state transportation finance bills as nostalgic examples of bipartisan politics or contemporary opportunities to set aside political differences to invest in the future. We should really be more careful about the kind of future we are investing in.

Erick Guerra is Associate Professor of City and Regional Planning at the University of Pennsylvania Stuart Weitzman School of Design.

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