As an engineer and a regular reader of your splendid column, I was interested by two recent correspondents, who presented opposing views regarding road speed, safety and the law, to do a little research and a few sums. I offer the following as food for thought. Studies have shown that, depending on road conditions, it takes between 0.5 and 2 seconds to take one's eyes of the road ahead, refocus on a nearby object and then subsequently 're-acquire' the street scene. Rain and darkness lead to the longer times. I would suggest that, without recourse to the speedometer, even moderately experienced drivers can judge their speed to within 10mph when travelling at a nominal 30mph, and greater experience will improve this accuracy. The official stopping distances (which are generally considered to represent ideal conditions) for 30 and 40mph are 23 and 36 metres respectively. A car travelling 30mph covers about 13.5 metres in one second. Calculating using a figure of, say, 1 second to check a speedo reading and re-acquire the road ahead indicates that a car travelling at 30mph, with the driver accurately maintaining his speed by frequent speedometer checks, could have a total stopping distance of 36 metres (13+23) - equivalent to that of a car travelling at 40mph. Similar results are obtained for other speed-pairs with differences of up to 10mph. Thus, it would seem that a little more leniency in the enforcement of speed limits would lead to an improvement in overall road safety, particularly in urban areas where the road scene can be very complex and, therefore, take longer to take in.