Due to increasing traffic volumes on German freeways, widening of carriageways to four lanes becomes increasingly necessary. Within this thesis, traffic flow and road safety on freeways with four-lane carriageways were comprehensively investigated. The
empirical analysis of traffic flow characteristics covered the shape of the flow-speed diagram, the capacity, the lane flow distribution, as well as the drivers speed and headway behavior. The lane changing behavior of exiting vehicles at interchanges was
analyzed to identify specific requirements for signposting. The assessment of road safety was based on accident and incident data. In conclusion, recommendations for planning and design of freeways with four-lane carriageways are provided.