Abstract
The Meteorological Office, the Transport and Road Research Laboratory and the Home Office have combined in a trial to evaluate potential low-cost visibility Instruments as possible aids in motorway traffic control. The results of the trial show that the two instruments under test came close to the performance criteria lay down before the trial but that each had limitations. The short-baseline transmissometer showed a temperature drift which was sufficient to given an apparent visibility change of 7m oC-1 (at 200m), and the forward-scatter instrument gave poor results when obscuration was caused by factors other than fog.