It was a brave move to introduce congestion charging into central London in 2003.  Transport for London subsequently published detailed annual reports on how the scheme was affecting traffic conditions.  One trend observed has been that the impact on congestion tended to decline over time.

I thought I might have missed this year's report, usually published in the summer.   I made enquiries of TfL and was told that the five-year assessment programme has ended, so no more detailed analysis.  There will be some reference in the Travel in London report due next March.  However, because this covers all aspects, we can't expect much detail on congestion charging.

This is a pity.  The London congestion charging scheme has international status.  There is wide interest in its impact.  If the reduction in congestion declines over time, the the case for other cities taking up the idea is weak.  One reason why such a decline might be expected is that those drivers who pay the fixed daily charge to experience less congestion will tend to travel further to gain access to a wider range of destinations in the amount of time they allow themselves for daily travel.  In this respect, reducing congestion by charging is similar to reducing congestion by road widening - you get 'induced traffic' which tends to offset the congestion-reduction benefits.