A flow indicator on storage units

On the roofs of train station modules, there is an icon showing what the module contains as well as a simple arrow showing which direction the goods are flowing. i.e., Are these being loaded or unloaded. It would be nice to have the same sort of arrow on the roofs of basic storage units. Doesn't seem like it would take a lot of work and would be a simple QoL improvement.

This is already in the game