This link has been sitting in my tab bar for a week now, and this morning I said “enough already, write the post!” So here goes.
Courtesy of Autopia, a new study says that reducing choices that drivers have in complex road systems may reduce congestion. Besides having an awesome name (at least the part before the colon), The Price of Anarchy in Transportation Networks: Efficiency and Optimality Control, the study provides a nice summary example:
Say you have two options for reaching destination X. One of them is a narrow bridge, the other a longer but wider highway. The combined travel time for all drivers is shortest if half take the bridge and half ride on the highway. But drivers only know which route is most direct, so they all head for the bridge. The crush of vehicles creates a traffic jam, so the next batch of drivers choose the highway. Bridge traffic clears as a result, causing more drivers to choose this route until it clogs again.
This back and forth continues until each route ends up taking the same amount of time, a phenomenon known as the the Nash Equilibrium. The problem is that with this equilibrium, average travel time ends up taking longer than it would if the drivers had split the two routes 50/50.
This brings us to the local example. While there hasn’t been evidence that travel time was cut, there is some evidence that the removal of the I-35W bridge for over a year did not significantly impact commute times. There are all kinds of other variables to factor into that situation (increased bus routes, shifted commute times, additional lanes on certain roads), but we know that traffic levels have been steady over the time the bridge was down, so people were not traveling less. Maybe we should invite these guys to examine the rest of our bridge crossings and tell us which one to eliminate? (I vote 10th Avenue)