All fixed and variable costs associated with an airplane's operation are used, including pilots' salaries, cost of facilities, cost of assistance by officers on the ground, and depreciation. Cost data were obtained from sources kept by the Illinois Department of Law Enforcement. Total aircraft operating costs were $96 per hour. When the airplanes are used for line highway patrol, including the cost of assistance by officers on the ground, the average cost per mile of patrol is $1.33 compared to an average cost per of $1.08 dollars per mile for the same type of patrol performed by a solo police officer on ground patrol; however, the pilots produce more activity for the miles patrolled than an equivalent number of officers on the ground. From this perspective, aerial patrol is less costly than ground patrol for traffic law enforcement. When used for enforcement of traffic laws at a specific location, particularly speed enforcement, the airplane is far superior for the detection of flagrant violators. The primary benefit of the airplane is its ability to observe violations in different sections of the highway separated by several miles while remaining undetected by violators. Operation costs, however, are about 75 percent more than the costs of a ground detail using a radar operator and chase vehicles. Making the aircraft more cost effective would require greater productivity from supporting ground personnel. Tabular data, 19 notes, and 14 references are provided. (Author abstract modified)