So LALD, this is the layout I'm leaning towards for my diorama. Nothing is permanent though, it's a quick mock-up I made in MS PowerPoint to broadcast my idea.

The past couple of days I've been getting measurements and dimensions for the following; sidewalk, road width , parallel parking spot, a 90 degree parking spot, and the width of the parking lot aisle (with 90 degree parking). I should be able to put all I just said on my wood base (18.5" x 24"), but I'm not going to give out all the measurements yet, as I want to double and triple check everything.

Back on track. As you can see, this a roughly what I'm aiming for. There is a road to the south, and a parking lot to the north. Around the parking lot is a grassy area, meant as a barrier for the cars and to create contrast with the black pavement the cars will be sitting on. Between the lot and the road, I'm still hazy on. What I wish to do is put in some parallel parking (for the visitors to the show) and put in a sidewalk (to give it a sense of realism).

So, you guys like what you see so far? :)