In a strange juxtaposition of views, many environmentalists begrudgingly agree that auto emissions should decline over the next decade under strict new controls.

But at least one major auto industry expert is cautioning that Americans' desire to hit the open road will eventually erode those gains.

Starting in 2004, automakers face strict new auto emission guidelines that will require cars and light trucks to follow the same standards for the first time.

Cars and mainstream trucks will fully meet the much cleaner standards by 2007, while larger light-duty trucks have until 2009 to get with the program. The Environmental Protection Agency (EPA) also plans to significantly limit emissions of the largest trucks, those weighing more than 8,500 lbs. (3,863 kg).

"Anybody who knows anything about emissions should be able to predict that emissions are going down," says Dan Meszler, director of Energy & Environmental Analysis Inc., an Arlington, VA, consultant group that recently completed an emissions study for AAA.

Mr. Meszler's study determined that vehicles now contribute less than 24% of the smog in 25 major cities, even when you adjust the numbers to account for more trucks on the road, he said.

"In my opinion, car emissions already have reached a low enough level where the error in the measurement can be higher than the standard," he says.

While Mr. Meszler's study is seen by the environmental community as self-serving to the AAA agenda of encouraging more Americans to take to the roads, it's hard to suggest auto emissions are increasing.

The U.S. Environmental Protection Agency model shows that overall emissions from light vehicles will decrease out into 2030 with 2004 rules in place, said Candice Morey, a transportation analyst for the Union of Concerned Scientists in Berkeley, CA.

"If cars continue to get cleaner, pollution will decrease," agrees Ann Mesnikoff, a Sierra Club staffer who studies automotive environmental issues.

But that doesn't mean the environmental movement is finally at peace with the automobile - far from it. One big sticking point is the flexibility the EPA has built into new rules that gives automakers more time to perfect diesel engines.

The California Air Resources Board (CARB) already has passed rules that all but eliminate diesels from its light-duty fleet. CARB has gone so far as to attempt to classify diesel exhaust as a human toxin. The problem is particulates, which cause respiratory problems in relatively low concentrations. Gasoline produces few particulates; diesel fuel produces significant amounts.

Even CARB's own technical staff doesn't agree with the board's extreme position and suggests the EPA be more flexible - but that also means that diesels will be allowed to emit more particulates than gasoline. The EPA is giving diesels a break because it's more fuel efficient than gasoline.

Under 2004 rules, it's possible that diesel engines could produce six times more particulates than gasoline through 2010, says Ms. Morey. But that would only happen if 50% of the light-truck market was diesel. That isn't likely; current levels run less than 3%.

"Encouraging more diesel is a mistake," Ms. Mesnikoff says. "Americans love their cars, but I don't think people are interested in diesel anyway. It's a health risk."

Outside of the shadow of the red, white and blue, the automotive environmental concern of choice is global warming, and the weapon of choice is diesel. Where Americans in big cities fear choking in smog, Europeans imagine the ice caps melting and oceans rising as the Earth swelters.

Europeans encourage diesels, which burn less fuel and thus produce less carbon dioxide, the chief culprit in global warming.

But Kelly Brown, head of vehicle environmental engineering at Ford Motor Co., doesn't predict the U.S. will opt to shift its policy - at least not until vehicle emissions of smog forming components hit zero.

"We're getting closer and closer to zero," he says. "Either we have to get to zero, or come up with a different fuel source."

The auto industry is pushing the EPA to move more slowly on 2004 rules, and not because automakers can't meet them. "We're going to have to re-engineer every vehicle we make. We just don't have the people for that," Mr. Brown says. Automakers want the EPA to alter some of the requirements to make it easier for them to adapt vehicles already required in California for some of the less stringent requirements during phase-in.

But surprisingly enough, Mr. Brown doesn't agree that the new rules will assure that vehicles produce less pollution in the future - not unless someone figures out a way to get people to drive less.

The number of vehicle miles traveled each year is increasing between 2% to 3% a year, with only war or a recession causing any blips in the trend. If it continues, even the strict new rules will only hold the line for a decade, Mr. Brown predicts.

But he doesn't expect the 2004 rules to be the stopping point. It's likely that stricter standards will be in place before vehicle miles travelled starts to increase the percentage of automotive pollution, he says.

"A lot of people thought we had cleaned cars up as much as we were going to have to in the 1970s," Mr. Brown says. "We're not any closer to being finished now."