Reducing road space for cars can cut traffic

This is because people have choices in how they get around and many trips are induced by the perception that it is easy to get around.

Congestion is a good thing because people don't pay for the full cost of their trips, especially in terms of the costs they impose on others. Congestion is a non-monetary charge for the cost travellers impose on others. (Plus, some of this is about capacity constraints.)

From Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence (1998, Landor Publishing):

Prolonged, long-term gridlock is simply not reported, although there can be short-term disruption, and some increase in problems on particular local roads. In many cases, there were actually significant reductions in the total amount of traffic on the networks studied. On average, 14-25% of the traffic that used to use the affected route, could not be found on the neighbouring streets. However, the results varied substantially, depending on the context. For example, where schemes made public transport more attractive, they were more likely to encourage people to change mode than those which did not.

In explaining what was happening to the traffic, the following model of behavioural response emerged. Initially, when road space for cars is reduced, drivers simply change their driving styles in ways which pack more vehicles in, for example, by driving closer together. As conditions deteriorate, they then take the next easiest options - swapping to neighbouring streets, or changing their time of travel, leaving a bit earlier or later to avoid the worst of the traffic. As such adjustments also become problematic, a whole variety of responses is triggered, ranging from people altering how they travel, or where they carry out activities, through to people moving house or moving job, where the change in travelling conditions ‘tips the balance’ in a decision that was being made for other reasons anyway. Taken together, this third set of responses accounts for the measurable ‘disappearance’ of a proportion of traffic from the networks studied.

The project also highlighted the amount of variability which underlies apparently stable traffic flows, and which enables people to change their travel habits.

2. "Math whiz had I-5's number," from the Seattle Times, reports on the lack of gridlock as a result of construction closures on I-5 in the Seattle region.