Comment: only driving less solves the problem

But it does illustrate some of the problems of trying to green our transport system at the same time as reduce congestion, not necessarily the same thing.

Encouraging more electric cars reduces CO2 emissions, provided the drivers have switched from driving petrol or diesel motors. It certainly helps our air quality.

But it doesn't necessarily reduce congestion or competition for parking spaces, especially if people who once took public transport go electric.

Confusion over cutting emissions and cutting congestion is the problem with the Mayor's criticism of councils who charge owners of gas-guzzlers more for parking permits. It's questionable how much impact the charges will make on emissions: it takes a while for people to change car and if you can afford, say, a £40,000-plus Porsche Cayenne, then an extra £200 a year isn't going to make much difference.

But Boris Johnson's alternative focus - "reducing emissions by reducing congestion" - is no more effective. Even if rephasing traffic lights reduces congestion - hard to see, given that if some lights are on green more, other cars are sitting on red - it won't reduce emissions. He also wants people to "drive more efficiently". Good luck, but again it won't affect emissions.

Mr Johnson's only real solution is the more obvious one - getting us to walk and cycle more - because the only foolproof way of reducing congestion and cutting emissions at the same time is simply to drive less.