Why doesn’t air pressure crush an empty soft-drink can as you drive down a mountain?

The air pressure on the inner side of the can balances that on the outside of the can.

Explanation:

Indeed the air pressure might crush a properly-sealed air-tight can during the descent. The reason is that the air pressure on the outside of the can exceeds that on the inside. The force that pushes the wall inwards exceeds the force that pushes the wall outwards. It's the pressure difference that crushes the can. However, that's not exactly the case for the "empty soft-drink can" here.

The question states that the can is already "empty." Therefore, it is quite reasonable to assume that the can was already opened. In other words, the can would no longer be air-tight. Air particles could freely enter and leave the can. As a result, when the can is brought down a mountain, air enters the can. As long as the can stays open, there would be enough air particles on the inside to balances the pressure. A pressure difference will not build up. That ensures that the can stays intact.