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Heavy Traffic: The Great Merge Debate

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A quick true/false quiz:
– Late merges at lane closings are rude and make traffic worse.
– Variable speed limits only confuse drivers.
– EZ Pass-style automated pay lanes cut down on accidents near tollbooths.
– The nearer you are to a major city, the deadlier the highways.

All false, as you may have guessed.

Technology marches on, but traffic still crawls. Moreover, our knowledge of what makes traffic stop and go inches forward at roughly the same rate. According to a study by the Texas Transportation Institute, total traffic delays in the U.S. increased by 655 percent in the nation’s 26 largest urban areas from 1982 to 2003. It turns out that a new generation of traffic analysts, more computer geek than slide-rule engineer, have demonstrated rather convincingly that everything we know about how to make traffic flow smoothly and efficiently is dead wrong.

More highways don’t help. Fewer highways don’t help. More bike lanes don’t help. No bike lane at all doesn’t help. Raising the speed limit doesn’t help. And neither does lowering it.

What, for the love of what’s left of our lives, can the nation’s engineers, in this, the year 2009, do about traffic? Or are we destined to average two or three hours a day immersed in it, fighting road rage, letting our attention drift, or alternately steaming or shouting behind the wheel, traveling at 10 miles per hour on the world’s finest interstate highway system?

Start with the late merge:

The concept of delaying a merge when two lanes are funneled into one, as often happens during road repair work, caught on with traffic engineers after studies showed that traffic flow speeded up as much 15 percent over the old “merge early and politely” philosophy. The driver you shake your fist at while he speeds by in the lane to be closed is actually making your trip shorter. What happens is that all that early merging creates an underused lane — everybody crammed into one lane while there are still two useable ones. Merging when necessary, in zipper fashion, is the better way to go.

As for variable speed limits:

While automobile manufacturers are busy making smarter cars, traffic engineers are now insisting on a similar effort at building “smart highways.” One way of doing this is with dynamic speed limits. Sensors in the roadways relay information about slowed traffic, and speed limits upstream are adjusted by computers to avoid the dreaded “shock wave,” in which a line of cars are all quickly forced in succession to decelerate to zero. Until they get used to it, drivers tend to feel like they are going slower under variable speed systems, but tests show that variable speeds result in less stop-and-go driving (and thus a reduction in fuel emissions), as well as an overall reduction in trip times.

In an overall sense, slower is faster, as Tom Vanderbilt explains in Traffic: Why We Drive the Way We Do (and What It Says About Us). Since traffic is “nonlinear,” accurate traffic predictions are fiendishly difficult to make. Albert Einstein, according to Vanderbilt, observed that “any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.”

And the EZ pass?

Nope. Vanderbilt writes that auto-pay systems should reduce vehicular mayhem at the tollbooth by eliminating all that fumbling for change, if nothing else. But it doesn’t work that way: “Drivers approach at a higher speed, with nothing to stop them from zooming through the toll plaza, while other cars, finding themselves in the ‘wrong’ lanes, dart out and jockey among lanes more than they would have under the old system.”

Counter-intuition is the rule of the road. Those dangerous rush hour highways? It turns out that the slower speeds caused by traffic congestion lower the risk of fatal accidents. Statistically, the most dangerous driving is done on secondary roads in low-traffic rural areas, where high speeds, blind curves, and excess alcohol combine to cause carnage on rural two-lane highways.

The entire field of traffic engineering abounds in counter-intuitive findings. A study in Finland showed that adding reflector posts to a curved highway actually increased accidents. It turns out that a curve marked with reflectors and advisory speed signs can lead motorists to go faster around the curve than they ordinarily would. Similarly, “Children at Play” signs do not slow drivers down, or reduce accidents, and some transportation departments have stopped posting school zones for this reason.

As Vanderbilt accesses the matter, “There is a simple mantra you can carry about with you in traffic: When a situation feels dangerous to you, it’s probably more safe than you know; when a situation feels safe, that is precisely when you should feel on guard.”

–Patrick T. McCoy, Geza Pesti, et. al. "Dynamic Late Merge-Control Concept for Work Zones on Rural Interstate Highways." Journal of the Transportation Research Board. 2001 1745 20-6. DOI 10.3141/1745-03

–Vanderbilt, Tom. Traffic: Why We Drive the Way We Do (and What It Says About Us). New York, Toronto: Alfred A. Knopf, 2008.

Breton, P. et al. “Shock Wave Elimination/Reduction by Optimal Coordination of Variable Speed Limits.”Transportation Research Part C: Emerging Technologies. June 2005 13 3 185-209

Kallberg, V.P. “Reflector Posts—Signs of Danger?” Transportation Research Record. 1403 57-66.

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