Hi Smuj.
That's the fellah! If I remember correctly the device was driven at 40 MPH along paths 10 metres either side of the runway centreline in either direction. In the early days the operator, (a fireman at the first airfield I encountered it), would read off the recorded figures in thirds of each run. We would then convert these to co-efficient of friction figures that would be passed to aircraft as a verbal report, i.e. Good-Good-Medium Good to describe the braking action to pilots who needed the info. Later on, the machine was operated by airfield operations guys who would do all the converting for us which was just as well as I seem to remember that in the early days the 'conversion device' was an enormous metal circular slide rule like a ping-pong bat!
Spiney.