The speeds and flows achieved by traffic on road links, and their relationship with road geometry provide an important basis for Highway Link Design and the economic assessment of road improvements. The current relationships used in the DTp economic appraisal package COBA are based largely on the results of a study carried out in 1977-78. Changes in driver behaviour, improvements in vehicle performance and other factors make it necessary to review these relationships periodically. The report describes the observations and analysis carried out during 1990 to update these relationships for rural dual-carriageways and motorways. Speeds and flows were measured for 16 hours on each of 26 sections of road carriageway chosen to be representative of the national road network, both in link geometry and traffic flow conditions. A form of the moving observer method was employed to provide speed information, while flows were obtained from static counts. The surveys were carried out for light and heavy vehicles separately in good weather conditions during daylight. Geometric parameters were obtained from Ordnance Survey maps and site measurements. This data was analysed using step-wise multiple regression to relate the speeds and flows to the corresponding geometric characteristics. The relatively small sample, and the wide range of potential explanatory variables meant that the data required careful analysis to avoid misleading results arising from cross-correlation of the data. The results of the analysis were generally satisfactory, but for low flows it proved necessary to rely to some extent on the results from previous studies. There was no evidence to suggest a need for a major revision to the form of the existing model connecting speeds and flows to the geometric characteristics. At the detail level, the main changes since the previous study are that the speeds of both light and heavy vehicles have increased significantly. There have also been some minor changes in the effect of the geometric parameters, such as a change in the effect of hilliness on vehicle speeds. Updated relationships connecting speed, flow and link geometry are recommended for both light vehicles and heavy vehicles.

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