Stephen Rees's blog

That is the way that the Texas Transportation Institute puts the question in its latest report comparing congestion across US cities.

You can read the way they answer the question in this excerpt but I am going to copy the whole of the conclusion

The analysis shows that changes in roadway supply have an effect on the change in delay. Additional roadways reduce the rate of increase in the amount of time it takes travelers to make congested period trips. In general, as the lane-mile “deficit” gets smaller, meaning that urban areas come closer to matching capacity growth and travel growth, the travel time increase is smaller. It appears that the growth in facilities has to be at a rate slightly greater than travel growth in order to maintain constant travel times, if additional roads are the only solution used to address mobility concerns. It is clear that adding roadway at about the same rate as traffic grows will slow the growth of congestion.

It is equally clear, however, that only five of the 85 intensively studied urban areas were able to accomplish that rate. There must be a broader set of solutions applied to the problem, as well as more of each solution than has been implemented in the past, if more areas are to move into the “maintaining conditions or making progress on mobility” category.

Analyses that only examine comparisons such as travel growth vs. delay change or roadway growth vs. delay change are missing the point. The only comparison relevant to the question of road, traffic volume and congestion growth is the relationship between all three factors. Comparisons of only two of these elements will provide misleading answers.

And you thought I was long winded! They are being careful, and they only talk about “slowing the rate of growth” – as though congestion getting worse is inevitable. Moreover it only deals with the transportation aspects of the problem and as we all know (or should do) transportation and land use are two sides of the same coin – and just studying one of them as though it has no effect on the other is pretty bloody silly. BUT it does put the lie to what Kevin Falcon has been saying. No one has managed to cure congestion by building roads. The only thing that a few cities have managed to do by building roads frantically is to keep up with the growth of congestion – so it doesn’t get any better but it gets worse more slowly.

There is also a logical fallacy here. All the TTI has done is look at what has been done in a lot of US cities. It does not actually tell you very much about what can be done, other than point to the failure of road building on its own as a solution. It also does not look at any other indicators of liveability other than traffic congestion – and I would venture to suggest that we need to be more concerned about a lot of other things – like the quality of life or the environment or the effectiveness of other spending programs that could have used the money that otherwise gets wasted on ineffective highway expansions.