When the battery is gone, all of your electronics including the computer system will stop function and your engine will shutdown immediately even your car is going 70 mph. Your car will become completely powerless and the speed will drop to zero in a very short period of time (maybe less than 10 second). The car can be down at ANY TIME, even when you are in the middle of making left turn...
Do NOT drive that car until the problem is fixed for sure. Do not take the risk because you may be losing your life at any time.
Remember, when the power is gone, means the emergency light will not work too.