A simple and easily-installed system for vehicles equipped with an extensible pole, mast, boom, or other structure, providing an audible alarm when the operator attempts to drive away without first completely retracting the pole, mast, boom, or other extensible structure. The system includes a sensor for sensing a first position and a second position of a vehicle's gearshift lever, and a control module. The control module receives an electrical signal from an extensible structure when the extensible structure is fully or partially deployed and communicates the electrical signal to an audible alarm if the sensor senses that the vehicle's gearshift lever has been moved from the first position to the second position.