Basis of the Swiss Design Standard for Roundabouts

Abstract

The roundabout form of intersection underwent a huge boom in Switzerland in the 1990s. So-called compact roundabouts have proved particularly successful with regard to traffic safety; in many cases it was possible to eliminate accident "black spots" by converting existing intersections. Analysis of accident data has shown, however, that safety improvements have not produced satisfactory results at all roundabouts. Indeed, before-and-after comparisons indicate certain roundabouts actually experienced increases in the number of accidents. The causes of these negative effects appear to lie in local circumstances. A research project was launched to investigate the relationships between accident occurrence, traffic flow, and geometry in the area of roundabouts. The goal of the project was to develop basic principles for a Swiss design standard. The central importance in roundabout design of the deflection of vehicle pass-through tracks by means of the roundabout's central island was established. An insufficient deflection of the vehicle stream from the straight direction of travel results in failures to give way, increased pass-through speeds, and underestimations of these speeds by the other parties in conflict situations. These consequences are manifested in increased accident frequency.

Identifying crash contributory factors at urban roundabouts and using association rules to explore their relationships to different crash types

“…Furthermore, consistent with Swiss studies (Spacek, 2004), because of the high speeds of the circulating vehicles, the major contributory factor of too large radius of deflection of the approach to the left is associated with angle crashes at entry (S = 6.9%, C = 54.3%, L = 2.61). Consistent with the same Swiss study (Spacek, 2004), which found a negative correlation between the entering-circulating crash rate and the deviation angle, the major contributory factor of a small entry angle is associated with angle crashes at entry (S = 6.2%, C = 41.5%, L = 2.00). The association discovery has also shown that the combination of the major contributory factors radius of deflection and deviation angle is associated with angle crashes at entry (S = 4.7%, C = 32.2%, L = 2.23).…”

“…However, in two approaches, a negative deflection angle is present. Low values of the deflection angle contribute to failures to give way, increased pass-through speeds, and underestimations of these speeds by conflicting parties in the subsequent approach on the right (Spacek, 2004).…”

“…The use of roundabouts improves intersection safety by eliminating or altering conflict types, reducing crash severity, and causing drivers to reduce speeds (FHWA, 2000;Highways Agency, 2007;Lenters, 2005;Rodegerdts et al, 2010;SETRA, 1998;Spacek, 2004). Indeed, large and highly significant crash reductions were observed following a conversion of signalized and stop-controlled intersections to roundabouts.…”

Dernières avancées sur les carrefours giratoires

“…Selon le ministère des Transports du Québec (2002), il semble qu'un angle de visibilité trop important pourrait nuire au fonctionnement sécuritaire d'un carrefour giratoire car il inciterait les automobilistes à circuler à vitesse plus élevée. La réduction du champ de vision, notamment par l'obstruction de la vision à travers l'îlot central, serait donc né-cessaire pour forcer les automobilistes entrant dans le carrefour giratoire à adopter un comportement prudent et une vitesse convenable (Spacek 2004).…”

“…Chaque région doit donc adapter le giratoire à ses propres besoins, ses propres caractéristiques. Ainsi, en Suisse, l'établissement de normes pour la construction des carrefours giratoires s'est appuyé sur une analyse des accidents observés au droit des giratoires au pays (Spacek 2004). Cette analyse a en effet pu mettre en lumière les principales caractéristiques du carrefour giratoire qui permettent son fonctionnement performant et sécuritaire dans l'environnement suisse, soit la déflexion, l'obstruction de la visibilité à travers l'îlot central et le rayon d'entrée.…”