In The Per-mile Costs of Operating Automobiles and Trucks, Gary Barnes and Peter Langworthy provide a spreadsheet model for calculating the costs of operating cars and trucks, specifically for use in planning highway projects which change the conditions under which people drive. The specific costs addressed area fuel consumption, routine maintenance, tires, repairs, and some depreciation. Their approach to estimating costs is innovative in several ways. They have developed a way to determine the marginal costs of actually driving exclusive of the fixed costs of ownership. They also offer methods for adjusting the costs for different conditions and guidance on how to adjust costs in the future. The authors conclude that in a case of highway driving on smooth pavement at $1.50 per gallon for gasoline, personal vehicles average 17.1 cents per mile to operate and trucks average 43.4 cents per mile. City driving conditions increase these costs by 3.9 and 9.5 cents per gallon, respectively.