And it's not simply the ability of the cabbie to get from A to B that colours our popular wisdom about them. The Knowledge is far more than mere navigational skill; it is a test to ensure that the driver understands every point a passenger might wish to pass along a particular route. As one driver from north London tells me, "Learning a route's like learning a song, word for word. And there's 400 of them." The training is so comprehensive that black-cab drivers, scientists recently discovered, develop bigger brains. When I tell onehe might have an unusually large hippocampus (the part of the brain we use to navigate), he drily responds: "My wife been telling stories about me again?" You don't get that banter from a GPS.

But not everyone wants banter, and every day new advancements make GPS a more efficient, quieter alternative to the cabbies we love or loathe. Even Roy Ellis, the chairman of the Public Carriage Office, which grants licences to prospective taxi drivers, admits that times are changing. "We've had an external report done on these systems, and the upshot is, they work. They're good." Problems concerning delays or traffic-congestion response times will be ironed out in time. Voice activation, more complex traffic-prediction software, and route organisers with greater flexibility are all imminent developments. So what will become of The Knowledge?

Ellis discusses the introduction of a topographical test for London's private-hire drivers that would be, he imagines, "nowhere near as tough as The Knowledge", but this is yet to be confirmed. With more sophisticated technology on the horizon, it's easy to see nationwide licensing authorities reconsidering how much taxi drivers actually need to know. Presumably, black-cab drivers could also take the watered-down test by 2006. A drop in standards would precipitate a rise in numbers applying for a licence, and the crowded London taxi market might become saturated.

It's a problem Ellis and the PCO would be powerless to tackle. "We put no cap on the number of people who we give licences to. If they're good enough to earn their badge, they should have it. We let market forces decide how many cabbies are out there."

The GPS debate masks a greater vulnerability that sits at the heart of the licensed taxi trade. One cabbie who has been driving for over 30 years complains that, over the years, everyone has taken a chunk out the taxi industry: - minicabs, couriers, delivery vans.Particularly in London, cabbies have seen a sharp decrease in their earning potential over the past few years, as running a cab grows more expensive.