Anne Johnstone: Will yesterday be remembered as the day Gordon Brown took the bull bars by the horns? Why do people buy gas-guzzlers anyway?

Will yesterday be remembered as the day Gordon Brown took the bull bars by the horns? I wonder why people buy gas- guzzlers. Sophisticated marketing? A fashion statement of the greedy? Few would admit to it. An advertisement for the new Freelander 2 dropped on my mat this morning. It told me that motoring journalists had tested it across deserts and frozen wastes but there was no mention of its manoeuvring abilities in Tesco car parks. That's the problem. Country-dwellers are forever telling us they need their 4x4s but, with few exceptions, even they have only bought the hype. The tarmac stretches all the way to John O'Groats these days. That's why research suggests only 5% of these beasts ever venture off-road.

Safety? Certainly, many people, women especially, say they like them because they make them feel safe, high above the ground in their reinforced metal box. But, as Dr Lesley Walker of Imperial College, London, put it succinctly in the British Medical Journal recently: "In general, 4x4s may reduce the risk for their occupants but they increase the risk for everyone else." In crashes involving a 4x4 and a smaller car or pedestrian, the risk to the other party is substantially increased. It's an example of "risk compensation", where the safer a person feels, the riskier their behaviour becomes. This may account for 4x4 drivers being four times more likely to be seen using hand-held mobiles. A reward is offered for any reader never to have spotted one.

Carrying the kids? This one, too, is a bit of a mystery as many owners boast a brace at most and, besides, big families were far more common before gas-guzzlers existed. Family friendly? Not to families flooded from their homes in Mozambique, Bangladesh or fast-disappearing Pacific islands as a result of global warming.

People talk about "green 4x4s" but, let's face it, a heavy, fast- moving, high-rise car eats energy and excretes carbon. The monsters affectionately known as Hummers have poorer fuel efficiency than the century-old Model T Ford, the world's first mass-produced car. Driving a 13mpg car for a year will waste more energy than leaving a fridge open for seven years. In town, a big Range Rover will emit 389g of carbon dioxide per kilometre, double a two-litre fuel-injected saloon and nearly three times that of a Smart car.

Is the message starting to get through? For the first time, 4x4 sales fell slightly last year. Probably this had less to do with car tax than rocketing insurance premiums (because they are involved in more accidents) and threats from various city authorities to start charging them a premium to enter. And something else perhaps. A woman with an "outraged Tunbridge Wells" tone of voice phoned Radio 5 Live the other day to complain about a group of small children chorusing "planet killer" at her as she drew up in her Range Rover.

Their minds uncluttered by cynical counter-claims and rationalisations, our children feel passionate about our global stewardship. Could it be that pressure from them is driving this change? Perhaps it's no longer cool to be dropped off at the school gate from a Porsche Cayenne or VW Touareg.

When Gordon Brown entered the Treasury a decade ago, vehicle excise duty - car tax to you and me - was charged at a flat rate. Following yesterday's announcements in the Budget, car owners will soon be paying from nothing at all to £400 a year, depending on their vehicle's age and emissions.

What a difference a year makes. In 2006, urged by the green lobby to tax Chelsea tractors off the road, the Chancellor's response was craven. The creation of a £210 band (Band G) for the 225,000 most polluting vehicles amounted to the cost of an extra half tank of petrol. Like the equally ineffectual doubling of Air Passenger Duty, to those obliged to pay up it is no more than an irritation that will save not one gram of carbon dioxide.

Meanwhile, the gleeful amusement that greeted proposals from Ken Livingstone to slap a £25 extra charge on 4x4s and from various London boroughs to penalise their owners for parking in the street, plus increasingly convincing evidence of the speed and seriousness of man-made climate change, have produced a quiet revolution.

Inevitably, yesterday's package of announcements on car tax appeared to please nobody. The green lobby quoted Ireland, where high-emission cars attract a £1000-a-year duty, and argue that the differential between high- and low-carbon cars needs to be wider still if the objective is to transform the market. Petrolheads muttered about policies that have more to do with envy than environment and moaned about "tax grabs" (though the changes will generate less than £300m).

Brown has a double dilemma. The specific issue is that the government which unwisely poured millions into trying to salvage Rover will look a bit stupid if it then proceeds to tax the Discovery and the Range Rover off the road. More generally, as an astute politician, the PM-in-waiting knows that the British people talk with forked tongue on green issues. Like St Augustine on chastity, their silent prayer is: "Oh Lord, make me carbon neutral . . . but not yet."

Car sales offer a convenient illustration. As car manufacturers never tire of telling us, vehicle engines are far more efficient than they used to be, but what have car owners done with this green gift? They have squandered it on driving further, air-conditioning systems and bigger, more powerful models. That's why carbon emissions from cars in Britain have risen and now account for 12% of the total. Sales of 4x4s, most of which fall in Band G, rose from 78,000 in 1996 to 187,000 in 2005, while sales of the smallest cars have fallen. One car in seven is now a 4x4.

Of course, the debate doesn't start and finish with Chelsea tractors. They are merely the easiest targets. Over 6000 miles, even my bog-standard Golf will emit its own weight in carbon dioxide. So why not leave it at home? Well, one grubby, rattling, hourly bus, often 15 minutes late, with big gaps in the evening schedule and nothing to speak of on Sundays, sums it up. Not to mention a timetable that appears designed deliberately not to integrate with the trains at my nearest railhead, where, incidentally, it's impossible to park after about 8.30am.

Believe it or not, car ownership is lower in Britain than across most of Europe, but congestion is worse. Why? Because if you live in, say, Prague, you take public transport during the working week and reserve your car for occasional rural jaunts and carrying heavy loads. In Britain, we drive more because public transport is so poor. If Gordon Brown had announced yesterday that he was restoring the fuel duty escalator and hypothecating every penny to creating fast, free, frequent national bus services and improving capacity and subsidies on a high-speed, high-quality rail network, would anyone feel inclined to blockade fuel depots as they did in 2000? Unlikely.

I'm told my grandfather owned the first private car in Ayr, but, having fallen on hard times, ended his working life travelling to work by bus. One day I hope to tell this story to my grandchildren and have them reply: "So what?"