In the wider Northern California region, five districts have chosen CNG for new purchases. Those districts currently run a total of 324 buses, of which 218-bus Sacramento Regional Transit is the largest. The combination of fuel and filters, currently used in a Southern California demonstration project that includes San Diego school buses, Ralph's Grocery trucks, Santa Monica city buses and Hertz equipment rental, has reduced particulate matter by 91-99 percent over vehicles not using exhaust filters. The technology has been widely used in Europe to help countries meet international air quality agreements to reduce greenhouse gases. Despite assumptions than diesel can't provide as much clean air as compressed natural gas, calculations done by each of the local transit agencies confirmed what Europe has experienced.
Clean diesel produces fewer hydrocarbons and carbon monoxide as well.
Both are greenhouse gases. Each of the transit agencies had to consider the added expense if they chose natural gas. But for many agencies, it was the increased yearly operating costs and service concerns that fueled the final vote for clean diesel, according to Diesel Technology Forum spokesperson Pam Jones who testified at hearings of the seven major Northern California transit districts. Paradoxically, the higher CNG price tag could result dirtier air because fewer dollars would be available to retire old, dirty buses. Fewer buses mean more people in cars.