Observations have been at a roundabout at Gants Hill, Ilford (Essex), of the numbers of drivers giving way to other vehicles at the points of entry, before and after the introduction of a right-hand priority rule. The results were analysed in a previous report (LN/175/EMH). In the present report, the analysis is simplified and extended. A `precedence index' has been constructed by expressing the number of entering vehicles giving way as a proportion of the total number of entering and circulating vehicles giving way. It was found that this precedence index is related to the relative flows of the two streams of traffic and a method is suggested of calculating a `corrected' precedence index, corresponding to equal flows in the two streams. The average value of this corrected index was 0.57 before the introduction of the priority rule and 0.87 afterwards. Some observations made at a single entry point of a roundabout in Manchester with a priority rule gave a corrected index of only 0.69. The reasons for the lower value may be that the first roundabout had white lines painted across the entrances and the second did not.
Abstract