Serious accidents regularly occur at intersections controlled by traffic signals, both inside and outside of urban areas, when traffic turning left (across the oncoming traffic) collides with oncoming traffic or with pedestrians or cyclists who have been released in parallel. According to calculations by the UDV, accident costs are halved if a separate signal phase for traffic turning left is introduced. Decision-makers, however, often resist such moves, as they claim that this slows down the flow of traffic. The UDV therefore commissioned a study from the Technische Universität Dresden to determine whether this argument is valid. Simulation of a wide range of types of intersection and traffic signal phases showed that an additional protected green phase for traffic turning left generally resulted in no relevant restrictions to the capacity. Intersections in the road network can be constructed and operated in different ways. The combination of the construction and the way in which the traffic is regulated is referred to as the intersection type (Eckstein/Meewes 2002 and Draft Guideline for Designing Roads outside of Built-up Areas, RAL). At locations with a high volume of traffic, intersections regulated by right-of-way are as a rule being replaced by intersections regulated by traffic signals. Traffic signals make the road space within an intersection available to the competing streams of traffic alternately in a cyclic sequence (phase sequence). The different streams of traffic then cross the intersection in sequence. If a free-flow solution or a roundabout cannot be considered, intersections and junctions controlled by traffic signals are regarded as intersection types that provide both high capacity and a high level of traffic safety. Studies have, however, shown significant discrepancies in this view: Traffic signal control does not always make an intersection safer than control on the basis of right-of-way, as it is possible that initially there is only a change in the structure of the safety deficiencies, depending on the type of control and the number of phases, but that there is no resulting improvement in traffic safety. According to the German Guidelines for Traffic Signals (RiLSA 2010), traffic signals are installed to “increase traffic safety and/or to improve traffic flow quality. […] The set-up of a traffic signal system has to be considered if accidents which may have been prevented by traffic signal control have occurred repeatedly and if alternative measures (such as speed limits, overtaking prohibitions or constructional crossing aids to pedestrians or cyclists) have proved to be ineffective or not promising.” According to Section 37 of the German “General Administrative Regulations on the Road Traffic Regulations” (VwV-StVO), situations in which traffic signals are necessary include those where accidents frequently happen because visibility is restricted and there is no possibility of improving visibility or prohibiting traffic which is crossing or entering the road, and those where there are frequent infringements of right-of-way without this being related to the intersection being difficult to identify or with the right-of-way being difficult to understand. The VwVStVO also states that the following principles apply to sections 39 through 43: “The free flow of traffic is to be upheld using the means available. In applying this principle, the safety of all road users takes priority over the free flow of traffic.” This study is therefore intended to clarify the impact on traffic caused by different traffic signal control methods deployed to protect traffic turning left. In particular, it is intended to show the extent to which any claimed negative impact on traffic flow actually occurs and how it can be minimized or offset by an improvement in traffic safety. (Author/publisher)
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