5. Conclusions
Researchers have stated that in order to achieve equity and
efficiency, motorists should be charged MR&R marginal cost
prices based on the axle weights of their vehicles raised to the
fourth power. However, this power is often inaccurate. Depending
on the definition of pavement deterioration, such as rutting, loss
of serviceability, roughness or cracking, the appropriate power
ranges from 3 to 6 or more.
This study assesses the importance of using the appropriate
units of traffic loading (i.e. appropriate DEF) in estimating the
marginal costs of highway pavement MR&R. It considers a
highway agency that uses a condition-responsive MR&R strategy
and assumes that the agency currently uses DEF4 (known as ESAL)
as the unit for traffic loading, whereas the appropriate unit that is
consistent with the MR&R strategy is actually DEFa.
If the highway agency switches from DEF4 to DEFa that affects
not only the marginal cost (per DEF), but also the number of DEF
units corresponding to a given axle of a weight w. Therefore, this
study compares the values of MR&R marginal cost per axle, rather
than per DEF, under different powers.
We find that under MR&R marginal cost pricing, the sum of
MR&R prices paid by all axles remains the same regardless of the
power p used for the DEF (at the current equilibrium). However,
the value of p has equity and efficiency implications because it
dictates how this total amount is distributed among the axles
belonging to all the different vehicles.
The marginal cost price for an axle equals the product of the
number of DEF units and the marginal cost per DEF for the axle.
The value of p has two effects on this price. First, it obviously