Motorists in Samoa have switched the side of the road they drive on, overnight. It's a move that Britain has considered - but how would it work?
It's the kind of interview question that has reduced confident job seekers to quivering wrecks.
Imagine you are the minister in charge of the UK's roads and you have to switch the country to driving on the right-hand side. How would you do it?
A study of Samoa, in the South Pacific, this week might offer some clues. The country is experiencing its first day of driving on the left on Monday, the start of a special two-day bank holiday to ease Samoans into the new regime.
What if the UK were to follow? Driving on the right would make trips to the European mainland easier, when taking or hiring a car. And cars with steering wheels on the left could be cheaper.
The idea is not as fanciful as it sounds. Although the Department for Transport says it has no plans to change, it did examine such a plan in the late 1960s, two years after Sweden successfully switched to driving on the right.
Its report rejected the idea on grounds of safety and costs. But that was before Britain's entry into the European Union and the opening of the Channel Tunnel, which for the first time established a land link between Britain and the Continent. So, if the UK was to think again about a switch, what would be the key issues?
SIGNS & JUNCTIONS
Road markings and roadside signs would have to be switched to the other side of the road, but ready in advance of the day of change, in a huge logistical exercise.
One-way streets would have to be reconfigured and traffic lights with filters changed, says Paul Watters of the AA. To get an idea of the cost, changing signs from miles to kilometres alone was estimated at £750m, he adds.
The biggest engineering issue would be highway building, says Benjamin Heydecker of the Centre for Transport Studies at University College London. About one in 10 motorway junctions is asymmetric or incomplete, so would need to be dug up and rebuilt.
"Motorway signs would have to be turned round and repositioned, so approaches to junctions would not be in the same place."
Accident blackspots would all need looking at too, because the signs there are site-specific and so would need to change.
ADJUSTING SLIP ROADS
"Entrance and exits to motorways are not symmetrical either, so there would be consequences there too," says Mr Heydecker.
Slip roads that were deceleration lanes would suddenly be used for accelerating, so their lengths would need to be extended; and vice versa.
Although many motorists would be used to driving on the right - thanks to trips abroad - a comprehensive retraining programme would be needed, according to Mr Heydecker. Particular emphasis would be put on negotiating roundabouts (which would run anti-clockwise) and left-hand turns, which would require cutting across oncoming traffic.
After years of driving, habits are well entrenched and it might take more than a few lessons to get used to the new arrangements. But where could "learners" practise, before the switch?
CAR STEERING WHEELS & BUS DOORS
Making life even harder for motorists is the fixed right-hand driver's position of cars sold in the UK - suddenly drivers would find themselves further away from the centre of the road.
Over time British drivers would buy cars with left-hand steering, so they would be changing gears with their right hands.
The global manufacturing of cars would be simplified if all countries were to opt for left-hand steering, says Mr Heydecker.
"If cars were all manufactured the same way, it would reduce the cost of design and improve the quality of vehicles."
Public service vehicles like buses would also have to undergo a massive overhaul so that their doors were on the right-hand-side of the vehicle.
LEARN FROM SWEDEN
Preparations were made long in advance, says Niklas Stavegard of Motormannen, which is the Swedish Automobile Association.
"All road signs were doubled, new signs on the right-hand side, which were covered until the day of the change. On the particular day, the left-hand side signs were covered and right hand side signs were used."
The change was made at 0500 on Sunday 3 September 1967. All private traffic was banned between 0100 and 0600 and there was a total stop on all traffic at 0450, with a countdown on the radio to 0500.
The speed limit in urban areas was lowered from 50km/h to 40km/h for a period of time after the change.
A majority of all cars already had the steering wheel on the left side, so no change was made to cars.
In 1969, the financial burden of making the switch was calculated by the government to be £264m - about £3.4bn today. But that would now be seen as a ridiculously conservative estimate.
"Since that time, the road network and the level of sophistication of the network and its controlling infrastructure has grown enormously," says a spokeswoman for the Department for Transport.
Additional costs would include all buses being changed, alteration to motorway entrances and exits, and traffic control systems.
"Casualties would also be likely to rise, and the current cost of a fatality is £608,580," she says. "This could be particularly true for elderly road users who are less able to adapt to changed conditions."
The Republic of Ireland, which has already changed its road signs from miles to kilometres, briefly considered this move to greater European integration last year, when a pro-Brussels political party suggested it. But it was swiftly rejected.
"When the question came up, it was followed by the question 'Is it needed?' and the answer to that was 'No', says Sean O'Neill of the Irish National Roads Authority.
"If it was needed or if the UK did it, and left us as one of the last European countries driving on the left-hand side, then we would think differently."
For a significantly bigger country, like the UK, with a relatively more developed road network, the challenge would be even greater.
The immediate fear would be of road chaos and mass casualties, says Philip Gomm of the RAC Foundation.
"Given that any change would have to be instantaneous, and the nation's roads are never quiet, only less busy - indeed Britain has some of the most congested roads in Europe - how do you seamlessly get everyone onto the other side of the carriageway?
"The whole concept is mindboggling. It would be a logistical nightmare involving huge public education, vast sums of money and a massive amount of staff - and all so we can be like the French.
"So probably the best advice to anyone even contemplating such a scheme is; don't bother. If it ain't broke, don't fix it."
There used to be a time when headlines like 'FOG IN CHANNEL. CONTINENT CUT OFF.' meant something.