THE EFFECT OF THE WIDTH OF SUBURBAN ROADS ON THE MEAN FREE SPEED OF CARS

The paper describes the results of field investigations carried out on untroubled roads in Belfast to derive an expression in quantitative terms. The spot speeds of cars under free flow conditions were measured using a cine-camera to film their passage along streets of various widths. The camera was placed in a side street perpendicular to the line of travel so that it could not be seen by approaching drivers. The results are tabulated and an expression derived showed that the mean free speed increased linearly with carriageway width within the range of 5.2 to 11.3 M. Speeds were measured in dry conditions during off peak periods on Friday afternoons. Measurements made with the road wet showed a decrease in speed of about 10 per cent. /TRRL/