Abstract: A highway line detection system which is insensitive to changes in road surface warns the driver of a motor vehicle when the vehicle is about to drift across either a center line or an edge line. A unit on each side of the vehicle directs light rays rich in infrared and red radiation downwardly toward the road surface, and includes a photoresponsive receiver responsive to the rays reflected from a painted line or mark. A valid center line or edge line has a width within a predetermined range of minimum and maximum widths and is recognized by the receiver to activate a suitable alarm, but other markings of greater or lesser width are discriminated against to prevent a false alarm. The receiver employs a series of photoconductive cells which are highly sensitive to radiation in the red and near infrared region; they are spaced horizontally on a line extending orthogonally with respect to the fore-and-aft axis of the vehicle in order to establish corresponding spaced points at which reflected rays are detected.