If you are driving at normal highway speed and the auto vehicle starts to overheat, turn off the air conditioner, turn on the heater and immediately pull over to the shoulder. Odds are if the vehicle starts to overheat at highway speed, there is a problem in the cooling system such as low coolant, a clogged radiator or a broken drive belt or burst hose. Once at the shoulder, shut off the auto-car engine, open the hood and let the car engine cool down – 20 minutes minimum. Once any overboiling stops and the car's engine has cooled, look for obvious signs of trouble. DO NOT attempt to open the auto-car radiator cap unless the car engine is off and the top of the radiator is cold. If there is no noticeable problem such as a broken drive belt or burst hose, you can then add a coolant/water mixture to the radiator or overflow reservoir, start the auto vehicle and drive slowly to a service facility.