Surfaces for "Keep Clear" and "Low Clearance"
A simple "Keep Clear" surface (or marking) which would mark a service area and thus prevent the accidental placement of buildings or pillars which would prevent access to important buildings.
In addition, there could be a "Low Clearance" surface which effectively would not prevent the placement of transports lower than 2 over it, i.e. the height required by trucks to pass under it.