Effects of Dynamic Speed Limits on a Dutch Freeway

This paper describes the effects of dynamic speed limit (DSL) control on a three-lane section on the A13 Freeway in Rotterdam, Netherlands. The main objective was to contrast the latest empirical findings from an experiment conducted in Barcelona, Spain, in which the main line metering capability (i.e., gating, or main line flow restriction) was questioned because of subcritical speed limits (e.g., speed limits down to 40 km/h). Moreover, according to the results of the study conducted in Barcelona, the validity of the current fundamental diagram models accounting for DSL control could be strongly compromised. This investigation took advantage of the huge amount of empirical traffic data recorded by the Dutch government and of the DSL control strategies present on most freeways in the Netherlands. The data were treated to identify stationary periods of traffic. To this end, a method that was found to be reproducible elsewhere and computerized into an algorithm was developed. The fundamental diagram was used as a graphical tool to assess the results. The main findings do not contradict the observations from the study performed in Barcelona. Flows of 1,850 vehicles per hour per lane were steadily observed at the subcritical speed limit of 50 km/h. The reactive nature of DSL control implemented in the Netherlands implies that very low speed limits do not affect a wide range of traffic states. This finding hampered the possibility to extend this conclusion. Further research with fewer limiting DSL control strategies is necessary to clarify the extent of the phenomenon considered.