The tyres of a lorry travelling at high speed on a wet road can pick up and eject many litres of water every second. Much of this is shattered into small droplets, and the resulting cloud of spray is a nuisance and a potential danger to car drivers. Research at TRRL aims to produce ways of measuring the practical significance of spray reductions to drivers, so that the effectiveness of alternative methods of spray control can be assessed. Part of the work involves developing improved physical methods for measuring spray. The usual method is to measure the degree to which the spray cloud attenuates a beam of light. As the lorry passes a trackside measuring station, a time-varying signal is produced at the beam detector, indicating the variation in spray density along the spray cloud. This report shows how the signal may be analysed to give the transmittance of any portion of the spray cloud swept by the light beam. It should allow research results to be linked more directly with what drivers actually see, and enable results from different research projects to be compared. (A)

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