Suppose the average family had to go only three-quarters as far to reach its places of work, education, shopping and entertainment, and that these were grouped more in town centres so more errands could be combined in each trip. This would reduce the total number of journeys to ¾ x ¾ = 9/16 of the current level - just over half.

If trips are shorter, more of them can be walked or cycled. If destinations are grouped together, buses become attractive alternatives for more journeys. If these two factors together reduce vehicle fuel use by another quarter, and more concentrated journey patterns allow an increase of say a third in the average occupancy rate of vehicles, we have another ¾ x ¾ = 9/16, another near-halving, of car miles.

If journeys are shorter and less frequent, and there is less congestion because there is only a third as much traffic (9/16 x 9/16 = 81/256), driving at relatively moderate speeds would be more acceptable, gas-squandering stops and starts less necessary, and violent acceleration would look ludicrous to all except those worried about the size of their genitalia. This could reduce the fuel consumption of existing cars by a further quarter. And, finally, suppose that this different pattern of driving made buyers prefer cars that were optimised for short, predictable, low-stress journeys: lighter, slower, simpler, less aggressive cars. These could reduce fuel use by another quarter, giving another ¾ x ¾ or 9/16 reduction from changes to driving and vehicles.

Multiplying all these savings - reduced and combined journeys, the shift away from cars, higher occupancy rates, more relaxed driving and vehicles designed for it - and we get a better than four-fifths reduction in the greenhouse emissions from personal transport. Of course, the ratio of ¾ is arbitrary. A more ambitious 2/3 at each step could reduce emissions by more than 90 per cent.

Read more from the New Statesman 'Heat and Light' energy supplement at

www.newstatesman.com/supplements/energy