Los Angeles County Metropolitan Transit Authority (Metro) has retired the last diesel bus in its fleet, becoming the world's first major transit agency to operate only clean fuel buses - nearly 100 per cent of which operate on compressed natural gas (CNG). According to Metro, compared with diesel buses, its CNG fleet reduces cancer-causing particulate matter by more than 80 per cent. And, because of the switch from diesel to CNG, Metro avoids emitting nearly 300,000 pounds of greenhouse gas emissions per da