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