You couldn't pull a tight U-turn on a two-lane street. Or easily fit five passengers in comfort in the back. Or have room to spare for a bale of hay for your horse.

Black cabs - a misnomer, since they're available in a dozen colours - are direct descendants of horse-drawn hackney carriages first licensed in 1643.

More than 20,000 ply the streets of London, the majority owner-operated, with thousands of others in cities and towns across Britain, as well as increasingly abroad as manufacturers LTI and Metrocab pursue new markets.

Long-term investment

They don't come cheap. The basic version of LTI's TXII goes for £27,000 pounds, though popular options can easily bring that up to £30,000 pounds (42,750 euros, $50,000).

But in return, you get a specially designed vehicle built to last. A typical London black cab, shared between two drivers, can easily build up 500,000-plus miles over a working life of 10 to 15 years.

"It's a working vehicle. It wears well," said Alf Townsend, a veteran London taxi driver and author of Cabbie, a memoir of his 40-plus years in the business.

Tourists to London can immediately appreciate what makes the black cab different from the Chevrolets, Peugeots, Toyotas and other off-the-rack sedans used as taxis in other parts of the world.

Throw the steering wheel all the way over, and a black cab should be able to swing from one side of a two-lane street to the other.

There's the wide back seat, plus two folding jump seats, with legroom galore and plenty of headroom - though not, as urban myth has it, quite enough for a man to wear a top hat.

But from a driver's point of view, the black cab feels more small van than big car.

The driver's seat, set high to make it easier to spot fares along the curb, is designed to be occupied for hours without bringing on backache, and is separated from the back by brickproof Plexiglas.

Baggage goes next to the driver, in a space that was originally intended by law to be large enough to accommodate a bale of hay - even after the demise of the horse-drawn cab.

The trunk, or "boot", has room for little more than a spare tire and a new mandatory feature - an aluminium platform for loading passengers in wheelchairs through the wide back doors.

U-turn

The engines - a Ford diesel for LTI, a Toyota for Metrocab - are built to last, not to accelerate, and the brakes are truck-sized to endure stop-and-go traffic on hot or rainy days.

For the first-time driver, the surprise is the 7.6m turning circle, dictated by "conditions of fitness" set by London's official Public Carriage Office which only LTI and Metrocab products meet.

Throw the steering wheel all the way over, and a black cab should be able to swing from one side of a two-lane street to the other.

When put to the test by a non-professional driver in the confines of a parking lot, it feels as if the cab is pivoting on the inboard rear tyre. A big grin is the inevitable result.