Sunday, May 24, 2015

Traffic myths by Tom Vanderbilt

Tom Vanderbilt, author of the excellent transportation book Traffic: Why We Drive the Way We Do (And What It Says About Us), wrote about some common myths about traffic in today's Post. In light of the current plan to expand I-66 we especially liked his first myth:
1. More roads = less traffic. This is the granddaddy of all traffic myths, one still held dear by the average driver and certain precincts of state highway offices. More funding for more roads is on the way in Texas, where the governor declared that residents are “tired of being stuck in traffic.” On Memorial Day, it will assume the stature of an intuitive truth: If they just built more roads, we’d be home by now.

But that reasoning doesn’t stand. First, Memorial Day is one of a handful of peak travel days. “You don’t build a church for Easter Sunday,” as the saying goes (a lesson most shopping malls in America have not heeded, judging by their acres of empty parking lots). More broadly, whatever short-term gain that comes with capacity expansion is generally eaten up by longer-term behavioral shifts. As University of Toronto researchers Gilles Duranton and Matthew Turner describe what has been called the “fundamental law” of traffic congestion: “People drive more when the stock of roads in their city increases.”

Aren’t planners simply keeping up with population growth? Perhaps. Except that growth in vehicle miles traveled has consistently outpaced population growth over the past few decades. And as transportation researcher David Levinson has noted, U.S. roads are already bristling with spare capacity and inefficient use is rampant (such as too-large single-occupant cars all traveling to work at the same time on too-wide lanes). He argues that we should focus time and resources on using the highways we already have more efficiently, rather than on building more.
His second myth greatly affects bicyclists and pedestrians:
2. Faster roads are more efficient roads. Speaking of efficiency, common sense says that when drivers are humming along at or even above the speed limit, highways are performing at their best. The German autobahn, with its (shrinking) speed-limit-free zones, is often offered as a shining example. It must be those slower drivers who are holding things up!

But as good as fast-moving roads might be for the individual driver, they are not the best for the most drivers. As data gleaned from in-pavement “loop detectors” on Washington state highways showed, those highways were able to achieve “maximum throughput” — pushing the most cars through one segment of road in a given time — at speeds that were roughly 80 percent of the posted speed limit of 60 mph. Why? At higher speeds, drivers need to allow more “headway” between vehicles, meaning more space is required per vehicle. And faster-moving traffic tends to break down more quickly, with more severe “shock waves”; it takes a lot longer to recover from a traffic jam than to get into one. I have been told, anecdotally, by traffic engineers that the left-hand “passing lane” can become congested first. (I’ll leave it to you to decide if karmic justice is at work there.)

In a “speed harmonization” experiment on Colorado’s I-70, the state highway patrol was able to improve traffic flow by enforcing, via phalanxes of patrol cars, 55 mph speeds — thus preventing the instability caused by people driving fast into packs of congested, slower-moving vehicles.

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