Impact of Compressed Natural Gas Fueled Buses on Street Pavements

Federal Clean Air Act Amendments of 1990 (CAAA) and the Energy Policy Act of 1992 (EPACT), together with other state regulations have encouraged or mandated transit systems to use alternative fuels to power bus fleets. Among such fuels, compressed natural gas (CNG) is attractive, although it must be stored in robust, heavy pressurized cylinders, capable of withstanding pressures up to 5,000 psi. Such systems are typically heavier than conventional diesel storage tanks. As a result, this raises gross vehicle weight, sometimes significantly, which then increases the consumption of the pavement over which CNG buses operate. Capital Metro, the Austin, Texas transit authority, is currently evaluating a number of CNG fueled buses. As part of the U.S. DOT Region Six University Transportation Centers Program (UTep), a study was instigated into the scale of incremental pavement consumption associated with the operation of these buses. The study suggests that replacing current vehicles with CNG powered models utilizing aluminum storage tanks would raise average network equivalent Single axle (ESAL) impacts by around six percent. This translates into an increase in total annual overlay rehabilitation costs across the network of over four percent. Finally, it recommends that a full cost study be undertaken with evaluating the adoption of alternative bus fuels which includes pavement and environmental impacts.