Archive for July 31st, 2008

One of the themes readers will discover in Traffic is the idea that slower can be faster. Ramp meters on highway entrances, for example, keep the mainline flow going more smoothly by temporarily holding up drivers on the ramps. The individual driver suffers a moment of time loss, but the whole system moves better.

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.

It’s counterintuitive, but slower is faster. As individual drivers, we pursue our immediate interest, which is to get ahead as quickly as possible. But in traffic, this works against the system as a whole. As Phil Goodwin once described it in another context, “It is one of those cases where Adam Smith’s individuals pursuing their own best interests do not add up to Jeremy Bentham’s greatest good for the greatest number.” These sorts of initiatives, which are lumped under the heading of “ITS,” are just one of the ways we can “think,” and not build, our way out of congestion.

The last time the U.S. saw a substantial drop in traffic fatalities was 1974, when the double whammy of the recession and the 55 mph speed limit (a reaction to the fuel shortage) saw the number of fatalities drop by some 9000 (it’s still debated to what extent this had to do with the economy and to what extent it had to do with the speed limit).

But this year, which promises to see the first annual drop in vehicle miles traveled in 28 years, is also shaping up, if trends continue, to see fatalities drop below 40,000 for the first time since 1961.

This comes from a new preliminary report from Michael Sivak, head of the Human Factors division at the University of Michigan’s Transportation Research Institute. What’s even more interesting, notes Sivak, is that the drop in fatalities we’re seeing seems to exceed what we might expect based on the drops in fuel consumption and miles driven. It suggests a fascinating trend: U.S. drivers might not only be driving less, but driving differently.

Sivak, whose work (especially this one) has been an influence in Traffic, points out several factors that may underlie this. For one, the mileage reduction has been greater on rural highways, which are statistically riskier. Also, he suggests, the mileage of lower-income drivers, who are also statistically over-involved in crashes, may be have disproportionately dropped. And more people may be driving more slowly to save fuel, further reducing the risk of a fatal crash. I might even suggest another possible factor: Larger, higher consumption vehicles like SUVs and pickups — which pose a greater risk to other drivers — seem to be being driven less, which could also improve things for everyone.

There’s more parsing of the data to come from Sivak, so stay tuned…

Astute listener Susan heard me talking (briefly) about Dutch bicycling on the Leonard Lopate Show and pointed out something interesting I neglected to mention: The existence of a law that puts a higher burden of responsibility on the car driver in crashes involving cars and cycles.

As John Pucher at Rutgers notes in a report, “motorists are generally assumed to
be legally responsible for most collisions with cyclists unless it can be proven that the cyclist
deliberately caused the crash. Having the right of way by law does not excuse motorists from
hitting cyclists, especially children and elderly cyclists.” (my italics).

One would intuitively think this would lead to a greater caution amongst the part of drivers (who are, after all, the only ones operating heavy machinery), and thus more safety for cyclists, and I wonder if there’s any state law in the U.S. that has anything remotely similar (I would suspect not). But I’m also curious about any good studies about the safety rate of Dutch cyclists before and after the law, which I believe was passed in the late 1990s. Anyone seen anything?