Now this is a surprisingly easy riddle to figure out, if you think outside the box a little.
There is a normal sequence of numbers, we are just looking at it in the wrong direction. The numbers of the parking spots must be viewed from the outside in, as a driver would see them, which is upside-down from the current view.
The shown numbers are: 86, ??, 88, 89, 90, 91. Therefore the answer is ’87’.
It’s that easy! 🙂