Why We Need Less Green Arrows
In reviewing the top ten questions asked this week, #7 actually had to do with traffic engineering (#3 was interesting but unrelated to traffic engineering — but if you’re curious, yes, the medicine is helping). Question #7 had to do with the appropriateness of left and right turn lanes at busy intersections. Also, when are green arrows truly appropriate and how long do turn lanes need to be, anyway?
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Getting back to question #7, turn lanes are recommended whenever the overall intersection operates more efficiently because of them versus without them. Note the word, “recommended.” This is because turn lanes are expensive and the associated roadway widening often requires buying land from adjacent property owners. Relocating utilities or ditches also impacts the cost of adding turn lanes. Cost/benefit analysis required.
Also, note the word, “overall.” Although adding a turn lane may improve the efficiency of turning vehicles, it’s possible the net efficiency of the intersection may diminish. Take, for example, a signalized intersection. Adding a turn lane (with a turn arrow) results in three things: 1) vehicles using the turn lane experience elevated moods, 2) the “green time” available for other vehicles is reduced, and 3) the “red time” available for other vehicles is increased.
The decision to add a dedicated turn arrow should be coupled with some serious signal programming to ensure the time allotted to said green arrow is appropriate based on time of day and, the concurrent demands of overall traffic (CDoOT). Fortunately, modern traffic signals are “traffic-actuated” and can detect the presence of all vehicles allotting green time accordingly and efficiently.
Regarding the physical length of turn lanes, well, as a minimum, they should be long enough to store the number of vehicles making that particular turn. And on high-speed roadways, turn lanes are often extended several hundred feet to allow turning vehicles to decelerate in the turn lane (instead of in the through lane).
So think twice when wishing for a green arrow. And when the car behind you starts honking, stop wishing and get moving…
Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience — online help available at TrafficEngineering.com/Services