a vehicular collision avoidance system includes a sensor mounted on a vehicle , wherein the senor is operative to measure the distance between an overhead obstacle ahead of the vehicle and the sensor ; 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 . the processor produces an alarm if the measured height of the obstacle is less than the height of the tallest portion of the vehicle .