Driving through central London is, like root canal dental treatment, something that should be avoided if humanly possible. Nevertheless, there I was recently rolling off the motorway and through the sociological layers of North London: Hatfield, Barnett, Golders Green ...

As the streets start to become terraced and increasingly multicultural, you eventually hit Islington which inevitably reminds you of Tony Blair, New Labour and expensive brasseries where they plotted. By this stage I was getting weary, frustrated and vaguely intoxicated by the fumes from the dense traffic. Vans bear down on you like big white dogs snapping at your heels.

Worst of all are the 4X4s. Only very rich people generally drive in London and they entomb themselves in Chelsea hearses; typically black Range Rovers with tyres as wide as cycle lanes. They take up half as much space again as a city car. The speed and sheer size of cars is one reason I've stopped riding my bike in Scottish cities. On London streets, where these monsters dominate the roads, cycling borders on insanity.

I was musing on monster truckism when I saw the sign: “Islington: the first 20 mph borough in London”. The words were in pastel colours and surrounded by balloons presumably to remind you of all the toddlers whose lives would be saved by this latest exercise in traffic calming. I groaned inwardly. Trust politically correct Islington to make it even more difficult to get through London.

Then I noticed something strange. I seemed to be moving more quickly the slower I went; not only that, the traffic seemed to have largely disappeared. It felt like driving through a small town, even a village. Where had all the cars and vans gone?

I looked around and they were still there but somehow more space seemed to have opened up between them, as if the road had become larger. Cyclists, who normally appear abruptly and perilously out of nowhere, were clearly visible and moving at around the same speed. This was quite remarkable.

Some critics of 20mph zones in cities say that they don’t reduce traffic speeds but I think they are missing the point. The average traffic speed in inner London boroughs like Islington is less than 10 mph. But the 20 mile limit seems to lift this significantly. The reason is that there is less bunching. You don't have the usual stop-go cycle of traffic racing away from lights and then having to slam on the brakes.

This is a race no one can win but it is normally difficult to avoid getting caught up in it. The weight of buses and vans pressing on behind pushes your speed over 30mph before you notice. At speeds of 20mph motorists experience capitulation. They no longer think they can catch up lost time by racing to the next junction.

At 20mph, white-van man settles back and listens to the football instead of trying to beat the lights. The traffic moves at a more level pace, which is good for fuel consumption, engine wear and drivers and pedestrians. You no longer have to brace yourself for the barrage of angry horns when you venture onto roundabouts or try to change lanes.

It is an exercise in creative group psychology and almost worth entering London to experience it. But let's not get carried away: this is still one of the most congested cities in the world.

At 20mph you start looking around you as well; not that there's a great deal of visual stimulation in an inner London shopping street but you start to notice how many beards and burkahs there are, as well as fixed wheel bicycles with coloured rims. But you don't have to look too long because you really do seem to move faster in 20 mph zones.

As soon as you leave and are allowed to go faster again, your speed drops and stress returns; at least that was my experience. Speed is largely to do with perception. But suddenly the traffic is all around you again, coming from all directions. You find yourself revving the engine again at traffic lights so that you will be able to keep up with the traffic flow when they change.

This counter-intuitive phenomenon is well known to traffic managers. On busy motorways such as the M25, the traffic flow is regulated by speed limits. When speeds are reduced to 50mph or 40mph the flow increases because there is less bunching, or “bouchon” as they say call it on French motorway gantries. The reason, apparently, is that vehicles can move closer together at slower speeds, thus effectively increasing the size of the road.

In Scotland, 20mphs zones are appearing in some urban areas, especially around schools. Edinburgh has committed itself to becoming the first Scottish 20mph city and Glasgow seems likely to follow suit. This is generally a good thing, and not just from the road safety point of view. Fewer people will be killed because accidents are rarely fatal at 20mph. At 40mph they are invariably lethal.

City centres are about to become much more pleasant places to be, both for drivers and pedestrians. However, I make one proviso. The congestion in Scottish towns and cities is as nothing compared to London. I suspect few people reading this column have direct experience of it but I used to live there and I know just how bad it can be.

True, Scotland has some of the most congested individual roads in the country, such as the M8 through Glasgow and the A8 into Edinburgh, but the areas of profound congestion are fairly small in the city centres. So the planners need to be just a little sensible here. They should avoid the temptation of imposing a blanket 20mph limit on all city roads irrespective of the traffic conditions. This is for the obvious reason that, if they do, people will simply ignore it.

In Queen's Park in Edinburgh there is already a 20mph zone in what is in effect a country lane. This perhaps makes sense on sunny Sunday afternoons when families are wandering around eating ice cream. But for the rest of the year it is unnecessary. When I cycle or drive on it I find taxis and even police cars ignoring the limit. This is pointless and just leads to frustration and disrespect for the law.

The point is that 20mph limits, like speed cameras, are not there to punish motorists or to provide revenue for local government but to save lives, improve the quality of life and have traffic moving quickly. It's not social engineering.

Personal transport is not going away. When we’re all driving electric cars the problems of congestion will still be with us. 20mph is a great innovation, and everyone can win but only if it is introduced with an ounce of that rare commodity: common sense.