The Parking Garage has one of the best ideas in the entire series. I always think of it as the sequel to The Chinese Restaurant: a bottle episode about a situation EVERYONE can relate. Just like The Chinese Restaurant, The Parking Garage has short gags about this situation in itself. Some of them work (double space park-er, nobody wants to drive them around) and some don't (urination). In my opinion, this episode has a much richer central idea than its "prequel". Because every parking garage is the same no matter where you go, while the restaurant situation is more uncommon.
Nevertheless, no matter how good the idea on paper may be, the script must be on the same level. And here it's not. The jokes aren't that funny, the episode has a slow-languid pace which may symbolize the feeling of being in that situation but on reality it just makes the episode quite tedious. On the much better episode The Chinese Restaurant the pace is much faster and there are more interesting and funny things going on which justifies the length of the episode. On this case, the length really drags and it feels like the writers are trying to stretch an idea into a 20 min episode.
In conclusion, this episode is great on paper (not script) and you will always recall it in a similar situation, but as a whole it's not quite up there with the Seinfeld classics.