One idea: Provide stalls that are just large enough for a single car to fit with a gap of 1m between the stalls. Put a hole in the wall on the passenger side where the doors would be so that they open into the hole and allow someone on the passenger side to exit into the gap.

Maybe a hump running down the middle of each stall, but offset toward the left, so when pulling in, the driver door is very close to the wall?

I had the same question, and am wondering what prevents someone from backing in

Make it narrow enough that when a car is inside, only one door can open? So either it’s the passenger door, so the prostitute can get in and out, or it’s the driver door, in which case she can’t get in in the first place?

Sounds to me like it's just spacing? Like pulling into a small parking space, where you have to park the drivers side in a way the door can't open...