Abstract
Three 1-Hour films of moving traffic on the M 1 motorway were used to present synthetic closed-circuit television pictures. Twelve police officers and 84 civilians monitored one or three simultaneous films, looking for vehicles parked on the hard shoulders. Over 75 per cent of incidents at or nearer than 440 yards were reported, compared with under 45 per cent of incidents at or beyond 660 yards. The police officers were better at the task than the civilians. More detailed analyses suggested that in monitoring more than one film, looks should be long enough to detect distant incidents, but not so long that the reporting of near incidents on other films is unduly delayed.