Getting signals in sync will help traffic flow
Fine-tuning controls on the nation's traffic signals — a project already being undertaken in Seattle — would cut U.S. road congestion by up...
WASHINGTON — Fine-tuning controls on the nation's traffic signals — a project already being undertaken in Seattle — would cut U.S. road congestion by up to 10 percent, transportation experts estimate.
It also would reduce air pollution from vehicles by up to one-fifth, cut accidents at intersections and save about five tanks of gas annually per household, according to the National Transportation Operations Coalition, an alliance of federal, state and local traffic departments and equipment-makers.
That's the good news. The bad news is that the average local traffic department earned an overall grade of D on the alliance's latest report card. Streamlining intersections is happening in only some cities and states, even though it's eminently doable.
"People who say we can't do anything about congestion are wrong. We can do lots," said Joel Marcuson, a specialist in urban intersections with the Jacobs Engineering Group Inc. in Phoenix.
Right now, however, three of four of the nation's 300,000 traffic signals need replacement or timing adjustments for optimum performance, according to the U.S. Department of Transportation.
In Seattle, workers retimed and synchronized 1,175 city intersections between 1998 and last year, according to Mayor Greg Nickels' office. The clearest result, said Alex Fryer, the mayor's spokesman, was a 40 percent drop in congestion on three of the city's major arterials. Emissions at intersections on those corridors were reduced by 20 percent.
This year, Seattle's transportation department has updated timing plans for 200 signals. Some major intersections are timed as frequently as every two years to reflect traffic-volume changes, the department said.
And Seattle says it works with adjoining jurisdictions and the Puget Sound Regional Council to provide seamless timing plans on several major arterials.
Nickels said the effort is not just about making traffic flow. "It reflects our continuing work to fight climate pollution wherever we can," he said.
Washington coordinates the timing of half the 1,000 traffic signals the state is responsible for. It checks the timing of signals on busy arteries every two-and-a-half years. That compares with a three-year standard unmet in many states.
Among the obstacles for many cities are a nationwide shortage of skilled traffic engineers, unfocused local political leaders with tight budgets and stodgy local traffic departments. For that matter, federal aid that could ease congestion goes mainly to building and maintaining roads.
Nonetheless, Seattle and lots of other cities in Washington and at least six more states — California, Florida, Minnesota, Maryland, Georgia and Texas — are finding ways to move traffic through intersections faster, according to the transportation engineers group.
Cities need improvement, traffic engineers say, unless:
• You can sometimes make it through six to eight consecutive intersections on green lights.
• There is useful traffic information on the radio and on roadside message signs.
• It is rare that there's no cross traffic when you're stopped at a light.
Most traffic departments can do better at each of three levels of traffic management, Marcuson and other experts said: individual signals, coordinated signals and regional traffic management.
Regionalized traffic management is the secret in U.S. metropolitan areas that move traffic best. They include Seattle, Las Vegas, Milwaukee, Minneapolis-St. Paul, Los Angeles, Kansas City, Denver, Houston, Miami-Dade County and the Washington, D.C., metropolitan area.
Among the most resourceful is Portland, which installed carbon-dioxide-emissions monitors at intersections before it improved their flow. The lower pollution that the monitors recorded enabled Portland to claim pollution-reduction credits that it sold for $560,000 on the carbon-offset market. The money helped pay for intersection improvements.
Technologically, most U.S. traffic signals remain very 20th-century, said Philip Tarnoff, director of the Center for Advanced Transportation Technology at the University of Maryland in College Park.
Centralized timers drive most of them by changing lights at scripted intervals, he explained. "They tell the signals: 'It's 6 a.m. Use timing schedule A until 9 a.m. Then use timing schedule B until 4 p.m."'
If timers are accurate, and the prescribed signal intervals are based on accurate and recent traffic surveys, these systems can do as well as fancier ones in typical situations.
That's a big if, however. Most timed systems aren't refreshed and adjusted at the three-year intervals recommended for busy intersections or ones that see big changes in traffic due to new homes or businesses.
Even perfectly tuned timer-dependent signal systems can't adapt to unpredictable roadway events such as accidents, construction and bad weather. Together, those factors cause half of U.S. traffic congestion, according to Transportation Department statistics.
For all these reasons, Tarnoff and many other traffic engineers favor adaptive signal-timing systems first adopted 30 years ago in the United Kingdom and Australia. They measure traffic minute-to-minute with cameras or in-pavement sensors and automatically adjust signal times to maximize flow for existing conditions, including accidents, construction and bad weather.
These adaptive signals are costly and challenging to program, however, and haven't caught on with local U.S. traffic departments.
Seattle Times staff reporter Charles E. Brown and McClatchy researcher Tish Wells contributed to this report.
Copyright © 2008 The Seattle Times Company
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