« Tail-gating tragedy reversed? | Main | Viruses as technical solutions: just science fiction? »

Driver behavior a tragedy of the commons after all?

Apparently some of what I wrote last week is wrong. I contacted Dr. Martin Treiber, a traffic expert in Germany who is working on an "adaptive cruise control (ACC) system that is designed not only to provide a comfortable acceleration and deceleration behaviour to the driver, but also to improve the overall traffic flow." He writes that there is sometimes a conflict between the two objectives of: 1) improving the experience of individual drivers and 2) overall traffic flow. Apparently the driving pattern that optimizes traffic flow feels unnatural to the driver.


Dr. Treiber writes that "the short-term user optimum is different from the system optimum or even the long-term user optimum. This adds 'less comfortable equals more comfortable' to the other dilemmas 'slower is faster' (speed limits may also be beneficial for traffic stability and capacity), and "less is more" (reducing temporarily the capacity by ramp metering leads effectively to more capacity since congested traffic entails a lower dynamic capacity)."

I also asked about excessive lane-switching as a possible tragedy of the commons. He writes, "The actual benefits are marginal, both in the simulations and in tests performed on real roads. Due to a statistical effect, however, the PERCEIVED advantage may be substantial" and cites a paper in Nature (vol. 401, p.35) explaining why people incorrectly think the next lane is moving faster, even when it's not. If lane switchers are slowing everyone else down and not getting there any faster but feel like they're getting there faster, is that a tragedy of the commons?

He says his models don't support the claim (see last week) that a single motorist can erase a jam by increasing his following distance and driving at a constant speed. "First, a single motorist can create a jam (by creating a large perturbation of traffic flow), but once a jam is created, beneficial effects due to different driving behaviour grow proportional to the fraction of drivers applying the different driving style, so a single driver has a minute influence. Second, by increasing the following distance, the capacity is reduced (since the capacity is always lower than the inverse of the average time headway). So, driving with (overly) long following distances will lead to more stable traffic but also to more congestions."

OK, looks like I should leave this problem to the experts!

Post a comment

(If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)


Type the characters you see in the picture above.