"If you build it, they will come" is a reassuring slogan if you're building a baseball field. But for the highway engineer, it's a disaster. The fact that so many new roads get congested so quickly has long baffled road builders trying to ease gridlock. This phenomenon -- often called "induced traffic" -- is well-known to the transportation sector. Studies on induced traffic in the U.S. date back to the 1940s, and it is now widely acknowledged that building more roads does not relieve congestion.

Across the Atlantic, the British government is reinventing its transport policy after an expert panel found that while expanded road capacity enables vehicles to travel faster, time savings are lost because people drive more -- results that prompted UK Transport Minister Gavin Strang to conclude that "We cannot tackle our traffic problems by building new roads."

However, the idea of induced traffic has never gained currency in the U.S., where many transportation planners still work under the Eisenhower-era assumption that travel and vehicle use increase only with population and economic growth and not because of reduced travel times or cost. This is partly because early research was unable to differentiate between new traffic induced by expanded capacity and redistributed traffic or traffic caused by changes in land use. But empirical research has improved, and new studies designed to make those differences clear have also found that new roads generate new traffic. Even the Federal Highway Administration found in a recent study in Milwaukee that induced traffic accounted for 11-22 percent of the area's increased traffic from 1963 to 1991.

Their findings showed an even greater induced traffic effect on the regional scale than on individual highway segments because when drivers perceive an increase in either travel time or cost they typically cope by altering travel routes, traveling at a different time or traveling less. When road capacity is expanded near congested routes the opposite happens -- drivers far and wide flock to the new facility hoping for reduced travel times, thereby increasing the total amount of traffic in the region. U.S. Transportation officials have failed to apply these findings, and evidence of induced traffic is rarely used in travel modeling, where it would have a big impact on deciding whether a project gets built.

Washington, DC -- An analysis of the respected Texas Transportation Institute's annual report on metropolitan congestion shows that the most common congestion-fighting strategy has had virtually no impact on the growth of traffic congestion in major urban areas in the last 15 years. The analysis by the Surface Transportation Policy Project compared metropolitan areas that have added extensive new road capacity with those that have not, and found no significant difference in the rise in traffic congestion.

The STPP analysis also found that between the two groups, the urban areas that added more new lanes spent roughly $22 billion more on construction, but their drivers are still paying high costs due to congestion delays. The average of TTI's "Roadway Congestion Index" for the two groups is almost identical, at 0.93 and 0.92.

"Widening roads to ease traffic congestion is ineffective and expensive at the same time," said Roy Kienitz, Executive Director of STPP. "It's like trying to cure obesity by loosening your belt."

Each year, as part of its study of congestion in urban areas throughout the country, TTI projects the theoretical increase in lane miles needed to keep up with traffic growth. STPP's report estimates that most urban areas in TTI's study would have to spend thousands of dollars per family each year to build new roads. Nashville, Tennessee would have to spend the most, at $3,243 for a family of four every year, followed by Austin, Orlando, and Indianapolis. Figures for all 70 cities are listed in the STPP report.

"If adding roadway capacity worked, cities that built and widened many roads would show a slower rise in congestion." says STPP Research Manager Don Chen. "We found no such correlation."

The STPP report says the problem may be partially explained by the phenomenon of "induced traffic." Several recent studies have documented that new roads actually encourage more driving and more automobile trips. A University of California study of 30 urban counties in the state found that every 1% increase in lane miles generates a 0.9% increase in traffic within five years, negating the congestion-easing effect of new roads.

"Communities that are investing in strategies that give people alternatives to driving, such as transit, bike lanes, and land use planning, are finding these techniques can be both a popular and effective means of fighting traffic congestion," says Kienitz.

The Texas Transportation Institute's annual report on the impact of traffic congestion in major urban areas has become the accepted national benchmark of congested conditions. Each year the report calculates hours of delay per person, excess fuel consumption, and congestion costs to reach a "congestion index" for each area. STPP used 15 years of TTI's data for its companion report.

STPP is a coalition of over 200 environmental and consumer groups interested in promoting a transportation system that is energy efficient and environmentally responsible.

For a complete copy of the report, visit STPP's website at www.transact.org.

Flying in the face of conventional wisdom, a British study released this month is creating a buzz in transportation circles by concluding that closing roads can actually reduce driving. The research team analyzed 60 cases worldwide where roads were either closed or capacity was reduced. On average 20 percent of the traffic vanished, but in some cases an astonishing 60 percent disappeared. Where does the traffic go? Transportation planning models assume that it shifts onto other roads, causing congestion elsewhere. But experts now say that in many cases it actually does disappear. An earlier British report went a step further, prompting experts to conclude that closing roads in city centers can actually boost local economies by creating jobs downtown and that, conversely, new roads can lead to job loss downtown. The U.S. equivalent of the expert British research teams, a panel convened by the Transportation Research Board, excluded empirical evidence on reduced travel from its 1995 "Expanding Metropolitan Highways" report, ignoring several compelling stories: The demise of San Francisco's Embarcadero Freeway, toppled in the '89 earthquake, and the city's Central Freeway, torn down in 1995, were predicted to cause gridlock, but didn't. When New York City's Westside Highway collapsed in 1973, 53 percent of trips disappeared. And when citizens in Portland, Oregon replaced their Harbor Drive freeway with a waterfront park there was no traffic chaos. Moreover, San Francisco's waterfront is now booming with residential and commercial activity and Portland's downtown has become a highly desirable location.

For a free copy of "Traffic Impact of Highway Capacity Reductions" contact Environmental Media Services at 202-463-6670.

Back to the Walkable Streets home page.