Highway-user fees for overweight trucks: Consequences of deviations from the fourth-power law

In updating road-user fees, highway agencies are motivated by the twin objectives of equity and efficiency. The concepts of marginal pavement consumption cost (MPCC) and the fourth-power law have been used in past research in a bid to establish efficient and equitable user fees. Over the past few decades, the use of the fourth-power law has faced some criticism that the actual power of the relationship between loading and pavement deterioration deviates significantly from 4.0. In addressing this issue, this article presents a methodology that investigates the consequences of the power variations to the estimated MPCC and the distributional (equity) impacts of road-user fees for overweight (OW) trucks. The methodology incorporates practical schedules of highway maintenance, rehabilitation, and reconstruction. The analysis suggests that there are extremely small differences between the MPCC values that result from using different values of the exponent in the power law. However, different values of the exponent in the power law result in significantly different equity impacts. This article demonstrates that the equity impacts are moderate in the case of OW trucks whose individual axles are loaded close to standard axle weights. For OW trucks that are loaded within 10% of the maximum permissible weight, this article estimates that variation of the power value from 3.0 to 5.0 results in a 1%-10% surplus or deficit of their road-user charges.