Although trucks pay significant taxes, and there have been large percentage increases over the last ten years, they still do not pay their full share of costs. However, the rate of underpayment is considerably smaller than most of the public believes, and in some states trucks are paying more than their share of estimated costs. Two federal studies have been conducted on this issue since 1982.17 The first of these two studies was done in 1982 by the Federal Highway Administration (FHWA), and was based on 1977 data. That study found that five axle tractor-trailer combination trucks were paying 68% of their cost responsibility on federal aid highways. In response, federal truck taxes were increased 231% on a combination of fuel, truck and trailer excise sales tax, federal use tax, and tire tax. A 1987 study by the FHWA found that five axle trucks weighing between 70,000 and 80,000 pounds were paying 86% of their costs. In response, federal fuel taxes were increased 5 cents in 1990 and another 4.5 cents in 1993, although not all of the funds were committed to the Highway Trust Fund. FHWA is conducting a new cost allocation study for release in about two years.

Fifteen states have conducted their own cost allocation studies since 198618 The average finding was that five axle tractor-trailer trucks were paying 96% of their cost responsibility. Based on a review of these states' total fuel taxes, but not considering registration fees, the average fuel tax was 23.6 cents per gallon including sales taxes and surcharges. Michigan's effective rate of 15.0 cents per gallon would be 61.4% of these other states' fuel taxes. Without any consideration of registration fees one might conclude that Michigan trucks are paying about 60% of imposed costs. This rough estimate may indicate of the magnitude of Michigan's truck underpayment problem.