OK, I think we've cracked it. I've spent huge effort on this blog trying to work out whether any forms of transport are truly low carbon.
It is an important issue - almost a quarter of world greenhouse gas emissions are from transport.
I have explored the relative carbon cost of almost every type of transport you can imagine.
I have even analysed the environmental impact of walking - and found that on one reckoning walking is actually more polluting than driving!
The fact is there are no straightforward answers.
Flying isn't as bad as you might think - pretty much the same carbon footprint as driving (except of course you are likely to go a lot further on a plane) - meanwhile public transport might actually be less carbon efficient than just hopping in your car .
But now I reckon I've got the definitive answer. I have found the world's most ethical form of transport.
Here's how it happened:
My blog on why cars might be greener than public transport caught the eye of the FT's devilishly clever "undercover economist" Tim Harford, who moonlights here at the BBC as the presenter of the Radio 4 programme More or Less .
The programme had snuffled out some statistics from the Department of Environment, Food and Rural Affairs which confirmed my claim.
The figures show that the average occupancy of a British bus is just nine people. Even in London a typical bus has just 13 passengers.
Two people in a car produce lower emissions per person than nine people in a bus.
Indeed, the figures show that buses are only 20% more efficient than taking the car based on average vehicle type and occupancy.
But that's not what had snagged Tim's interest. He was intrigued by a claim I made at the end of the blog.
I argued that it is always more carbon efficient to take the public transport option because "it will be going anyway".
He wanted to know whether the same argument applies to flying. If the reason you catch the bus is that it will be going anyway then surely that's also true of your holiday flight to Malaga - it will fly with or without you.
In short, it is the perfect excuse for us all to continue flying around the world.
But will the buses and planes be going anyway? That's what Tim wanted to know and he invited me and the Independent newspaper's travel correspondent, Simon Calder to discuss the question with him.
Simon Calder is known as "the man who pays his own way" and is quite possibly the most travelled man in Britain.
He is also, it transpires, a mathematician. In short the ideal person to discuss this tricky issue.
You can listen to the programme here:
As our discussion demonstrates, the question is not as straightforward as it might first appear.
Are you actually responsible (at least in part) for persuading the bus, train or airline company to put on additional services?
Do you agree with Tim's argument that average marginal cost applies to buses and planes equally - or are you on my side?
(I should probably come clean here. I was very briefly an extremely junior economist at the Department of Transport. But don't let that affect your judgement.)
Impact of walking
You'll have heard from the programme that I suggest that for short journeys walking or cycling is preferable to even public transport, but my analysis of walking shows that it is actually pretty energy intensive - and the energy in our food tends to be pretty carbon intensive .
So what is the truly low carbon travel alternative? It was Simon Calder who suggested hitchhiking.
He says that his own personal offsetting scheme is to hitchhike wherever possible, arguing that hitchhikers do not generate additional traffic.
I think there a strong case for that. After all, the driver who picks you up at a service station on the M62 or on the verge of the A43 is extremely unlikely to have decided to hop in his car with the intention of picking up a hitchhiker (if he has I venture to suggest you are in real trouble).
What's more the additional carbon cost of you travelling with him will be very low.
I asked Simon why I hadn't seen him standing sodden in the rain at the end of the M1 and he said that's because now he's finally got rid of the German army greatcoat he wore as a teenager drivers readily pick him up.
But does anyone else hitchhike these days? You certainly see very few hitchhikers on the roads.
I haven't hitchhiked for years but I can certainly remember my hitching days, and my best journey. That was heading to the Glastonbury festival back in the summer of 1986.
My friend Tim and I turned up at the end of the M4 and there were already 20 to 30 shaggy festival goers there already. Hitchhiking etiquette required that we join the end of the queue.
At this rate we wouldn't be at the festival until after Level 42 had played their set on the famous pyramid stage on the Sunday. (That's right, Level 42 were a headline band at Glastonbury - and people made a fuss about Jay-Z!).
But minutes later a lorry pulled up and said he'd take us all. We bundled into the back and sat on our sleeping bag rolls and rucksacks passing round the cider all the way to Somerset and were there in time to see The Cure (not to mention Lloyd Cole and The Pogues).
Not bad eh?
Do you have a better hitching story?