2+1 Roads with Cable Barriers – Traffic Safety and Transport Quality Effects

With the objective to improve traffic safety, the Swedish National Road Administration decided to replace the traditional 13 m road with a 2+1 design, i.e. a middle lane changing direction every 1-2.5 km, with a median cable barrier separating the two directions of travel. The decision to implement this new concept as a standard was based mainly on the expected traffic safety effects, which are found to be substantial. The number of severe and fatal injuries is significantly reduced, yielding traffic safety effects not far from those of a full extension to motorway. However, the question is raised whether the new lane arrangement in combination with a physical barrier has a negative impact on serviceability. A model for vulnerability analysis is developed and applied to two road objects of 2+1 design with a median cable barrier, describing their traffic performance during abnormal conditions. Reduced serviceability is found to be the result to some extent from physical obstructions and quite often due to winter weather, while temporary increases in travel demand can cause rather great disturbances. The concluding discussion touches on the trade-off between traffic safety and serviceability in terms of transport policy goals.