Calibration of a method for selecting roundabouts as a function of the inner Island treatment

It is generally accepted that a driver's ability to perceive, recognize, and react to surroundings contributes to safer roads. The objective of this paper is to investigate the perceptibility of roundabouts as influenced by their most distinctive feature - the central island treatment. Previous work has shown how visual perception of central island treatment can improve road safety performance. Recently, the authors have suggested a roundabout central island treatment classification scheme with three categories: Reduced, Compressed and Slender. Classifying roundabout central island treatment types was necessarily somewhat subjective. This paper introduces a more quantitative measure for central island visual perception, based on the theory of visual intrusion as measured by the solid angle subtending the central island treatment. Data were collected using laser instruments, Google Streetview® and Google Earth®, and the related solid angles have been estimated for a large sample of roundabouts located both in Tuscany (Italy) and in the United States. The classification scheme is analyzed using the nonparametric Fisher's exact test which shows the central island treatment classification scheme is not associated with roundabout size. Therefore, central island treatment can be utilized as a unique metric for categorizing roundabouts. Descriptive statistics indicate that roundabouts with compressed island treatments have the largest solid angles (and implied visibility) when compared to either reduced or slender treatments, which have similar levels of visual intrusion as estimated by their solid angle.