Eric Jaffe, in CityLab, recently reported that Caltrans “admitted” that expanding highways increases traffic by posting a policy brief on the subject of induced demand to its website. He called it a rare admission from a state department of transportation.

State DOTs, as the country’s road builders, have usually responded to congestion, and even safety concerns, by expanding and widening roads, expecting that more lanes will solve problems caused by too many people wanting to drive on them at the same time. But research has shown that making it easier and quicker for people to drive somewhere just encourages more driving. By linking to the policy brief, “Increasing Highway Capacity Unlikely to Reduce Congestion,” [PDF] Caltrans is openly acknowledging the connection between building new capacity and more driving.

But linking to a policy paper doesn’t mean that California will stop building roads altogether.

There is still plenty of pressure to keep building roads—from rural areas that want wider highways, for example, to local areas that tax themselves for new highway expansions (as Placer County is considering doing).

Meanwhile the State Transportation Improvement Program, which is the blueprint for investing in highways in California, contains locally preferred projects including highway expansions, and the California Transportation Commission continues to approve funding for them.

Streetsblog reached out to Steven Cliff, Caltrans’ assistant director of Sustainability, to find out whether the department really has come to accept the concept of induced demand–that if you build more highway miles, more miles will be driven. Cliff said, in short: yes. “It’s pretty settled science that capacity expansion induces demand,” he said. “We know that while it relieves traffic in the short term, there’s pent-up demand that suggests it just fills up again in short order. There’s ample evidence that if you lower costs, demand increases.”

And what does that mean for the department formerly known as the state highway department? (Under Caltrans’ new mission and vision, it’s now the department that “provides a safe, sustainable, integrated and efficient transportation system to enhance California’s economy and livability”–not just highways for cars.)

Caltrans, within its new strategic management plan, developed goals to reduce vehicle miles traveled (VMT). “We can’t keep using single occupancy vehicles as our primary way to get around,” said Cliff. “Arguably, we’re not moving people efficiently now,” he said, and it will only get worse as California’s population increases.

“Our focus has been on strategic expansion, and on expansion of the transportation system in ways that won’t increase VMT–or at least, not increase VMT at the same rate as population growth,” which the state projects will rise about 17 percent by 2030. Some capacity expansion will still be necessary, he said. “It just needs to be strategic.”

For example, rather than adding an all-purpose lane to a congested freeway, a carpool lane might be added, as was done recently on the 405 in Los Angeles. That project has incited complaints that it hasn’t reduced congestion—but that should come as no surprise to anyone who understands induced demand.

Stuart Cohen, executive director of TransForm, has spent a lot of time thinking about induced demand as well. “We know it exists,” he says. “That’s why we put up new transit routes—because we want to induce demand for transit. But we also know that new roadways induce more car trips.”

“We already have enough lane miles in California,” he pointed out. “We just need to use them better. If we had more than 1.2 people in every car and everyone wasn’t trying to use the lanes for the same few hours every day, we could avoid the vast majority of road widenings.”

In 2010, Caltrans developed a “Smart Mobility Framework” to help guide transportation planning in the state, although implementation has been slow. “The idea is to reduce demand first, by providing alternatives and incentives to driving alone, before you consider road expansion,” said Cohen.

TransForm has also developed the idea of the “optimized HOT lane,” which would allow both high-occupancy vehicles (HOVs) and toll-paying single driver cars to bypass congestion in an already existing lane. The revenue raised by the toll would be used to improve transit and encourage people to cut their driving by joining carpools and vanpools or using alternatives–not pay back bonds. The Bay Area’s Metropolitan Transportation Commission did a study [PDF] using the concept on the 101 corridor in San Mateo and came to the conclusion that more people could be moved with less money invested—with no road bonds needed for new construction.

The idea has the added benefit of addressing equity concerns by sharing some of the time savings with people who can’t afford to use the HOV lane.

Currently, however, it’s illegal to convert an existing lane to an HOV/HOT lane. Testing the idea on the ground would require a change in state law.

And getting policymakers to understand the concept of induced demand has been a challenge. “Intuitively, it makes sense that if we give people more space they’ll be able to move around more easily,” said Cliff. “But a lot of people don’t take it to the next level, where it means that more people will want to occupy the space.”

Caltrans, by posting the short paper on its website, is encouraging policymakers to think more deeply about the unintended consequences of expanding highways. Those who are interested in the topic and want to delve more deeply can access the research behind the brief, which was conducted by Susan Handy at UC Davis for the National Center for Sustainable Transportation and the Air Resources Board.