Londoner Patric McGuinness wakes in the morning thinking about roads, goes to bed thinking about roads and shouts out road names as he passes them.

A former gang member and ex-drug addict, the 47-year-old is studying for "The Knowledge of London", the notoriously difficult test drivers of iconic black taxis must pass to ply their trade.

"To be a London taxi driver. London's finest. That's what they're called... Why would I not want to be a part of that?," said McGuinness, who spent many years in prison before turning his life around.

Black cabs are being squeezed by an onslaught of phone-based satellite navigation and ride-sharing apps, including the ever-more popular Uber.

They suffered a further setback earlier this month when they lost a legal challenge against Uber's use of GPS technology to calculate fares instead of the traditional taximeters.

City authorities are now planning regulation that would include a map-reading test for ride-sharing drivers -- although "cabbies" say this would still be a far cry from the stringent exams for taxi drivers.

"No one else studies anything like they do in London with the Knowledge. It's historical and it's renowned throughout the world," McGuinness said.

Undeterred by the competition and the dauntingly low 20-percent pass rate, McGuinness studies London's confusing maze of roads by puttering around the city on his moped with a map pinned to the windshield.

"I wake up in the morning sometimes thinking about roads. I go to bed at nighttime thinking about roads. In the middle of my day wherever I am going, whenever I am on my motorbike I'll call the road name which I am on," he said.

- 'Beat the sat nav' -

The driver's dedication is a far cry from his troubled youth and he has been in recovery for 13 years, working in drug rehabilitation and helping young offenders.

"I got into trouble with substance misuse quite young, spent a considerable amount of my life in prison. I was in a gang and all that kind of stuff.

"Since then I have done significant changes in my life," he said.

Taxi test applicants take an average of four years to learn 320 routes -- themselves made up of chains of streets, intersections and traffic circles -- as well as 30,000 points of interest to pass the exam.

The routes start from central London's Charing Cross train station, and "cabbies" must know all roads within a six-mile (10-kilometre) radius -- the distance within which horses laden with luggage would traditionally have had to stop for a rest.

The origins of the test are unclear, although various sources date the start of the gruelling exams to the latter half of the 19th century.

The Knowledge "would beat the sat nav hands down... there's no competition," McGuinness said.

"There's nothing like getting in the back of a black cab and asking the driver to take me somewhere and they know you know where you're going."