It's a pity the two most pressing issues in motoring have diametrically opposed solutions. An environmentally friendly car is invariably a light one; a safe car is indubitably a heavy one. So do you protect your family or the planet? Simple, your family. That's one reason why cars are getting fatter.

Cars are also becoming more frugal, primarily because of the booming use of electronics to monitor and control engines. Yet much greater progress would be made if cars were also getting lighter. While most industries have managed successfully to miniaturise, with a concomitant boost in reliability and energy saving, the car industry continues to think big. Check out the small and light batteries used in mobile phones or laptops. And then try to pick up a car battery - if you're strong enough.

Vent controls and associated trunking are as bulky and old-fashioned as ever, ditto wiring looms (although the gradual move to multiplex wiring will help). Engines and gearboxes are still far too bulky, and so are brakes and axles and lights and wipers and glass and, for that matter, just about every other bit of your car. And as if cars weren't heavy enough in the first place, buyers seem obsessed in making them fatter by adding sunroofs and electric seats and, if you've got one of those big, daft 4x4s, bull bars and various other bits of butch-looking addenda.

The move to make cars cushion occupants better in crashes is one of the main reasons for this obesity. Side impact beams are heavy, so are stiffer structures better to absorb batterings. Airbags add weight. Conversely, more weight makes a car less wieldy, and less able to avoid an accident. As in most other areas of our lives, the priority seems to be on protecting us once we've made an error, rather than encouraging us to avoid the mistake.

I've met quite a few car company safety gurus, and usually ask them who makes the safest cars. Most bosses answer "Mercedes-Benz", while giving honourable mentions to Volvo and Saab. But they invariably add: the bigger, the better.

Sure, there are some differences, manufacturer to manufacturer, in how well a car is engineered: some have better crumple zones than others, some catch fire more easily, on some you can open the doors after a smash and on some you can't. But the physics is immutable: when a big car hits a little car, the big car usually comes off better.

On this basis, and on the basis that the greatest responsibility a manufacturer has is to protect customers, all car-makers would ditch lightweight aluminium or composite body technology (on which some makers are already well progressed), component miniaturisation, and small cars. They'd set out to design and build tanks that weighed as much as possible without completely ruining the driving experience, and tell the punters: "Hang the cost of fuel, if you want to protect your family, this is the way to go". In other words, they'd follow Volvo.

American makers, now the world's most profitable, have also traditionally behaved this way. Americans have never fallen out of love with big cars, despite what you may have heard to the contrary. They would still rather be surrounded by 16 x 6ft of steel and chrome than 12 x 5ft of Japanese metallised origami and, with gasoline still hovering around a dollar a gallon, they'd have no trouble slaking their thirsts.

No: the only reason Americans are buying more small cars is because of the American government's one sop to car energy conservation. It's called CAFE (corporate average fuel economy) and it is legislation that ensures all makers selling cars in the States must average 27.5mpg across their range. If you want to sell gas guzzlers (and most do) then fine. But you have to sell some sippers to counteract the slurpers.

Most American makers reckon this is a crazy compromise. They want fuel prices increased, better to conserve energy and also to bring America in line with most other (potential export) markets, where petrol is much pricier. No American government - fearing electoral suicide - has ever agreed. Bill Clinton, liberal background notwithstanding, is no different.

No matter how unpopular the move may be in Europe, or in the States, one way to stop cars becoming more and more like designer tanks, and encourage the move towards much lighter and more efficient cars, is to raise fuel prices. (The only other way might be to introduce a maximum weight, as we have with lorries, but there's already too much legislation stifling car engineers.)

Large increases in fuel prices will force punters to take mpg more seriously, with the upshot that they'd think smaller (more accurately, they should think lighter, but let's take one piece of re-education at a time). Renewed efforts will go into new materials and into miniaturisation, to make cars more frugal. Efficiency will become important in design once again - just as when the Mini was conceived in the wake of the Suez crisis. And efficient design is invariably intelligent design.

Plus: if most cars on the road were light, then there wouldn't be the same urgency to buy a tank. It's only when the light car hits a big car that the lightweight looks vulnerable.

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