This paper presents an algorithm to detect road lines on digitized map images. This algorithm detects road lines based on object shape (line thickness) and gray level values. The road detection process is accomplished in two steps: road line extraction and road tracking. The road line extraction consists of level slicing, morphological filtering, and connected component analysis. The road tracking routine is capable of connecting broken road lines caused by the overlapping of text labels. The algorithm has been implemented on an IBM PC/AT-based image processing system and applied to various map images.