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Dr. Delay

Traffic signal gizmos work from above, below

By LORRIE LYKINS
Published July 10, 2005

The topic of stop bars at intersections came up a few weeks ago in this column, and a reader wrote to remind me that motorists should know that stopping past stop bars or not close enough may fail to trigger the monitoring systems that control traffic signal phases. (Stop bars are the 2-foot thermal lines made of a plastic material that are applied to roads and indicate where you should stop at an intersection.)

This got me wondering about what sort of technology is used these days to monitor intersections, so I called Pinellas County traffic engineer Ken Jacobs.

Jacobs told me that in most parts of the county, traffic signals are controlled by underground "loop detectors."

"The way we do it right now is bury a piece of wire in the asphalt and energize it so that it senses the metal in vehicles passing through. The metal affects the magnetic frequency, and that's how the detector senses that a vehicle is at the intersection."

Jacobs said that all left turn phases and most main streets use loop detectors.

Jacobs said that the white stop bars at intersections are relative to the loop detectors and if someone rolls across the stop bar, the detectors will not register the vehicle.

Jacobs recommends that motorists stop at intersections with the nose of the vehicle on the stop bar.

"If you roll over the bar, the detector will essentially think the car went away and will not give the signal to change the light." This is the reason why signs that read "Stop here to get green light" have been erected, Jacobs said.

"Isn't it sad that some of us are so thick-headed that we need such signage?" I asked Jacobs. He declined to comment.

Jacobs said that while loop detectors usually work fine, there are drawbacks.

"Part of the problem with the loops buried in the roadway is that any time there is construction that goes on, the loops get torn up. The only recourse we have is to set the signals in those intersections on a phase we call "max recall,' which means that they always assume a car is there," Jacobs said.

Larger intersections can be a hassle too. "If you have six-lane roads with traffic going in both directions, the loops get expensive because we can end up with over 20 loops to cover one intersection," said Jacobs.

The latest in traffic monitoring is digitizing technology - video detection cameras that are mounted on traffic signal poles. The video cameras are programmed to memorize colors and register color changes as different vehicles pass through the detection zone, which is how they signal the light to change. Jacobs said that the video cameras pay for themselves quickly and are much less hassle than burying the loops.

The new cameras have been installed on McMullen-Booth Road at Mease Hospital between Curlew Road and State Road 580, and Jacobs said cameras are up on signal poles at 10 intersections on U.S. 19. Jacobs said the cameras do not record and are used strictly for traffic control and ensuring smooth flow, not law enforcement.

Cyclists will rule the road at least for a few hours this morning during the third annual Share the Road Ride downtown. One of the goals of the annual event is to raise community awareness about cyclists on the road.

The event Web site notes that there are about 85-million bicyclists in the United States and too many are seriously injured each year. More than half a million bike riders visit emergency rooms annually; 800 are killed in cycling accidents, many of them involving motor vehicles.

Some of the heavily traveled roads that will be affected by the Share the Road Ride include 62nd Avenue N, Coffee Pot Boulevard, Beach Drive, First Avenue N, Haines Road and 16th Street N.

For hale and hearty cyclists, an additional 20 miles has been added for the 40-mile "social ride." I don't know how much socializing I could do while trying to huff my way through 40 miles of cycling, but my hat, er, helmet is off to everyone riding this morning!

Be safe. And motorists, please be patient and alert.

Another community event that will affect downtown traffic next Sunday, July 17, is the Reebok Women's Triathlon. Event planners expect 900 participants at this year's race, which will begin at Spa Beach.

The 10K bike route will loop along the waterfront from Bayshore Drive through Snell Isle out to 34th Avenue N, onto First Street to 22nd Avenue N, then south on Coffee Pot Boulevard back toward the Pier. The 2.5K run will take runners south on Bayshore Drive toward the Bayfront Center and back. The event will start at 7 a.m. and should finish about 10:30 a.m.

Until next week, happy and safe motoring!

Please share your traffic concerns, comments and questions with Dr. Delay via e-mail at docdelay@yahoo.com

[Last modified July 9, 2005, 23:34:17]