The Pavement Serviceability Rating is a way of measuring the quality of the ride over pavement sections by a panel of raters (usually 10) based on a scale of 0 - 5, 5 representing the best ride quality. The average of these ratings gives the Present Serviceability Rating (PSR) for the pavement. A good-conditioned car is usually chosen as the common vehicle for use in the tests.

These ratings can be vehicle-type dependent because a more "sensitive" vehicle, such as a car instead of a bus, is often preferred for the tests. The car, with a smaller tyre imprint, is able to detect slighter pavement distresses, unevenness and irregularities in the pavement surface. A more sensitive vehicle gives a more accurate PSR (i.e. more applicable and "true" to the real situation). The bus, on the other hand, will give relatively low PSRs. Other vehicle conditions such as the vehicle suspension quality, the speed and skill of driving can also affect the ratings. Thus, PSRs conducted using different vehicles over the same pavement stretch gives different sets of values and should not be combined as an average. The PSR for a pavement only indicates the ride quality for that particular vehicle used, and is not representative for all vehicles.

But PSRs can also be vehicle-type independent. To obtain consistent PSR results that can be used to grade and compare with other pavements, it is necessary to specify and fix the "other variables" as constants throughout all the tests. If the vehicle used is the same throughout (independent of vehicle-type), we will be able to obtain more reliable PSR results, which can directly reflect the true pavement serviceability for the tested pavement.

Furthermore, past tests have shown that different sets of PSR values, done on the same pavement with different vehicles used, have the same general trends and differ only in the level of rating. In other words, the PSRs conducted with a bus are always lower than that with a car over the same pavement stretch, although they have similar trends. The use of a bus or other vehicles of poor sensitivity contribute to inaccurate and lower PSRs. But if the raters have very good experience and sensitive judgement during the tests, the choice of vehicle type will be indifferent and the PSRs will not differ significantly from the otherwise more accurate values obtained using a more sensitive vehicle.

In conclusion, rating a pavement involves a pre-determined vehicle for use throughout the tests. The choice must be a relatively sensitive-enough vehicle that allows the raters to rate the pavement's ride quality with a high level of accuracy. To compare different pavements' quality, the PSRs used should be from tests with similarly sensitive vehicles.