An Investigation of Piecewise Structural Equation Modelling to Understand the Effect on Collisions of Shorter Visibility Designs at Urban Three Arm Junctions

  • Mustafa Ekmekci

Student thesis: Doctoral Thesis


Road Traffic Collisions (RTCs) are a leading cause of death and injury globally and are a considerable economic cost. Priority give-way junctions are the most common form of traffic junction in the UK and are associated with a high proportion of all reported collisions. Initially, the form or “design” of priority junctions evolved in the UK, based on the movement needs of particular vehicles. In the early to mid-twentieth century, their designs were formalised into standards. These were then updated to cater for increased car movements, by enlarging the dimensions and visibilities at these junctions; thus, increasing their capacity. It was believed that greater visibilities at the junctions would also improve safety. In the latter part of the 20th century, it was suggested that an unintended consequence of greater visibilities at these junctions was that drivers’ risk perceptions reduce, they drove through the junctions faster, and hence the high visibilities were leading to a greater number of collisions and increased severity. Design guidance was updated to reduce visibilities and dimensions of priority junctions in Manual for Streets (MfS and MfS 2). This study contributes to understanding of the precise nature of the associations between junction visibilities, geometries, vehicle speeds and the frequency of collisions for different road users, making different turning movements. A sample of 120 junctions in Portsmouth UK was used to develop a novel-to-the-field piecewise Structural Equation Model and an Artificial Neural Networks model. Results support the reductions in visibilities proposed in Manual for Streets and suggest particular combinations of geometries and visibilities as being important determinants of specific vehicle-movement accident combinations. The findings may be of interest to traffic engineers, and highway authorities, and could inform further updates to design guidance.
Date of Award29 Apr 2024
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorLee Woods (Supervisor), Djamila Ouelhadj (Supervisor) & Banafsheh Khosravi (Supervisor)

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