A black cab revolution is just around the corner. David Bowen reports
Whatever the merits of London cabbies, they are not usually considered to be driving at the cutting edge of technology. But they will be from next year if they work for the Computer Cab network.

In the relentless battle against minicabs and rival black cab "circuits'', Computer Cab - which controls 2,500 taxis - is linking into a satellite navigation system hitherto used in Europe only by tanks, ships and aircraft. The company will know where every one of its vehicles is to within 100 yards, and when a call comes in, it will be able to send the nearest cabby speeding to the scene.

That, anyway, is the theory - and after investing more than £3m of their own money in the system, the drivers must hope it will work. Or they really will have something to swear about.

Computer Cab is an offshoot of the London Taxi Drivers' Association, a co-operative formed 27 years ago to combat the growth in minicabs. It now has a turnover of £35m, an operations centre that looks like a City dealing room, and its own research and development department - which is why it is able to install such an advanced control system.

The aim of every taxi driver is to keep as busy as possible. Unlike minicabs, black cabs are allowed to pick people up on the streets. But the most lucrative business comes from accounts - companies ferrying people around town. Here, the cabs are in direct competition with radio-controlled minicabs. The best service wins the plum contracts.

The LTDA's first weapon in the war against minicabs was the radio. The dispatcher at headquarters relied on the drivers to broadcast their position, and so tried to minimise the delay before each pick-up.

In the Seventies the association installed its first computers to handle accounts and, in 1984, when Computer Cab was formed as a separate company, a more powerful system was activated. This could give priority to important customers and present them with properly itemised accounts - even on computer disc if they wanted. From the driver's point of view, though, this was still a radio service: a dispatcher read out the journey details, and the driver would call back to say how much he had been paid.

During the Eighties, business grew at 30 per cent a year, and Computer Cab and its rivals started to look at other ways in which technology could give them an edge - in particular, by putting computers into taxis. In 1984 Geof Kaley, the managing director, went to look at systems in Canada, and examined embryonic Automatic Vehicle Location systems. There was an obvious advantage in knowing where each taxi was (rather than where its driver said it was) so that the nearest could always be sent. Computer Cab's lab started work on AVL.

In 1989 every taxi was given a computer. The dispatch information appeared on a screen, and the driver typed in the financial details. Linked to this was a credit-card system - the driver swiped the card through the computer, and the amount was registered at headquarters.

Even with the computers on-board, the operators had to talk to the drivers by radio to find out where they were. Two rival circuits were threatening to leapfrog Computer Cab by fitting "zonal" systems, which divided the city into a grid and knew which taxis were in each square.

Computer Cab did not believe zonal systems worked well, because they still relied on the drivers saying where they were, and continued to work on AVL. In 1990 the recession slammed in, and the work was put on hold. But two years later, though London was still in the grip of the downturn, the drivers started calling once again for better technology.

"We were getting a lot of pressure to move to an AVL system," says Mike Galvin, head of operations. "We said we didn't think we needed it at the moment, but if they wanted it they were going to have to pay for it." A compromise was reached. The company would pay half the £6m cost; the rest came from the cabbies - at £15 a week each for seven years.

The system, which uses Logica software, will be introduced shortly. The telltale sign of a Computer Cab will be a plastic mushroom on the roof that will pick up messages from one of 28 satellites in the Global Positioning System, a US military network that was used to track tanks in the Gulf War.

On flat ground, GPS is accurate to within five yards; in undulating London, it will be closer to 100 yards. The whole fleet will be "polled" every 10 seconds so that, in theory, Computer Cab could have a giant Battle of Britain-type map with thousands ofcabs moving across it. In fact, the operators will content themselves with detailed maps of smaller areas on their computer screens.

If you ring Computer Cab, the operator will immediately send out a signal to the nearest cab. If you want to smoke, or are disabled, the signal will go to the nearest suitable taxi. If you are ordering in advance, the computer will store the booking and reactivate it 10 minutes before the booking time.

"The system will be capable of taking and dispatching 18,000 jobs an hour," Mr Galvin says. "The peak now is about 1,500 - we're playing it for growth.''

The computer also analyses where bookings are coming from, and can guide drivers to areas where there is most work. To make this easy to check - drivers are not supposed to look at the screen while driving - icons (pictograms) will be used.

The tracking system has one other advantage. If a cab is hijacked or has a troublesome passenger on board, the driver can press an emergency button and other taxis - or the police - will be able to follow it.

The GPS could be used to help direct drivers. Computer Cab has no intention of doing this. Mr Galvin says the Knowledge, the black-taxi drivers' formidable training, makes this redundant. He acknowledges that it might be possible for a minicab company touse AVL to give its drivers some of the navigational skills that cabbies carry in their heads, but is sceptical how well it would work in a "dynamic" (roadwork-plagued) city like London.

But there are still doubts about the system. Mike Holleyoake, a driver, wrote to Mr Kaley wondering why Computer Cab could not have bought an off-the-shelf system, and described AVL as "the bottomless pit syndrome". The economics look extravagant, especially as Mr Galvin says it will be difficult to sell the system to other cities. But that is for Computer Cab to worry about. For passengers, it will be comforting to know that they are being lectured by one of the highest tech taxi drivers in the world.