1. Apparatus for warning the driver of a motor vehicle when the vehicle is about to drift across a line on a road surface having a width within a predetermined range of minimum and maximum widths indicative of a center line or an edge line, said apparatus comprising: a light source adapted to be carried by said vehicle adjacent a side thereof for directing light rays downwardly toward the road surface, a photoresponsive receiver adapted to be carried by said vehicle for receiving said light rays reflected from said surface, said receiver having photoelectric means for producing an output corresponding to reflected rays received at a plurality of points spaced horizontally transversely of the fore-and-aft axis of the vehicle, and means responsive to said output for producing an alarm-activating signal when said output is indicative of the reflection of said received rays from a line on said surface of at least said minimum width but no greater than said maximum width, and alarm means responsive to said activating signal for warning the driver, whereby to alert the driver to the detection of a center or edge line and discriminate against other markings of greater or lesser width.