There is a body of evidence that suggests that driver fatigue increases crash risk and is a contributing factor in motor vehicle crashes; this is especially critical when operating and driving commercial tractor-trailer trucks due to their weight and size. A National Transportation Safety Board study on nighttime truck driving--where fatigue and drowsiness are expected to be more prevalent--found that fatigue may have been a contributing factor in 30 to 40 percent of reported crashes and 31 percent of fatal crashes. Federal Hours Of Service (HOS) rules mandate regular rest periods for heavy commercial vehicle operators. However, without timely, accurate information on parking availability, drivers are often left with the dilemma of choosing to continue driving fatigued or to park illegally along road shoulders and ramps. The need for truck parking information has been supported by many survey studies that have found a gap between driver perception and parking availability; for example, a 2002 Federal Highway Administration (FHWA)-sponsored survey indicated that 89 percent and 67 percent of commercial motor vehicle drivers perceived shortages of adequate information at public-sponsored and private truck stops, respectively.