On the main road network of South Holland part of the congestion cannot be readily explained as a consequence of road works, accidents, or rush hour traffic flow. The aim of this study was to account for this congestion type. On the basis of the Dutch Department of Transport's congestion classification, twenty locations have been selected where such congestion is found relatively often. For some of the locations a closer look showed that standard congestion causes were applicable after all. In other cases, major reconstructions have been planned, or have already taken place, so that the traffic situation at the time of congestion could no longer be observed. Finally, for three locations the road environment layout has been analyzed in detail. For each of these locations, possible congestion causes have been indicated, and where possible, improvements have been proposed. It was concluded that a number of basic elements that affect drivers' uncertainty, and consequently local sensitivity to non-standard congestion, were found across the selected locations. It is recommended: (1) to take combined effects more into account; (2) to avoid narrow road layouts; and (3) to remain alert with regard to `classic' sources of uncertainty, like threatening or unknown road elements, and factors that affect the viewing distance. (A)
Abstract