Abstraction, which is defined here as the increase of the degree of simplification and emphasis, is of major importance for object extraction. In this paper it is shown theoretically as well as empirically, how so-called scale-space events for lines define multiple abstraction levels for roads in images. The investigations are based on different scale-spaces as well as on a sophisticated model for line extraction for which the scales where events occur can be determined analytically. As one result of the investigations, a model is derived which is used in a multi-scale, multi-abstraction level road extraction system.