Everytime i go out with my buggy i first turn on the electronics and see how they work.You can tell if the batteries are low from the servos speed and power when turning the wheels or braking(without running the car).Also find a small spring and put it against the carb or servo horn so that it pulls the brakes when the servo is not responding.If you run out of batteries or for some reason the connections fail the spring will pull the servo or carb and stop the runaway.