Arizona has built a national reputation on its incentives to encourage businesses and individuals to use vehicles that run on alternative fuels. They recently introduced tax credits that offer the biggest benefit to the cleanest cars. This could erase the extra cost of buying an alternative fuel vehicle. So far, the persuasions have yielded virtually nothing. Of 3.3 million registered cars and trucks on Arizona's roads, only about 6,600 use alternative fuels. With numbers like that, the environmental effect of such vehicles is obscured.

The federal government is pressing motorists and states to buy significantly more alternative fuel vehicles. When the year 1997, ended, federal agencies fell 17 percent short of their four-year mandate to buy about 34,500 vehicles that run on alternatives to gasoline. And most states are at risk of lagging. Congress set this goal in 1992 as a way to help spark demand for cars and trucks propelled by alternative fuels. Now automakers and environmentalists say the government's failure to adopt alternative-fuel vehicles is stunting consumer interest.

In Denver, with a car that uses natural gas, a lone driver is allowed to drive through high-occupancy vehicle lanes on freeways and city streets any time of day. Although natural gas vehicles are 98 percent cleaner than their gasoline- powered counterparts, only two cars have been sold in Colorado. The only thing cleaner is an electric car. Since natural gas vehicles have a range of about 150 miles in the city and 250 on the highway, drivers must plan fuel stops carefully if they travel away from Denver' s 20 natural gas fueling stations. Nationwide, there are 1,200 natural gas fuel pumps.

A new clean fuel that is up to half tap water has been approved for marketing as a primary fuel. The new fuel is still being tested to verify its low emissions, cold-start ability, and engine wear, but tests on multiple engine designs in Nevada, at Caterpillar Inc.'s Illinois testing facility, in California show it performs as well as conventional fuels but with a 60 percent reduction in key emissions.

The fuel does not meet the definition of an alternative fuel because of its petroleum content. But it is legal to purchase in the United States, and engine modifications to use it are legal as long as they don't boost emissions. For cars and light trucks, the only engine modification needed is a new set of spark plugs; for diesel buses and trucks it's a two-hour retrofit job costing less than $1,000. In either case, modified engines could run on this mixture or conventional fuels.