The pilot of the twin engine airplane reported that, while on a visual approach for landing, he transmitted the appropriate radio calls, and noted that all the airplane lights were illuminated. He added that the airplane touched down about 400 ft down the runway, at which time he noticed a vehicle approaching the runway from the south side. The pilot observed the vehicle hesitate, then cross the runway towing a large trailer with a tractor loaded on the back. The pilot swerved the airplane to the left to avoid impacting the trailer head on. The pilot was unable to avoid the vehicle, and the right wing impacted the trailer. The pilot reported that after the collision, the driver of the truck apologized, and stated that he thought he could beat the airplane across the runway. The driver of the vehicle reported that he looked both ways down the runway and did not observe any aircraft. He added that while he was crossing, he saw an airplane. He accelerated the vehicle, but the airplane impacted the end of the trailer. The airport manager reported that the driver of the vehicle was not one of the airports employees but had volunteered, without compensation, to cut and bale hay at the far corner of the airport. He added that all weather station or other repair technicians are escorted to their location by airport staff. He also reported that this was the first time that he had allowed the driver of the vehicle to cut hay on the airfield, and that he had thoroughly briefed the driver to stay 50 ft away from the runway. The driver reported to the airport manager, that he took the quicker and drier route, opposed to taking the longer and safer route because he was concerned with getting his truck and trailer stuck in an area that was known to be more saturated after the previous night's precipitation. A witness located at the airport reported that he observed the vehicle cross the center of the runway four times, with a truck and trailer, and two times while driving a tractor with a cutter attached. He noted that the driver of the tractor was traveling more than safe speed around the hanger areas. After the collision, the airplane was destroyed during the post-impact fire. In an effort to improve safety, the Precinct 4 Commissioner created a Standard Operating Procedure (SOP) that encompassed operating procedures, training curriculum, and a training manual that was derived from Advisory Circular (AC) 150/5210-20 Change 1.