On October 27, 2004, about 1000 eastern daylight time, a Bombardier CL-600-2B19, N592ML, operated by Mesa Airlines Inc. as US Airways Express flight 2924, was substantially damaged while taxiing at Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. The 2 certificated cockpit crewmembers, 1 cabin crewmember, and 38 passengers were not injured; while 1 airport employee sustained minor injuries. Visual meteorological conditions prevailed for the planned flight to Montreal, Canada. An instrument flight rules flight plan was filed for the air carrier flight conducted under 14 CFR Part 121. The flightcrew reported that the airplane was taxing for takeoff at PHL. While on taxiway "SA," they observed an airport vehicle parked on the grass, off the right side of the taxiway. The flightcrew also observed a larger airplane that had taxied past that point, and thought the clearance was adequate. However, as the airplane continued to taxi, the right wing struck the rear of the airport vehicle. The driver of the vehicle stated that it was yellow in color, with lights flashing, and parked in a grass area. He was seated in the parked vehicle when it was struck from the rear, forcing the driver backward into his seat. According to a Federal Aviation Administration (FAA) inspector, the taxiway was 50 feet wide, with a safety area that extended an additional 34 feet from the edge of the taxiway. The wingspan of the airplane was 69 feet, 7 inches. The driver of the yellow airport vehicle was escorting two other vehicles, which were parked on the grass, outside of the safety area. However, the driver of the yellow vehicle parked about 6 feet from the edge of the taxiway; about 28 feet in the safety area. Additionally, the driver did not inform air traffic control (ATC) that the vehicle was parked there, and buildings obstructed the view from the control tower. During the collision, the right wing sustained substantial damage, and the driver of the vehicle was injured. The FAA inspector further stated that the driver of the vehicle was subsequently counseled by PHL Airport Operations. After several requests from the FAA inspector, PHL Airport Operations failed to provide information on vehicle operator training and procedures. The inspector added that the training was not regulated. The reported weather at PHL, at 0954, was: wind from 350 degrees at 5 knots; visibility 10 miles; few clouds at 4,500 feet; ceiling broken at 14,000 feet; ceiling overcast at 20,000 feet; temperature 55 degrees F; dew point 44 degrees F; altimeter 30.22 inches Hg.