A collision avoidance system for use with vehicles includes a sensor mounted on a moving vehicle, wherein the sensor is operative to measure the distance between an overhead obstacle ahead of the vehicle and the sensor itself; a processor connected to the sensor; means for determining a reference height which is the height above ground level at which the sensor is mounted on the vehicle, and wherein the reference height is entered into the processor; means for determining the height of the tallest portion of the vehicle above ground level, wherein the height of the tallest portion of the vehicle is also entered into the processor; and wherein the processor adds the reference height to the distance between the obstacle and the sensor to determine a measured height of the obstacle, and wherein the processor produces an alarm if the measured height of the obstacle is less than the height of the tallest portion of the vehicle.