A method to operate a motor vehicle is provided. The method comprises capturing an image of the road ahead of the vehicle; determining a limit point on a first side of a lane of the road in which the vehicle is travelling, at which the first side appears to substantially converge with a second side of the lane within the image; determining a limit line extending across the road from the limit point in a direction perpendicular to the first and/or second sides of the lane; determining a limit distance between the vehicle and the limit line; determining a stopping distance of the vehicle according to the current speed of the vehicle; and alerting a driver of the vehicle if the stopping distance is greater than the limit distance. A safety system for a vehicle is also provided.