By Robert Plummer BBC News business reporter

They're big, they're bold, they're broad - and as far as US environmentalists are concerned, they're public enemy number one on the roads.

Thirsty SUVs are proving less popular when fuel prices are high

The gas-guzzling SUVs, or sports utility vehicles, saved Detroit's automotive industry from being wiped out in the 1990s as Japanese rivals took over the market for smaller cars.

Now Ford and General Motors are both reporting a slump in SUV sales for September.

Sales are down 50% compared with the same month last year, as consumers look for transport that offers more miles to the gallon.

Soaring gasoline prices in the wake of Hurricanes Katrina and Rita are being blamed for the dramatic decline, with some analysts predicting that America's love affair with the bulky vehicles has now reached a turning point.

"Could Katrina kill the SUV?" is the question being asked in some quarters.

However, this is not the first time that the end of the SUV has been predicted - and previous reports of its death have proved to be greatly exaggerated.

'No demise'

Back in March 2003 at the Geneva Motor Show, GM's Vice-Chairman Bob Lutz was moved to mount a spirited defence of the 4x4 vehicles in the face of an earlier sales drop.

"I believe the anti-sport utility faction is trying to create the demise of the SUV by reporting its demise when in fact no such demise is taking place," he said.

SUVs generally produce more carbon dioxide than normal cars

On that occasion, the "axles of evil", as some in the green lobby call them - because of the extra pollution they tend to produce - eventually roared back into favour with US consumers.

But this time, industry analysts are less sanguine about their long-term prospects.

"The market changed as it often does," Art Spinella of CNW Marketing Research said. "Frequently, that means Detroit gets left behind. It never fails."

Lighter vehicles

One possible way forward for US carmakers is to invest more in what are known as "crossover" vehicles - SUV-style vehicles offering greater fuel efficiency that are based on passenger cars rather than pick-up trucks.

However, this may offer little comfort to Detroit, since Japanese firms such as Toyota and Nissan have a lead over them in producing smaller and lighter sports utility vehicles.

In any case, hardcore greens are not expecting US motorists to desert their SUVs any time soon.

Activist Todd Hymas recently pointed out on environmental news blog Gristmill that "many Americans go into debt or spend beyond their means to drive the vehicle they believe best defines them as a person".

But a reduction in the number of 4x4 vehicles on the roads may have more positive effects than you think.

According to research carried out by US scientists in 2003, and published in the journal Accident Analysis & Prevention, pedestrians hit by large SUVs are twice as likely to die as pedestrians struck by cars.