From Aaron Renn in New Geography comes this graph:
...supply of lanes creates its own demand by drivers to fill them. Hence building more roads to reduce congestion is pointless. But if we’ve really reached peak car, maybe we really can build our way out of congestion after all.
Renn points out that projections of growth in car use haven't been matched by actual use growth since at least 1999. Whether all this applies in the UK is moot but the National Traffic Survey suggests that car use is declining - the survey reports that since the mid-'90s trips by private modes of transport fell by 14%. The graph doesn't show the same steep decline but there is no doubt that travel habits are changing. And, as Renn points out this has significant implications for transport policy.