ALEXANDRIA, Va. (AP) Nighttime drivers on the Capital Beltway near the Woodrow Wilson Bridge now encounter the latest in traffic management technology — a speed limit that changes depending on the degree of congestion.
Virginia transportation officials hope the variable speed limit, which went into effect July 28, will help avoid the funnel effect that occurs when lanes are closed for construction. The technology has been used successfully in Europe but has had only limited trials in the United States, said Ronaldo T. Nicholson, who coordinates the Wilson Bridge and other major projects for the Virginia Department of Transportation.
“With the number of major interstate projects going on right now, we have to look at new technology. We have to look at new ways of managing the congestion that will definitely occur or be exacerbated,” Nicholson said.
The phenomenon the variable speed limit aims to prevent is familiar to anyone who has ever encountered a lane closure on a busy highway. Ordinarily, when the road narrows, smoothly flowing traffic can slow to a creep and the chance of rear-end collisions and sideswipe crashes goes up.
That has been happening lately on a 2-mi. stretch of the beltway at Telegraph Road, on the Virginia side of the bridge, which connects to Maryland over the Potomac River. In the spring, the road was reduced from eight lanes to six, and construction will require additional short-term lane closures between 9 p.m. and 5 a.m.
Officials illustrated the problem at a news conference July 24 by dumping rice into a funnel. Instead of being pulled through quickly by gravity, much of the rice got stuck.
The idea of the variable speed limit is that if the grains of rice are poured gradually or vehicles approach slowly, all of them will get through more quickly.
Just how slow they need to go depends on how much traffic there is. Traffic operators will monitor conditions using cameras and sensors along the road. If congestion occurs, they will lower the speed limit in increments of 5 to 10 mph.
The speed limit along the affected stretch will range from 35 mph to 55 mph and will be posted on signs that look like regular, white speed-limit signs, only with a light board in the middle.
VDOT plans to use the technology only during nighttime lane closures at first. If it’s successful, officials hope to deploy it near the Wilson Bridge during the day as well.
Ultimately, it could be used around the region both “for recurring congestion and definitely incident management,” Nicholson said. It could be useful even when there are no lane closures — for example, in an area like Tysons Corner, where a large number of drivers try to get on the highway all at once, he added.
Of course, the success of the variable speed limit depends on whether drivers obey it.
“Our biggest challenge on this, of course, is (people saying), ’You’re going to tell me, type A driver, that you want me to slow down in order for traffic to move more efficiently,’” said John Undeland, a spokesman for the bridge project. “That’s why we’re getting the word out.”
Officials are using advertising and highway message boards to get drivers to pay attention to the fluctuating speed limits. Virginia State Police are promising an active enforcement campaign.
Along with the variable speed limit, project officials are rolling out a new feature on the bridge Web site to provide real-time traffic information. The color-coded map will tell people what the travel speeds are at any given moment, allowing them to avoid congestion by postponing trips.
The feature uses the same data from camera and sensors that operators will use to change the speed limit. But the Web feature will be accessible 24 hours a day, not just during lane closures.
The work on the Telegraph Road interchange, scheduled to be completed in 2013, is the final phase in the reconstruction of the 7.5-mi. (12 km) Wilson Bridge corridor, a massive project aimed at relieving one of the nation’s worst bottlenecks.
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