BOULDER - Boulder Yellow Cab rolled out two types of environmentally friendly additions to its 40-car fleet: two Prius hybrids and two propane-powered cabs.

"Boulder's an environmentally aware city. The company is responding to customer demand with the green expansion of its fleet," said Sean Duffy, a spokesman for Boulder Yellow Cab. "By investing in cutting edge technology we're reducing the city's overall carbon footprint while reducing on fuel costs."

The hybrids sport a gingko leaf that runs the length of the cab, and the tag "Powered by Propane" flanks the two other cars. According to Duffy, the Prius costs more upfront than similar nonhybrid choices, but enhanced fuel efficiency makes up for it over time. The propane-powered cabs are retrofits. The propane tank is installed in an existing car's trunk and presents a less expensive initial capital investment.

"The propane-powered cabs will run at between 15 and 20 more miles per gallon with half the CO2 emissions as compared to a nonretrofit vehicle," Duffy said. According to Consumer Reports Top Picks for 2008, the Prius logged 44 mpg overall, winning the green-car category.

The green fleet unveiling follows the company's decision to organize its taxi stand at 11th and Pearl streets in the egress around the corner from the Boulder Bookstore.