LONDON—Steve Scotland had better reason than most for thinking he knew London like the back of his hand. Not only was he a native Londoner, born and bred, but he'd spent years working as a chauffeur in the city, driving his passengers wherever they wanted to go, finding the shortcuts, negotiating the city's traffic-clogged streets swiftly, accurately, and with a minimum of fuss.

So he quietly fancied his chances of passing "The Knowledge" test—the demanding test of London's back streets and landmarks that confronts anyone who wishes to join the elite ranks of London's cabdrivers.

"It was something I always wanted to do," Scotland says.

View Images Robert Lordan sports the hard-won prize: the coveted green badge of a London cabbie.

In pursuit of this dream he went to the Public Carriage Office, which regulates taxis in London, and signed on for The Knowledge. After paying his fees and taking a map test and some written exams, he got hold of a motor scooter and set off to familiarize himself with his city in a whole new way.

Although "Knowledge Schools" exist to offer advice and help would-be cabbies prep for the series of examinations you have to pass along the way, the intensive learning of the city's streets and landmarks, the thousands of miles of exploration by scooter or on foot, is very much a do-it-yourself affair, all of it on your own time and at your own pace.

Nearly five years later, and with more than 10,000 miles (16,000 kilometers) clocked on the scooter, Scotland is still at it—although now, at least, it's with an end in sight. A good enough score on his next test, or "appearance," in a fortnight's time, and he'll have done it—cracked The Knowledge and earned himself the coveted green-and-white badge of a London cabbie.

"I had no idea how tough this would be," he says. "I really thought I knew the city well, but what I knew, or thought I knew, was nothing compared with what it takes to do The Knowledge." The five years he has spent on the quest is fairly typical. The Knowledge does not come easy.

Ultimate Brainteaser

Forget Mensa and armchair brainteasers. The Knowledge of London is a real-time, street-level test of memorization skills so intense that it physically alters the brains of those who pass it.

To qualify for that elusive green badge, you need to learn by heart all 320 sample runs that are listed in the Blue Book, the would-be cabbie's bible. You will also have to commit to memory the 25,000 streets, roads, avenues, courts, lanes, crescents, places, mews, yards, hills, and alleys that lie within a six-mile radius of Charing Cross.

Add to that the locations of another 20,000 landmarks and points of interest—pubs, clubs, museums, parks, monuments, railway stations, tube stations, hospitals, schools, police stations, government buildings, embassies, cemeteries, churches, guild halls, theaters, cinemas—any place, in other words, a fare-paying passenger might conceivably ask to be taken or an examiner might challenge you to find.

View Images NGM Staff. Sources: London councils; London Black Cab; Open Street Maps

You'll need to know your way around so well that, when asked, you can calculate the most direct legal route between any two addresses anywhere in the entire 113-square-mile (293-square-kilometer) metropolitan area within seconds, without looking at a map, and be able to rattle off the precise sequence of streets, junctions, roundabouts, and left- and right-hand turns necessary to complete such a journey.

And you'll have to be able to do this consistently, not just once or twice, but in a potentially endless series of one-on-one oral exams, called "appearances," taken at regular intervals until the examiners are satisfied that you do indeed possess The Knowledge.

Who Needs Sat Nav?

Even in this time of GPS and Google Maps, satellite navigation, or Sat Nav, is no match for a cabbie with The Knowledge. In May, London's Guardian newspaper pitted a cabbie against a Sat-Nav-equipped driver from Uber, the new "taxi" company that's taking the world's cities by storm by allowing passengers to book cars via their smartphones. The Uber driver did the run from the newspaper's office in King's Cross to Big Ben, in Westminster, in 22 minutes; the cabbie did it in 18, by taking a slightly longer route he knew to be quicker.

Such victories are points of pride these days within the ranks of London's cabbies, who have launched legal challenges against the upstart Uber. They claim that Uber drivers are acting as de facto taxis by using smartphones like taximeters to calculate fares. Only licensed taxi drivers—who possess The Knowledge—are allowed to work London's streets as taxis, metering their fares and being hailed from the curb.

A temporary ruling by London's transport authority last month found in favor of Uber on the grounds that the smartphone app that calculates Uber's fares was not connected to the vehicle in the manner of a traditional taximeter and was therefore not a taximeter within the meaning of the law. A British court will make a final determination on the matter next month.

View Images Robert Lordan navigates his new black cab in the vicinity of Big Ben and Parliament.

Proponents of Uber say Sat Nav technology makes The Knowledge obsolete. Not surprisingly, London's cabbies disagree. They're quick to point out that Sat Navs have a knack for getting things wrong, do not always pick the best or quickest route, and that having thousands of cabs idling curbside while their drivers punch in addresses for their Sat Navs will further clog London's streets, where average speeds have already dropped below nine miles an hour.

It's not simply a matter of speed, either, cabbies say. A driver who relies on Sat Nav doesn't know the city. "I like to put it this way," says 18-year veteran David Styles, who writes a blog about life behind the wheel: "When gentlemen have enjoyed supper at their club with their old regimental chums, they need a taxi to take them to the station. As they can generally afford to live in East Sussex, their station, Victoria, is only six minutes from Pall Mall. Depending on which entrance they want, they ask for The Shakespeare, Old Gatwick, or Hole in the Wall. Show me a Sat Nav which not only has that database but can be programmed in seconds, and I'll buy shares in it myself."

He continues: "And actors don't want to arrive at the front of the theater. They want the stage door. And yes, we have to learn those too."

Hail one of London's iconic "black" cabs (which nowadays can come in any color) from anywhere you please within the greater London area, tell the driver where you want to go—it doesn't matter whether it's the Tower of London or some obscure pub in an outer suburb—and by the time you've climbed in the back seat and closed the door, he'll have already calculated the most direct, swiftest route, without ever looking at a map.

What if you're not quite sure where you want to go? Say you're visiting London, you've reserved tickets to see The 39 Steps, and you're picking them up at the box office, but you can't recall the name of the theater. Just name the play, and your cabbie will take you—in this case, straight to the Criterion on Piccadilly Circus.

The "Nervous Wreck"

London cabbies don't just happen to know these things: For more than 150 years they've been required to be the consummate experts on their city. The whip-cracking drivers of those Victorian hansom cabs Sherlock Holmes was forever hailing all had to bone up for the world's toughest geography test, just as the roughly 25,000 drivers of London's cabs must today.

View Images Students study maps at London's Knowledge Point School, one of many that have sprung up to help the 6,000 or so people who are candidates at any one time.

The final series of tests, known as the "required standard," or "req" for short, is known among cabbies as the Nervous Wreck. Here's where the last-minutes jitters creep in.

Indeed, Steve Scotland would have had his badge a few weeks ago, at his last appearance, had he not miscued a turn and dropped his hypothetical passenger off on the wrong side of the street. "Just nerves," he recalls. "All I had to do was pick up at the cab rank at Sainsbury's [supermarket] on Liverpool Street and drop off at the emergency entrance at the Moorfields Eye Hospital. That's a run I can do in my sleep."

But he bobbled when it counted. As a result, instead of spending this Sunday afternoon sitting home watching the World Cup, as he'd been looking forward to, he's once more astride his scooter, still a "Knowledge Boy," puttering along Great Swan Alley, just off Copthall Avenue, in London's financial district, brushing up ahead of his next—and hopefully final—appearance.

"A new restaurant has opened up around here, and I want to get it fixed in my mind—just in case," he says. "You just never know what the examiners are going to ask you."

Knowledge Boys (and Girls)

Cabbie apprentices such as Scotland are Knowledge Boys. There are Knowledge Girls too, but fewer than 2 percent of cabbies are women.

Cabbies come from all walks of life—students, tradesmen, lawyers, teachers. David Styles was a typesetter for one of London's oldest printing companies. Most grew up in or around London, but people from elsewhere in Britain, and even a few foreigners, have successfully completed The Knowledge.

"I was studying biology," says 24-year-old Osman Jamal Zai, who left school six months ago and began studying The Knowledge. "This just seemed like a better idea, and I have to say I'm loving it."

View Images "Knowledge Boy" Stuart Moore makes a "pointing run" on his scooter—tracking down the locations of some of the thousands of points of interest he'll need to know by heart if he's to earn his green badge. Scooters are the preferred Knowledge vehicles because they're cheap to run and highly maneuverable.

By all accounts, being a London cabbie pays well. Although the cabbies themselves are cagey about what they earn, it's widely accepted that incomes of $100,000 (U.S.) a year aren't unusual, with some operators—those working extremely long hours—believed to be making that figure in pounds sterling (roughly $170,000 U.S.).

Aside from the money, the draw for many is the ability to set their own hours and achieve an enviable work-life balance. And unlike many cities—Paris, for instance, which imposes strict limits on the numbers of cabs—London is wide open. Anyone of good character can get a cabdriver's license, as long as he or she passes The Knowledge.

The nearly five years Scotland has spent on that quest isn't unusual. "It took me four years, 11 months, and 13 days," Styles says.

While a fortunate few—those who can afford to pursue the training full-time—can complete it in as little as two years, most have to fit The Knowledge in around work and family commitments.

"I'm guessing it'll take me around five years," says 53-year-old David Greenhalgh, an IT specialist who's spent the past two years juggling street explorations and his day job.

In Pursuit of Worshipfuls

This weekend, Greenhalgh is taking advantage of the relative lack of bustle to learn the tangled arteries in the neighborhoods between Cannon Street Station, London Wall, and St. Paul's Cathedral. He's on foot, with a rucksack slung over his shoulder, and a guide to London's 110 livery companies—just some of the thousands of points of interest whose locations he will need to know by heart.

In the past couple of hours, he's tracked down the Worshipful Company of Mercers, the oldest of London's liveries, founded in 1394, as well as the grandiose building housing the Worshipful Company of Vintners (on Upper Thames Street) and the Worshipful Company of Stationers and Newspaper Makers, in a secluded square off Ave Maria Lane.

View Images A massive salamander—replica of early jewelry from the Cheapside Hoard—on the rotunda outside the Museum of London makes a fanciful backdrop for Knowledge boy David Greenhalgh as he does a pointing run for livery halls.

On such a fine, warm summery Sunday afternoon as this, a quixotic nosing about for London's old livery halls seems like the pleasantest kind of tourism—sightseeing with a purpose.

"It's not always like this," Greenhalgh laughs, as he pauses to photograph the ornate portal to the Worshipful Company of Apothecaries, whose 17th-century headquarters is hidden away down a picturesque cobbled lane not far from Blackfriars Station. "You have to get to know the dodgy neighborhoods too."

The Winnowing

Only about one in five of those who attempt The Knowledge ever make the grade. "You can never actually fail," Styles says. "There's only quitting. You're allowed to keep trying as long as you like." The overwhelming majority drop out in frustration after a year or so.

View Images Tools of the trade: Maps, pens, magnifying glass, pins, and cotton thread for linking points and finding the shortest routes are some of the study aids for those doing The Knowledge.

To put the success rate into perspective, the percentage of people who successfully complete The Knowledge is roughly the same as that of candidates who make it through the training to become a U.S. Navy SEAL.

"There are no shortcuts," says 79-year-old cabbie Alf Townsend, who did The Knowledge in 1962 and still drives his cab a few hours a day to mingle with old friends and keep his hand in.

"You can't do it by sitting at home, memorizing maps and street names, and hope to pass that way. You have to get out on the streets, putting in the miles, seeing and experiencing everything firsthand. There's no other way."

Royal Roots

It was King Charles I who, in 1636, launched London's taxi service—the world's oldest—by granting royal permission for 50 hackney carriages to "ply their trade."

A few years later, in 1654, Oliver Cromwell put in place the framework of regulations under which the hackneys operated, but it was a stern Victorian police commissioner, Sir Richard Mayne, who dreamed up The Knowledge in the early 1850s.

Dismayed by the number of complaints from visitors at the Great Exhibition of 1851 that London cabbies didn't seem to know where they were going, Mayne made it a requirement that anyone seeking a cabbie's license be an expert on the city. And so The Knowledge was born.

Although the city has changed past all recognition since the 19th century, when a series of sample cab runs were formulated by the Public Carriage Office to create a framework for studying and testing, the demands of The Knowledge have hardly changed.

A candidate who mastered all the runs and knew them by heart, and who knew all the streets and landmarks in a quarter-mile radius around each start and end point, could be considered to have acquired The Knowledge.

The precise number of runs has varied over the years, but today there are 320, and they're found in the Blue Book (which is pink).

Being "On the Cotton"

"In some ways, people doing The Knowledge today have it easier than we did 50 years ago," Townsend says. "We had to figure out the shortest routes for ourselves, sticking pins in maps at the end points, tying threads between them, then trying to work out the route that stayed closest to the thread. Being 'on the cotton,' it was called. Nowadays you can buy books and apps that have the correct routes already worked out for you."

There are also Knowledge Schools, taught by veteran cabbies, to help candidates learn the runs and prepare for the exams. The examiners are said to be more reasonable now—still austere, but a bit less like Royal Marine drill sergeants and a bit more like Mr. Chips.

There's better gear too, according to Townsend—no small consideration for anyone who has to ride thousands of miles around the streets of soggy old London town on a scooter.

"I nearly froze," he recalls. "All I had was a garbage bin liner over me to keep out the rain, a waterproof fisherman's hat, and a couple of my wife's scarves wrapped around my neck to try to keep me warm. Nowadays they have Gore-Tex and heated gloves."

Some things never change. Manor House Station to Gibson Square is still the very first of the 320 Blue Book runs a would-be cabbie is expected to know.

"I started out very early one Sunday morning," says Robert Lordan, a 33-year-old former schoolteacher who, like Styles, writes a blog on cabbie life. "I remember it was eerily quiet. I felt as though I had the entire city to myself. I was full of excitement, very much looking forward to exploring and learning London."

The traditional first run, he discovered, was gratifyingly easy to learn—a reasonably straightforward journey, 2.9 miles (4.7 kilometers) long, between a nondescript Tube station on the Piccadilly Line and a quiet square in fashionable Islington.

"I'd prepared considerably beforehand, poring over the map, and being a beginner, I drove it several times to make sure I was familiar with every turn and junction." Then he moved on to the next run, and the next. "On average, I would spend three to four hours on each," he says.

To master a route for testing purposes, a student has to not only memorize the streets linking the two end points but also be intimately familiar with the back streets and landmarks within a quarter-mile radius around those points.

"An examiner quizzing you on a run is never going to ask you anything straightforward like, 'Take me from Manor House Station to Gibson Square,'" Lordan says. "He'll always pick some address that's just around the corner or a couple streets away."

Initial enthusiasm soon wanes in the face of the mind-boggling complexity of London's labyrinthine streets and the sheer frustration in trying to learn them all.

"There comes a time, about a year into it, when you really begin to doubt what you're doing," Lordan says. "For me, it was the intricate one-way systems and myriad dead ends in parts of North London, especially around Islington. They had me pulling my hair out. I didn't think I was ever going to get those straight."

Eventually though, with persistence, he says, there comes a tipping point, when it all starts to make sense. "It's like putting together a jigsaw puzzle. Suddenly you see it. You spend so much time on the streets and studying the map at home that it etches itself on your brain. An analogy I like to use is that it reminded me of starting secondary school as a child. At first, the building seemed huge with its many rooms, wings, and corridors. But after a while, you get to know the place and navigating it becomes second nature."

Reshaping the Brain's Geography

Starting school is a good analogy for learning The Knowledge. A study by neurologists at University College London found that the hippocampus, the part of the brain responsible for spatial navigation, of a London cabby is significantly larger than those in the rest of the human population—a result of the intense memorization and route-finding undertaken while doing The Knowledge.

The study involved taking regular brain scans of Knowledge-seekers undergoing their training and comparing them with scans taken of a control group of people who had no interest in becoming cabdrivers.

At first, the hippocampi of all the study subjects were of similar size, and all subjects performed similarly on routine memory and route-finding tests. By the end of the study, though, those who'd passed The Knowledge had larger hippocampi, and the longer they worked as cabbies, the larger their hippocampi became.

"We don't know what is changing in the hippocampi of taxi drivers," says Eleanor Maguire, who led the study. "Whether it's new neurons that are being produced, new connections between neurons, proliferation of other cell types, or all three.

"There's been a lot of research looking at trying to associate different brain areas with certain skills—musicians or linguists, for example," she says. "The key point about London taxi drivers is that they acquire their navigational expertise when they're adults, unlike musicians, who often start when children, and so there's the added factor of the interaction between brain development and skill acquisition."

"Calling Over"

Certainly, the hippocampi of London cabdrivers get a lot of intensive exercise. Every day David Greenhalgh recites at least 30 of the 320 runs he has memorized—every turn, junction, and roundabout.

"I work my way up through the list and then start all over again," he says. Reciting these runs is known as "calling over." Knowledge students often get together to recite them to each other. The call over for Manor House Station to Gibson Square would be:

Leave on left—Green Lanes

Right on Brownswood Road

Left on Blackstock Road

Forward on Highbury Park

Forward on Highbury Grove

Right on St. Paul's Road

Comply Highbury Corner

Leave Upper Street

Right on Barnsbury Street

Left on Milner Square

Forward on Milner Place

Forward to Gibson Square

After the 320 runs are memorized, The Knowledge student begins the long battery of oral tests. The first are called 56-day appearances, given every eight weeks.

"The examiner asks you to do four runs," says Greenhalgh, who's made it through two 56-day appearances thus far. "Each run is worth ten points. If you get a perfect score of 40—something phenomenally rare–you get an A and advance straight to the next level, your 28-day appearances."

Lesser scores are awarded B or C or D grades, and the student returns in 56 days to try again. Grading is strict. Points are deducted for "hesitancy," and making an illegal turn or going the wrong way on a one-way street earns you a big fat zero. To advance to the next level of testing, a candidate needs the equivalent of two B's or four C's.

View Images London cabbie Robert Lordan takes a break at the venerable Victorian cab shelter on Russell Square, one of 13 scattered around the city where cabdrivers can stop for a break, a bacon sandwich, or a cup of tea and a chat with colleagues. The shelters were originally set up in 1875, in the horse-drawn days, to provide cabbies with shelter from London's damp and chill.

A grade of D gets you nowhere.

If after seven attempts you've not scored well enough to move on to your 28-day appearances, the slate is wiped clean, and you start the 56-day tests all over again. This setback happens to as many as 80 percent of first-time Knowledge students.

When you get to your 28s, the exams come every four weeks and proceed along exactly the same lines as the 56-day ones, only now the questions are even more demanding.

As before, a rare perfect score of 40 advances you to the next tier, but most muddle through—if they get through at all—with combinations of B's and C's.

As before, a D earns you nothing, and if you fail to advance after your seven appearances—"red-lining," in the vernacular—you go back and start your 28s again. Fail twice, and you go back to your 56-day appearances.

Eventually, if you persist, you reach your 21-day appearances—the final tier. Scoring here is just the same, with the same number of tests, only now the questions and expectations are tougher still.

Grown Men Crying

"It's a very emotional moment when you realize you've done it and get that handshake from the examiner," says Lordan, who passed four-and-a-half years ago.

"I know I got quite teary. They tell me a lot of guys cry when they get their badge—you've invested so much of yourself, your time and your life, into doing this, to reach the end is just incredible."

Lordan's victory came on December 22. Two nights later, on Christmas Eve, he went out as a London cabbie for the very first time, driving a cab he'd leased. His first fare was a group of South African tourists who congratulated him on his achievement. Following the long-standing first fare tradition, he told them there was no charge. "They insisted on paying me anyway," he says, "and so I donated the money to charity."

Lordan and I are standing beside an old, green Victorian cabman's shelter on Russell Street—one of 13 still in existence around the city where cabbies can get mugs of tea and bacon sandwiches and take a rest. Parked a few feet away is the gleaming, built-in-Coventry black cab he bought eight months earlier, and on which he's already clocked 14,000 miles (22,531 kilometers).

With the ease and fluency of a man who has The Knowledge, and the passion of a true historian, Lordan explains how the shelters were founded by Captain George Armstrong in 1875 to give London's taxi drivers somewhere to keep warm and dry.

Armstrong had "sent his manservant out to hail a cab in a blizzard," Lordan says. "He came back an hour later and said they were all in pubs, and none of them were in any fit state to drive. And so he established these shelters."

View Images Lordan behind the wheel of his new black cab. "The best thing about being a London cabbie is the people you meet," he says. "I meet people from across the globe—ambassadors, celebrities, WWII veterans, politicians, parents taking their newborn baby home, people who've survived genocide in their homeland. Ninety-nine percent of the people I meet in my taxi are wonderful. To meet such a wide array of people every day is truly life affirming."

Lordan is one of a small percentage of cabbies who have gone on to complete the Worshipful Company of Hackney Carriage Drivers' course to become a licensed tour guide—a sort of post-doc among those who have The Knowledge. He offers tours on London's famous murders, Harry Potter landmarks, and American sites of interest. "Did you know Benjamin Franklin had a house here?" he asks.