The Knowledge: the test behind London’s black cabs

Tell a London black cab driver where you’re going and they won’t reach for a sat nav. They already know the way. Every licensed cabbie in the city has passed the Knowledge, a test that dates back to 1865 and requires learning roughly 25,000 streets within a six-mile radius of Charing Cross.

Preparing for it usually takes three to four years, and nothing is guaranteed. In a University College London study that followed 79 trainees, only 39 ended up with a licence.

What candidates have to learn

The backbone is 320 standard routes, known as “runs”, set out in the official guide from the licensing office that now sits within Transport for London. Each run links two points in central London. Candidates must know every street along it and everything within half a mile of each end.

“Everything” means it. The list covers squares, hotels, theatres, embassies, hospitals, police stations, courts, stations, places of worship, cemeteries, parks, schools, restaurants and historic buildings. Examiners can ask about the order of the theatres on Shaftesbury Avenue, or the side streets and traffic lights passed along a route.

There’s also a narrower licence for a single suburban sector, with 30 to 50 runs. Suburban drivers get a yellow badge; the green badge covers all of London, and it’s the one that gives the test its reputation.

How the test works

Applicants first have to show they are of good character, including a criminal record check, then pass a written exam. After that come the “appearances”: one-to-one oral exams with an examiner, and no map.

The examiner names two places in London and the candidate has to work out, from memory, the shortest sensible route between them. They call out every road, where they cross junctions, where they turn, which roundabouts they take and what they pass along the way. Most candidates need at least twelve appearances before they finish.

You can spot trainees on the street. “Knowledge boys” and “Knowledge girls” ride around the city on scooters with a clipboard fixed to the handlebars, listing the day’s streets.

What the Knowledge does to the brain

Neuroscientist Eleanor Maguire of UCL spent years studying London cabbies. Her best-known study, published in Current Biology in December 2011 with Katherine Woollett, followed 79 trainees and 31 people who weren’t studying for the test.

At the start, brain scans showed no difference between the groups. Years later, those who had qualified had more grey matter in the posterior hippocampus, an area tied to spatial memory. Trainees who dropped out or failed showed no such change.

It came at a price. The qualified drivers did worse than the control group on some tasks involving complex visual information. For the researchers, it showed that the adult brain can still reshape itself through learning, with trade-offs.

If you’re visiting London

• Hail a black cab on the street when the “TAXI” sign on the roof is lit, or find one at a rank. They are the only hire vehicles allowed to pick up passengers who flag them down.
• Minicabs and ride-hailing cars are a different category (“private hire”), with a different licence, and must be booked in advance.
• Black cabs aren’t always black. The colour isn’t required, and many carry advertising. The name stuck to tell them apart from minicabs.
• Check current fares on the TfL website before you ride. They change from time to time.

Cabbies know London above ground. For the city below it, see why you stand on the right on London escalators. And if you’re still planning the trip, many visitors now need a UK ETA.

FAQ

What is the Knowledge of London?

It’s the test every London black cab driver must pass. Candidates memorise around 25,000 streets and the places of interest in central London, then plan routes from memory with no map.

How long does it take to pass the Knowledge?

Usually three to four years of study, with at least twelve oral exams along the way.

Do London cabbies use GPS?

The test exists so they don’t have to. Drivers are expected to choose a route on the spot, based on the passenger’s request and the traffic.

Does the Knowledge change drivers’ brains?

A 2011 UCL study found more grey matter in the posterior hippocampus of trainees who qualified, and no such change in those who didn’t.

Sources

• Taxis of London, Wikipedia: the test’s 1865 origin, the 320 runs, the 25,000 streets, points of interest, the yellow and green badges, appearances, Knowledge boys and the rule that lets cabs pick up on the street while minicabs take only bookings.
• Changes in London taxi drivers’ brains driven by acquiring “the Knowledge”, Wellcome Trust, December 9, 2011: the 79 trainees, the 39 who qualified, training time and the study’s findings.
• Acquiring “the Knowledge” of London’s layout drives structural brain changes, Woollett and Maguire, Current Biology, 2011: the full paper.