powered vehicles, dedicated NGVs have lower exhaust emissions of carbon monoxide, nonmethane organic gases, nitrogen oxides, and carbon dioxide.

Natural-gas engines are more fuel efficient than gasoline engines are, and CNG in the past has had a low price (about 80 percent that of gasoline on a gasoline-equivalent gallon basis). Also, transport and distribution are relatively inexpensive because infrastructures already exist for delivery both to households and to industries (Yborra, 2006). Despite these advantages, however, NGVs still face many hurdles. The two main hurdles are insufficient numbers of refueling stations and inconvenient onboard CNG tanks, which take up most of the trunk space.

An NGV market can be analyzed using the vehicle-to-refueling-station index, or VRI, defined as the ratio of number of NGVs (in thousands) to the number of natural gas refueling stations. According to Yeh (2007), “Using techniques including consumer preference surveys and travel time/distance simulations, it has been found out that the sustainable growth of alternative fuel vehicles (AFVs) during the transition from initial market development to a mature market requires [that] the number of alternative-fuel refueling stations be a minimum of 10 to 20 percent of the number available for conventional gasoline stations.” A thriving NGV market tends to have an index of 1; this gives rise to a problem: new stations are not being opened because of the lack of users, but few people use NGVs because of the lack of refueling stations.

A key disadvantage of NGVs is their limited range. While the average gasoline or diesel vehicle can go 400 miles on a tank full of fuel, the range of an NGV is only 100–150 miles, depending on the natural gas compression. Given this fact, together with the shortage of refueling stations, the current prevalent choice is to use a bi-fuel NGV that can run both on natural gas and on gasoline. The problems associated with bi-fuel engines include slightly less acceleration and about 10 percent power loss compared with a dedicated NGV, given that bi-fuel engines are not optimized to work on natural gas. Further, warranties on new gasoline vehicles are strongly reduced if they are converted into bi-fuel NGVs. But perhaps the most important barrier to NGVs could be the public perception that compressed natural gas is a dangerous “explosive” to have on board one’s vehicle and that self-service refueling with a high-pressure gas may be too risky to offer to the general public.

About 22 percent of all new transit-bus orders are for natural-gas-powered vehicles. Therefore buses, together with corporate-fleet cars that stay in town,