This document describes a method to determine the in situ relative hydraulic conductivity, at specific locations, of a road surfacing that is designed to be permeable. An estimate of the average value for the surfacing is obtained from the mean value of a number of determinations on each section of road. The test measures the ability to drain water (drainability) achieved in situ of a surfacing. As such, it can be used as a compliance check to ensure that a permeable surface course has the required properties when it is laid. The test can also be used subsequently to establish the change of drainage ability with time. For the test to be valid, the surface of the test area should be clean and free from detritus. Measurements can be made when a road is either wet or dry, but not if it is in a frozen state.