If you are driving at normal highway speed and the vehicle starts to overheat, turn off the air conditioner, pull over to the shoulder. The 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 engine, open the hood and let the engine cool down for a minimum of 20 minutes. Once any over boiling stops and the engine has cooled, 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 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, then start the vehicle and drive slowly to a service facility.