Abstract: A vehicle lane guide and alerting device securable to a vehicle for monitoring a position of the vehicle on a lane of a roadway delimited by light-reflective lane markers wherein the markers have a different reflectivity than the roadway itself, the vehicle lane guide and alerting device including a photo sensor mechanism coupled to a front end of the vehicle for capturing light that is reflected from the roadway; a differentiator mechanism coupled to the photo sensor mechanism for discriminating a sharp change in the reflected light above a given level when the vehicle crosses the lane markers on the roadway; a buffer mechanism coupled to the differentiator mechanism for registering the change in the reflected light; and an alarm mechanism coupled to the buffer mechanism for providing an alarm indication once the change in the reflected light is registered to thereby indicate that the vehicle is moving out of the lane.