Same thing happened to me on the highway a few months ago. I pulled over just before overheating. There is probably a reason why your accessory belt went out. Your mechanic should check out the components that run off the belt. The water pump is one of the things powered by the accessory belt and your car will overheat very quickly without a functioning one. In my case, my water pump failed and caused the belt to break. I definitely wouldn't drive the car in that condition, just have it towed to a shop.