Friday, August 1, 2008

Variable Speed Road Tolls

This post by Tom Vanderbilt, in which he talks about the successful impelementation of variable speed limits in the UK, got me thinking about variable speed limits.

I’m in D.C. for the day, and I’ve been interested to note that an idea that’s had success in places like England’s M25 motorway is being introduced here. It’s called “variable speed limits” (wait, aren’t they all variable, you’re asking?), and the basic idea is that when a section of highway has become congested, rather than having upstream vehicles simply drive at full speed into the gelling pack, those drivers are given instructions to drive at specific speeds, lower than the typical speed limit. Instead of driving into a stop-and-go mess (in which a lot of time and fuel is wasted stopping and restarting), following cars approach at a slower, smoother pace. When the new speeds are obeyed (in the U.K. they’ve mounted cameras to enforce this), engineers have found they can achieve greater “throughput” through bottlenecks.

I agree with the idea that "slower is faster," but I cannot get behind instituting traffic cameras to enforce speed limits. Telling drivers to reduce their speed "for the greater good" or else get punished with a huge speeding ticket seems like a rather heavy-handed, clumsy way of achieving the desired goal.

I think a better way to accomplish variable speeds would be to financially reward people for driving the speed that optimizes the flow of traffic. Assuming that a “pay-per-mile” tolling scheme is in place, drivers could be charged not only for where and how far they drove but also how fast they drove. As drivers approach a traffic jam, there could be one rate for traveling at 65 mph, another for 55 mph, and another for 45 mph, with the cheapest rate assigned to the speed that optimizes traffic flow.

This solution provides a transparent set of incentives for drivers to reduce their speed without resorting to some draconian enforcement mechanism (i.e. cameras that rigidly enforce lower speed limits).

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