Many a time I'll find my traffic gets gridlocked not due to an insufficient number of parking structures,and I build plenty, but because all of the people want to use the same one, whether it has space available or not. Then everything gets stuck and can't move.

The A.I. shouldn't all try to rush the same parking structure just because it's the closest one to where they want to go all the time, every time. If it's full, have them reroute to the next closest, and then the next closest if that one's full, and so on.