Two wheels good, four wheels bad? That would seem at least to be the judgement to make about the mobility choices of developing countries. A week in Cambodia - following time in China and India - has persuaded me that if the citizens of developing countries can be persuaded to stick to mopeds, there are big benefits to their societies.

Phnom Penh's population is pushing two million, median age is 18 and wealth per head such that cars are an impossibility even for the middle class. So everyone rides on push-bikes or mopeds. But there is none of the cacophony of similar cities, the air full of horns and buzzing two-strokes. Everyone glides about on mopeds, two, three, even four up, along the boulevards the French laid out in the first half of the last century. There is none of the twitchy boorishness of Mumbai, where car drivers beep their way past lesser brethren, nor the might-is-right attitude of the new car owners of Beijing.

True, the traffic is hugely disorganised but it is also the best-mannered of any place I have been to. There are no high-powered motorbikes, so none of the aggression they encourage. The mopeds are 50cc to 125cc jobs, tuned for silence rather than speed. Standard speed is 15mph. There seem to be few accidents, which is a good thing, since helmets are rare. There's no showing off, no cutting up, no bad tempers, no self-righteousness, no machismo, not even much obvious pollution - everyone just gets around as swiftly and calmly as practicable.

How do they do it? It must be partly policy. Fuel is expensive, which encourages small engines, and there must be noise and pollution controls because rarely do you see smoke. But the explanation is more in social controls. There are hardly any traffic police - which must improve tempers; and the one bit of legislation that I tracked down (that only three people can ride on a moped) is not enforced.

I suspect that road manners are improved because there are few cars and hence so few people shut off in their steel boxes. And since hardly anyone wears helmets, the moped riders are not cut off from each other either.

Two questions. Will this good-tempered world last? And what can we learn from it?

On the first, there is a little hope, for one reason. Car drivers behave as though they were on mopeds - they weave in and out but they give way, they don't cut up other users and never get angry. As car ownership rises, let's hope that this benign attitude prevails.

And can we learn? If they could bottle up their road courtesy and sell it, the Cambodians could make a fortune. The real lesson for us is the case for using regulation to keep mopeds and motorbikes as silent and pollution-free as possible, and use tax to keep their engine size down. But once you become a four-wheel society, there is no going back to a two-wheel one - despite Red Ken's efforts to make us do so. London's mayor would feel at home in Cambodia.

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