Frequency and safety of small radii on motorway-like roads in greater Stuttgart area - recommendations for future guidelines of motorways

Abstract

The German Guidelines for the Design of Motorways (RAA 2008) assume sufficient safety of a circular curvature if a vehicle does not slip of the road at a given design speed. Earlier researches indicated a higher accident-risk of small radii R < 1000m frequently used on motorwaylike road with a design speed of 100 kph given in German Guideline for Motorways RAA 2008. With regard to the “Vision Zero”, no fatal accidents, it was examined whether minimum radii should be increased to improve safety. For this purpose, 174 km from the entire 200 km motorway-like road network in Greater Stuttgart Area were selected. The existing circular radii were evaluated according to their frequency and accident risk using databases of the State of Baden-Württemberg about road geometry and accidents including the accident cost rate for each 100 m section. Within the selected network 212 circular curves exist. All radii R ≤ 1000 m were examined for their accident cost rate. Radii with an accident cost rate above the average were classified as risky. The investigation showed that 33 % of all applied radii are below R < 1000m. Circular curves with R < 650m showed a significantly higher accident risk and should be excluded. Circular curves between R ≥ 650m and R < 900m showed a higher accident risk if the ratio of adjacent radii is considerably above R1/R2 ≥ 1.5. After a long straight line the minimum radius should be R ≥ 1000m. The study results in recommendations for the geometric design introduced by the follow-up of German Guidelines for Motorways.