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May 9th, 2012 MARTY SMITH | Dr. Know
 

Dr. Know: Idling Thoughts

If idling causes pollution, why do we have to stop at lights before entering the freeways?

drknowILLUSTRATION: Hawk Krall
     
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On the drawbridges in Portland, there are signs telling you not to sit with your car idling because of pollution. So why do we have to stop at lights before entering the freeways, forcing thousands of cars to idle? 

—Confused

Fasten your seat belts and keep that heart medicine handy—we’re about to head into the racy, forbidden world of ramp metering! “Ramp meter” is, of course, the official name for the signals you’re describing, C., and according to the latest in traffic-control science, they save you both time and fuel in the long run.

As you’ve probably noticed, once traffic gets heavy, it doesn’t take much to interrupt its smooth flow. One guy taps his brakes, two people behind him hit theirs, and pretty soon a wave of stoppy despair, called a “traffic shock,” is propagating backward up the road.

Experts sometimes use the equations of fluid dynamics to model these traffic waves. The analogy isn’t exact (possibly because individual molecules don’t pause to scream obscenities at each other after each interaction), but in theory ramp metering reduces the opportunities for such shocks. Without it, you’d burn more fuel in stop-and-go traffic than you’d save by skipping the signals.

“Theory, schmeory!” I hear you saying. “Those signals just slow me down!” In 2000, the citizens of Minneapolis felt the same way, and, in a failure of democracy comparable to determining prime numbers by popular vote, they successfully pressured their state legislature to have the meters disconnected, just to see what would happen.

With a weary sigh no doubt audible in Iowa City, officials set about disconnecting the signals for a month. Sure enough, they found that average travel times rose 22 percent, accidents increased by 26 percent, and emissions increased, albeit only by the equivalent of 400 cars. Science 1, Democracy 0.

 
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