And this is the road to Hell ...

If you were driving up Grosvenor Place to Hyde Park Corner at about 11am a few weeks ago, there is a chance that Sergeant Jim Hills and I were watching you from a video camera mounted high up on an overlooking building.

Sgt Hills is one of about half a dozen Metropolitan Police officers who observe the flow of traffic at key junctions in London, and adjust the changing of the city's traffic lights to keep it flowing efficiently. From his Olympian vantage point in a roomin New Scotland Yard (the same room where all London's 999 calls are answered), facing a battery of video screens, he is the man responsible for the longer-than-average wait you had at the lights at the junction of Hyde Park Corner and Grosvenor Place.

"Our job is to cure traffic problems before they develop," Sgt Hills says. "You have to be brutal with traffic: you have to say, I'm sorry about this lot, they're just going to have to sit."

This was one of the mornings the Household Cavalry crosses Hyde Park Corner on its way from Buckingham Palace to Hyde Park. It takes only about five minutes, but in that time the traffic has to be halted completely into Hyde Park Corner, one of the city's busiest "gyratories" (roundabouts), creating long tailbacks of fuming vehicles on the roads leading into it. Clearing the ensuing congestion is one of the chores of Sgt Hills's unit, the Area Traffic Control signals section.

They do it by prolonging the "green" period for the traffic penned up inside Hyde Park Corner while the Cavalry passes, and lengthening the "red" period on Grosvenor Place. This is why you have to wait up to 50 seconds for a green light, and then might not get through because the light remains green for only 10 seconds.

"Sometimes you have to create congestion for the greater good," Sgt Hills says. "Once you've got Hyde Park Corner cleared, you can clear up the tailbacks."

The entire operation takes about three minutes. Sgt Hills's unit, however, does not have the ultimate power to toy with us as they please. They can only introduce pre-determined alterations to the normal rhythm of the lights. These "tweakings" of the system and the grand scheme of traffic-light timings in London are set by the Traffic Control Systems Unit, a post-GLC quango controlled by the Corporation of London.

To gain an understanding of this scheme, I am briefed at the TCSU by a man under a cloak of anonymity. Even the location of the TCSU is supposed to be a secret. "Just say it's in Westminster. In the event of disturbances we may be a terrorist target," hesays. There are about 3,300 traffic lights inside the area once covered by the GLC, and they are all connected by telephone lines to the TCSU. The telephone bill for these lines alone is about £1m a year.

Some of these lights are "vehicle-actuated", portioning out "green time" according to the density of traffic, as determined by traffic detectors, but within fixed minimum and maximum green periods.

Others are part of the Scoot (Split Cycle Offset Optimisation Technique) system, state-of-the-art traffic control. It groups junctions together in groups of up to 50 and alters traffic-light timings on the basis of prevailing traffic conditions. For example, in the Marylebone Road in central London you cannot predict the length of a red light, but it has reduced the number of complaints about long waits on red. Scoot doesn't work everywhere in London, particularly in complicated areas such as Hyde Parkand Parliament Square, where there are multiple entrances and exits, and no single identifiable flow of traffic in any one direction.

The grand scheme is "minimum delay" - a policy of minimising waiting time equally across the city, giving priority to no single area.

Human nature must be accommodated in an age of decreasing public obedience. Traffic lights at some of the widest junctions have a long "all-red phase": the endless, annoying moment when all the lights at a junction are red. This is intended as a safety measure, or to give pedestrians a fighting chance. As human impatience grew over the last 10 years or so, this "all-red phase" was extended to as much as five seconds. In the past two years, traffic engineers have conceded defeat at these locations: the all-red phase is being reduced to about three seconds. Enforcement cameras are being introduced that are connected to computer systems. These identify jumpers of red lights and a fine is sent to the home address of the offending vehicle's owner.

In London, the maximum length of the traffic light's cycle of red to green and back to red is 90 seconds. After two minutes at a red light, "people think the lights have got stuck", and contemplate mutiny. After three minutes, they do mutiny. In the United States and the Far East, there are cycles of up to four minutes. People are accustomed to waiting for longer at traffic lights in car-oriented societies where little quarter is given to pedestrians. The 90-second maximum in Britain is also designed to give pedestrians a chance to cross the road.

Back at Area Traffic Control, the TCSU calls. A woman has telephoned to complain about the lack of a green-arrow, right-turn signal ("a right-hand overlap" in the jargon) at 3.30pm at the junction at the Embankment end of Albert Bridge. There is usually such a dispensation in the mornings during the peak period.

"My immediate reaction is that it isn't necessary," Sgt Hill says. The driver will have to be content with turning right on a normal green light after the usual wait. They will not establish a right-hand overlap just for her, no matter how impatient she gets. While saving her 20 seconds, a right-turn overlap creates a build-up of traffic in the opposite lane, which is blocked for its duration. This is the principle of "minimum delay" in application.

Inevitably, there are politics behind the length of time you spend at a traffic light. Current transport policy in London favours buses, because there is little political will in favour of investment in the Underground. So London buses will soon contain a transponder, which will cause the light to change to green as they approach: a cheap way to promote bus transport.

The defunct GLC had a policy of discouraging car commuting into London. This was occasionally reflected in a traffic-light regime that made driving into the city tedious. The 33 London boroughs that are "customers" of the TCSU all have the right to tailor their traffic-light regimes to suit them. Left-wing Islington, for example, is a paradise for pedestrians. There you can expect a lot of red lights at pedestrian crossings, even if no one is standing at them.

Most people are not aware of any of this. "The public does phone up to suggest refinements occasionally," Sgt Hill says. "But it's not often that they take the trouble. Joe Public doesn't even know TCSU exists. Well, not until he's been sitting at the red light for more than 60 seconds ..."