PRICE4 refers to the marginal cost per axle of weight w if the
highway agency continues to use DEF4, whereas PRICEa refers to the
marginal cost per axle of weight w if the highway agency switches to
the appropriate DEFa. If the power a associated with the appropriate
DEF is different from 4, the current estimate PRICE4 will be very
inaccurate (i.e. significantly different from the appropriate estimate
PRICEa) for the relatively small values of w. Furthermore, if a
significantly differs from 4, the current estimate PRICE4 will also be
inaccurate for the relatively large values of w. An example is when
the scheduling of MR&R is based on rutting (a ¼2.98).