Have we fallen out of love with the car permanently?
Ten years ago, a woman called Barbara Noble asked an important question.
Barbara was a statistician at the Department for Transport and her report was trying to untangle a mystery.
For decades, the richer Britain got, the more people drove. But somewhere in the 1990s things changed. The economy was bouncing along nicely, but our car mileage stayed flat.
In fact, if you singled out young people, especially young men, we were driving a lot less.
What Barbara's report had touched on was something that eventually became known as Peak Car - the idea that we had permanently fallen out of love with our cars.
It was not just happening in the UK, but in lots of rich countries, including the home of the Cadillac - America.
In 2012, the RAC Foundation got together with a group of academics. They analysed Britain's stats between 1995 and 2007, stopping there to leave out the impact of the subsequent recession.
And this is what they found. About 70% of us were driving more. But the average mileage was being dragged down because young men, company car drivers and Londoners were driving less.
- Men in their twenties drive 1,912 miles a year less than they did in the mid-1990s
- The number of men in their twenties holding a full driving licence also dropped by 11% between the mid-1990s and 2005-07
- Company car mileage dropped by nearly 40% between 1995-97 and 2005-07
- Driving mileage across London fell by 20% in the decade leading up to the recession
But why was it happening? Well, the government took away the tax breaks for company cars.
Public transport is good in London and bus use has been going up, while parking and a congestion charge made driving more expensive.
The increased pace of overseas immigration has also played a big part in the capital. The surveys show immigrants don't drive as much.
But young men abandoning their cars is harder to explain.
You would imagine that they are most likely to revere their vehicles. I grew up obsessed with the Lamborghini Countach and Lotus Esprit S1 and I have drooled over supercars ever since.
Cars used to be TV and film stars: the General Lee from the Dukes of Hazzard, KITT (which I'm told stands for Knight Industries Two Thousand) from Knight Rider and the new Bond car always used to be big news. But getting your driving licence just isn't the big deal it once was.
Some say it is cultural - that phones have replaced cars as the must-have status symbol.
There is also a suggestion that the internet has reduced the need to drive so much. No-one actually has to drive to the shops if they don't want to. And you don't need a car to talk to friends; you can do it all online (one of the biggest changes for young men was the fall in driving to see friends and family).
Meanwhile, more of us live in cities. The average speed of a car in Beijing is 7mph. Exactly the same average speed as a horse and cart.
It is likely that all of these things play a part. But they're not the key reason young people, especially men, aren't driving.
"It's not about desire, it's about how difficult it is," says Scott Le Vine, at Imperial College London. He was one of the academics behind the 2012 report.
"There's little support for the argument that young people have fallen out of love with the car."
Instead, Scott says it is just more expensive and more difficult to pass your test than it used to be. Lessons alone can be £1,000. And pass rates for the practical test are well below the levels of the 1990s.
According to the National Travel Survey (a brilliant piece of yearly government research that helps make England one of the best places to study travel trends), more than half of 17 to 29-year-olds without full licences say they are either learning to drive, put off by the test, or put off by the cost, especially the cost of learning.
Scott sums it up in a more recent paper: "Put simply, while older British people were getting richer in the 2000s, younger adults were getting poorer."
This is relevant because there has historically been a positive relationship between income and car ownership/use.
The big question then; is it permanent?
"We still have leverage over what happens," Scott told me. "Policies matter. Technology matters. It's difficult to say if it has peaked or not peaked."
As Scott says, it is still impossible to pin down exactly what's happening. For example, as the economy has picked up and fuel's got cheaper again, traffic volumes have just hit a record high in Britain.
Up 2.2% last year, the provisional figure of 316.1 billion vehicle miles is 0.6% higher than the pre-recession peak.
A collapse in fuel prices also means Americans are driving more too. They managed a whopping 3 trillion miles in 2014, although that is still short of the pre-recession peak.
But drill down a bit and there's another interesting stat. The average American drives fewer miles now than they did in 1997. And just like here, the proportion of young people getting licences is dropping off.
Why is it so important?
The way we will be travelling in the future has huge implications for the present.
Stephen Joseph, from the Campaign for Better Transport, said: "For at least 50 years, transport policy in the UK has been based on the assumption that car use would carry on growing.
"If car use has peaked, this will radically change transport policy - and in particular should lead to a complete reassessment of the government's £15bn road-building plans."
Unfortunately, predicting the future is critical to how governments spend taxpayers' money. I say unfortunately because it is notoriously hard to get right.
When they built the Channel Tunnel no-one saw budget airlines coming. As a result, trains to the continent have never carried anywhere near the number of passengers everyone predicted.
At a roads conference the other week, everyone I spoke to, from carmakers to road planners, was marvelling at the meteoric rise of Uber and the impact the company was having on transport, seemingly overnight.
We all scratched our heads trying to guess where the next game-changing innovation would come from.
But it seems ridiculous to try to predict 50 years into the future based on the world we live in today.