A couple of weeks ago the transport department, headed by Darling and ManLove, announced that they’d be opening a car-sharing lane on a busy stretch of motorway near Leeds. They argued that making people join forces for the trip to work would result in fewer cars on the road, a greener future for our baby children and 1,000 more wonderful years for our glorious leader.

Right. Well, if we’re going to share cars then it makes sense to buy something with a large number of seats. I, for instance, run a seven-seater Volvo XC90 because you can cram kids in the back on the school run and this means their parents can stay in bed. How sociable and public spirited is that? Not very, according to Gordon Brown who, only two days after Darling and ManLove’s initiative, announced that because I run a large car that’s ideal for sharing I was to have my head kicked in by the taxman.

This seemed strange but there’s a reason. Apparently when the Labour party came to power, it made a manifesto promise to cut the amount of greenhouse gas by 20% by 2010.

To do this they’d have to get rid of every car, bus, train, factory, aeroplane and power station. And then they’d have to kill every cow, horse and sheep. And then they’d have to exterminate everyone in China and India. But no matter.

They made the promise, the voters believed them and they had to be seen to be doing something about it.

That’s why Gordon Brown imposed his new tax on “gas guzzlers”. The plan is that we stop buying off-roaders, the ice caps heal, the polar bears are saved and all will be well when Blair’s 1,000-year Reich comes to an end.

Right. I see. So someone is going to walk away from a £70,000 Range Rover because of an £80 tax surcharge? Seems a bit far-fetched to me.

And now guess what? It obviously seems a bit far-fetched to the government as well, because just a week after the budget it announced that it wouldn’t be keeping its promise to cut carbon dioxide emissions. In the same way that it hasn’t kept its promise to sort out Iraq, the National Health Service, the education system, immigration, benefit fraud or anything at all.

So there you are. In the space of three weeks you are encouraged to do car sharing. Then you’re handed a Range Rover tax for doing just that. And then you’re told it makes no difference.

It’s not just Range Rovers either. You’ll be clobbered if you have a BMW 1-series or a Citroën C5. Which means nearly everyone with a reasonable car will be paying Gordon Brown to look like he’s trying to achieve a goal that’s simply not possible. And those who downsize to something more economical will find themselves banned from the new car-sharing lane proposed by Darling and ManLove. And this lot honestly think they’ll get re-elected.

Jesus.

The best thing we can do is treat our leaders as bluebottles. There’s no point waving our arms about and getting agitated because it’ll make no difference. They will continue to buzz about being annoying. So let’s just relax and think about this car-buying issue logically.

If you’re part of a school-run syndicate, you need a lot of individual seats with individual seatbelts. But not a people carrier obviously, because this will give other road users the impression you have no social life and no exciting underwear.

The next obvious choice is a large 4x4, but this is not an option for the weak. That’s because some people, usually on bicycles, bang on your roof as you go by and say they find your conspicuous consumption offensive.

What I want to do at times like this is bang on their cycling helmets and say I find their poverty offensive. But I’m made from stronger stuff so I turn the other cheek and run them down.

You may want to avoid this ugliness and go for a conventional estate car, which in many ways is wise and sensible. It’ll be nicer to drive than any MPV or off-roader. It’ll be easier to park, more stylish and much cheaper to run as well. But it’ll only have five seats. And this brings me back to the Volvo XC90. The first car to have been designed by someone who had children, not an engineer who’d read about them in a book.

You get seven seats, as well as space in the boot for dogs and bicycles. And yet it is not much larger than a normal estate car. It’s a triumph of packaging and yet it doesn’t look like a mumsy MPV or a gittish Chelsea tractor. That’s why it’s such a smash hit, a de rigueur must have accessory for every yummy mummy in the land. I’m on my second.

Next out of the blocks was Audi with the Q7, which is expensive, ugly, impractical and therefore irrelevant, and now we have the Mercedes R-class.

This car was listed as “good” in the recent Good Car Bad Car supplement. But because it is the policy of this column to correct mistakes as soon as possible I feel duty bound to tell you it isn’t.

And this is why. It is billed as a crossover vehicle combining elements of an estate with the best bits of a 4x4 and it is a measure of Mercedes’ success in muddling the two up that we managed to get our own wires crossed.

In the supplement it was judged as an estate, and was therefore up against the Audi A6 and Merc’s own E-class. Here it may well be a winner, and first impressions were pretty good. But last week I drove one for 800 miles and realised that this wasn’t an estate at all, it was a 4x4, and should be judged as such. And by this reckoning it wasn’t pretty good. It was, in fact, rubbish.

The model I drove was a standard length, entry-level 320 CDI, which is about £42,000. Plus Bluebottle Brown’s £80 punishment. This makes it nearly £10,000 more expensive than an entry-level Volvo and it’s hard to see why.

Yes, Volvo’s new diesel is no match for the creamy engine found in the Merc and yes, the XC90 wobbles where the Merc is smooth and flowing, but in 4x4 load luggers, handling, power and refinement must play second fiddle to the toys you get as standard and how much you can cram inside. The Merc fails on both counts.

First of all, just about every single gadget on my test car was an optional extra, which took it up to £46,000, and even then you get only six seats. This might be a bonus for an estate car but it’s one fewer than most 4x4s.

It’s the same story with boot space. With the third row of seats down, the boot is large. Bigger in fact than the Audi A6 or the E-class estate. But with them up there is no boot at all. What’s the point of that? No, really. What is the point of a car that takes up four spaces all on its own, but has no space in the back for so much as an after-dinner mint.

Mercedes points out that for an extra £1,500 it will sell you a version that’s 10in longer, which makes it even harder to park. It argues that in this you get an extra 200 litres of boot space. But how much dog can you get into 200 litres? The front half of an Irish wolfhound? So what do you do with the rest? Leave it at home?

I have no idea how many litres of boot space is offered by an XC90 but I know I can get two labradors and a small bike in there. And I know it has one more seat than the Merc and I know it’s cheaper, so why should I care if the engine’s a bit rough?

The worst thing about the Merc, however, is that it’s such a bore to drive. The larger engined R 500 (which was reviewed in Good Car Bad Car) at least has a burbling V8 to have some fun with. But in this version I went all the way to Co Durham and then back via London, Bedford and London again, and it never did anything remotely amusing. Except chew quite a lot of fuel.

It rode quite badly, failed to have the power for overtaking, gave me a hint of backache and was considered “ugly” by those who saw it. I will therefore do what the government cannot do and apologise for being a muddleheaded nitwit.

This, contrary to my earlier reports, is not an estate, it is a 4x4. And not a very good one. You’re better off buying a condom.

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