First thanks for this mod. Ever since they mentioned they would be adding bridges I had been thinking about a parking system for idle trucks and such. Yours is almost exactly like the one I envisioned.
I have a couple of minor suggestions/tweaks for your consideration.
For the transparent versions could you make the markings only show when holding/selecting the building, have the parking tab opened (similar to the belts and pipes building tab showing connection when opened), and when designating terraforming squares? It's quite contrary to the idea of a transparent version if you can always see the parking box. If you need something to mark it you could use the white center paint marks on the top end, the P paint mark centered in the parking space in white, or (and I think this is the best option) a concrete stopper used in actual parking lots - possibly could repurpose the top part of the in-game barrier for the texture/model.
The option to assign a spot to multiple building instead of just one
Default vehicles changes based on in-game width requirements and sub-category name changes:
T1 renamed to Trucks and include Pickups (blue) and all small Trucks (yellow) as defaults.
T2 renamed to Normal and include tree planters, small and normal excavators, and both tree harvesters sizes as defaults.
T3 renamed to Mega and include all amphibious and mega vehicles (excavators and trucks) as defaults.
T4 can stay as Super for as optional spot for mega vehicles and larger modded vehicles.
If you could do any of these or all of them that would be great.