rarely used than a quiet one that is used more frequently. Theoretically, Miller said, the Park Service could use noise level as a way to prioritize which vehicles need the most maintenance and which should be retired and replaced by quieter vehicles. A national, park-specific inventory database could also support noise management.

Weather and temperature are important considerations in managing noise from vehicles. Noise propagates differently depending on meteorological conditions, and understanding those effects when deploying vehicles is helpful in minimizing excessive noise.

The group also discussed various technologies for quieter vehicles and more efficient engines, such as hybrid vehicles, electric vehicles, and engines that use alternative fuels or means of propulsion, such as hydrogen helicopters with quieter rotors, aircraft with quieter propellers, or watercraft with four-stroke engines. Tracking these technologies is a good way to make sure that parks consider all options.


When making purchases, noise standards are available from the International Organization for Standardization (ISO) and SAE International for almost all types of vehicles. Manufacturers may be reluctant to provide exact numbers for vehicle noise but should be encouraged to do so. Partnerships with transit authorities and/or large private operators, such as FedEx and UPS, could be beneficial to the Park Service. Other government agencies, such as the US Army Tank Automotive Research Development and Engineering Center (TARDEC), can help with detection specifications, vehicle technologies, and measurement procedures.

Commercial purchasing partnerships may also be a strategy for the Park Service to consider. Leasing rather than purchasing vehicles could in some cases be beneficial, eliminating the need for maintenance and seasonal storage while allowing the lessee to set criteria for noise levels. Procurement specifications also could tie in with other initiatives such as NASA’s Buy-Quiet program or efforts to reduce fuel use and emissions of pollutants.

Quiet tires, which have been investigated extensively in the European Union, are another solution worth exploring. Putting limits on idling time, or adding smaller engines (e.g., auxiliary power units) for climate control on vehicles such as tour buses, also could have an impact. When practical, transportation vehicles should not be left idling when not in use, although there are some cases (e.g., diesel engines in winter),

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