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Negotiating the turn lanes

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POSTED: June 21, 2013 9:30 a.m.

Today’s public-service message is a bit different from past ones.  While neither natural-resource or garden-based, it still fits the mission of Cooperative Extension to give information the people of Georgia that they need to improve their lives.  
This offering is about driving and knowing about how traffic signals work. I think we both can agree that you and I are the best drivers on the road today (but I’m not too sure about you). Apparently, there is a dearth of knowledge in Richmond Hill about how traffic signals at intersections work, based on my personal observations over a long series of enforced idle spent at each approach to Highways 17 and 144 for lo these many years.
Traffic signals today are more responsive to moment-by-moment changes in traffic than ever before. The signal at 17 and 144 is an excellent example of state-of-the-art traffic management at a medium-volume signalized intersection. (Abercorn at White Bluff is the Big Dawg around here, and I do not think any of us want that volume of traffic in Bryan County). When you pull up to the intersection, you might notice — though clearly many have not — that there is a white horizontal stripe perpendicular to the lane of travel. This is called a stop bar. This is where the first car in the line is supposed to pull up while waiting for the light to change.  
The stop bar is not placed there randomly. Traffic engineers use their templates to put the bar close to the intersection but far enough back so through and turning traffic on other streets does not collide with waiting traffic.
In most signalized intersections today, just behind the bar, there is a loop cut into the travel lane. The cut is about the width of a pencil and makes a loop half a lane wide and at of varying length. Into this cut is placed a wire-detector loop that is connected to the traffic-signal controller.  When a vehicle passes over the loop, it disturbs the electrical current through the loop, which tells the controller that a vehicle is present. The electronics in the control box switch the program to run the protected left turn lamps (or whichever traffic lane is sensed) on the next cycle. If no vehicle is sensed, the control program skips that display and moves on to the next movement in the traffic rotation.
A traffic signal has completed a cycle when the program has rotated through all the traffic movements sensed and is back to where it started. The signal at 17 and 144 changes its programming moment by moment in response to current traffic conditions. For instance, the traffic-control program might translate as: green for 30 seconds maximum, less if no traffic detected.
At about 5 p.m. Friday, June 7, there was a backup of traffic on 144 westbound at 17 waiting to turn left onto 17 southbound. I was about eight cars back in what became a 20-car-long line waiting to turn left. After waiting through two complete cycles of the signals in which the left-turn arrow did not cycle, I did a little scientific-method exercise.  
Since I had travelled through the intersections a couple times earlier that day and all the signals seemed to be working, I tested my hypothesis that the lead car in the queue was not pulled up far enough for the loops to sense the presence of a vehicle. When traffic allowed, I pulled into the through lane and, sure enough, the lead car in the left-turn lane was sitting so far back the loops did not detect the vehicle. This fearless leader of the left-turners apparently had not yet started to wonder why the light had not changed, though everyone behind did. Eventually, the car crept forward and was detected.  
Whether this success was due to a cramp in the driver’s foot from holding down the brake pedal so long (probable) or a glimmer of enlightenment (doubtful) made little difference to the vehicles behind. I fear the drivers were cursing the city, Department of Transportation or anybody else rather than the guilty party: the ignorant lead driver.  
The detector loops also are used to tell how long a lamp for a movement should last. My favorite is the left turn from 17 southbound onto 144 eastbound. If left-turning traffic follows the vehicle ahead of them at a close (but still safe) distance, it is amazing how long the lamp will stay green and how many vehicles can be cleared through the intersection.
It seems that as long as the loop induction is disturbed, the signal will stay on. But let one driver dawdle and leave a four-second gap, and the light will change and only a few vehicles make it through. Most often, the dawdler makes it through but everyone behind them is made to wait until the next cycle.  
As is normal in America today, incompetence is rewarded. So it is not some faceless DOT engineer you should be upset with while you wait at the intersection. As Pogo said: “We have met the enemy, and he is us!”

Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.

 

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