Crowds of Londoners thronging to hear Mr Handel's big band rehearsing his new Music for the Royal Fireworks caused such congestion that traffic on London Bridge was brought to a standstill for three hours. That was in 1749; congestion is not new.

Crowds of Londoners thronging to hear Mr Handel's big band rehearsing his new Music for the Royal Fireworks caused such congestion that traffic on London Bridge was brought to a standstill for three hours. That was in 1749; congestion is not new.

A few Sundays ago I met it again. A journey into the City, which should have taken no more than an hour, took three and a half. This time it was no accidental stricture, but was organised by the police, doubtless under orders from government. There was some sort of protest going on (whether in favour of peace or in disfavour of Bush I am not sure - the agitators' banners seemed equally divided) and the presumably innocent population-at-large was clearly not meant to see it.

All manner of roads were peremptorily closed; the remainder were choked with frustrated and fuming traffic, finding every successive escape down a side street prevented by a stony-faced police motorcyclist.

The whole thing was superbly orchestrated, with all those diversionary notices on hand and squadrons of police and ambulances tucked away in those streets denied to us. No notice had been given, and -- which seemed even more sinister -- no mention of it was afterwards made in the news. Was that silence imposed to protect Bush, or the protesters, or us? Do not ask.

Apart from being an interesting sidelight on our democracy, it was a clear lesson on how to cause congestion: Reduce our routes, restrict our roads, impose frequent stops, do anything that will reduce velocity to viscosity.

It is as simple as that. To make traffic flow, make it go! The faster a car moves, the sooner another can come and whizz likewise. Speed limits are inappropriate, just as they are on motorways, where the natural limit on each lane's capacity for traffic is set by the natural (to the average human) safe limit to minimal spacing between vehicles. It is an interval of time, and it is about two seconds.

It follows that each motorway lane may carry no more then 1,800 vehicles per hour, regardless of how fast they go, except that when speeds drop below about 30mph the length of each vehicle (negligible at 70mph or more) becomes significant, and the prospect of the whole lane grinding to a halt becomes a real danger.

Reduce the number of lanes, put a speed limit on all or any of them, and the propensity to congestion increases; the principles of fluid dynamics apply to all roads, open or urban. Any constriction, no matter how brief, limits the flow capacity of the road to the remaining width. The only way to restore flow is to increase its velocity through the constricted passage.

The bus lane presents a typical case. Its real object is to make public transport appear efficient by preventing private transport from functioning as it should. If general traffic were encouraged to move as swiftly as it could and should, the buses would find it so hard to keep up that the question of them being obstructed by cars could not arise.

Much the same goes for all the other anti-car tricks: traffic lights where they are not needed, traffic lights with extended all-red phases, "traffic-calming" bollards and extended kerbing and islands, sleeping policemen, these and many more devices deliberately provoke congestion by reducing road capacity.

If you want to see a demonstration of traffic management where people are left to their own devices, find a crowded shopping street. Pedestrian traffic knows no speed limit: each does what he wants where he wants and you will see some walking twice as fast as others, some standing still, some running, They go in all directions and they neither give signals nor look in mirrors. They do not collide.

I remember blissfully a day long ago when all the London buses were on strike. How wonderfully fast and fluid was our driving that day! If that inducement to speed is not enough, try this: The faster the traffic flow, the less the contamination of the air by exhaust emissions.

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