Mazda's Davis told us that under the current test procedures used by the U.S. Environmental Protection Agency, iStop would show a gain in city gas mileage would be no more than 0.1 or 0.2 miles per gallon.

Talking to the EPA

But Mazda's data shows that in heavily urban environments (think midtown Manhattan or Tokyo traffic), iStop can cut consumption as much as 8 percent. That could mean a boost of up to 3 miles per gallon, a far more compelling figure in the U.S. market.

The company is currently talking to the EPA, Davis said, about additional types of testing under which the EPA might let Mazda show an efficiency increase for the iStop system if it were fitted to the 2011 Mazda2. A rough analogy, he suggested, might be upshift lights; the EPA tweaks the ratings to grant an additional 1 mpg to cars fitted with them.

Would 3 mpg be worth $300?

Mazda might market iStop as a mileage enhancing option, Davis said, in those markets with the worst traffic--perhaps the cities of the Northeast Corridor, and the suburban sprawl of southern California--at a price around $300. Buyers could do their own math.

The sticking point right now, he said, is that under current regulations, any car must perform a complex series of diagnostic checks on itself every time the engine starts. That would be clearly impractical if the engine were starting and stopping once a minute or more.