  Empirical observations and theoretical studies indicate that the overall
travel-time of vehicles in a traffic network can be optimized by means of ramp
metering control systems. Here, we present an analysis of traffic data of the
highway network of North-Rhine-Westfalia in order to identify and characterize
the sections of the network which limit the performance, i.e., the bottlenecks.
It is clarified whether the bottlenecks are of topological nature or if they
are constituted by on-ramps. This allows to judge possible optimization
mechanisms and reveals in which areas of the network they have to be applied.
