Two road rally checkpoints A and B are located on the same highway and are 12 km apart. The speed limits for the first 8 km and the last 4 km of the section of highway are 100 km/h and 70 km/h, respectively. Drivers must stop at each checkpoint, and the specified time between points A and B is 8min 20 sec. Knowing that a driver accelerates and decelerates at the same constant rate, determine the magnitude of her acceleration if she travels at the speed limit as much as possible.