OVERTAKING SIGHT DISTANCES ON SINGLE CARRIAGEWAY ROADS

One of the objectives of the UK Department of Transport's Highway Link Design (HLD) guidelines for single carriageway roads was to develop layouts, making it clear to drivers where overtaking is possible and where it is undesirable. This paper presents recent overtaking data, and reassesses the overtaking sight distances specified in HLD. Data were collected on a 1175m overtaking section, forming part of a 7.3m wide standard single carriageway with a design speed of 80kph; the section was almost straight and level. For each overtaking manoeuvre, many parameters of overtaking behaviour were measured from video recordings; the 30 hours of recorded data provided 1143 overtaking manouevres of different types. It was found that the HLD values for full overtaking sight distance (FOSD) and abort overtaking sight distance (ASD), for design speeds ranging from 80kph to 120kph, were less than the values calculated by applying the observed data to models previously developed by the authors. These methods were substantially different from those used for deriving and calculating the HLD values. The authors consider that the HLD values' apparent underestimates may lead to increased accident risk on overtaking sections, and reduce the efficiency of single carriageway roads, because their overtaking sections provide too few overtaking opportunities. For the covering abstract see IRRD 859533.