The 500 foot rule in the UK is this:
'Except with the permission in writing of the CAA, an aircraft shall not be flown closer than 500 feet to any person, vessel, vehicle or structure.'
This is a distance not a minimum height, so you can fly below 500 feet providing you are 500 feet away from 'any person, vessel, vehicle or structure.'
As I understand it (or interpret, possibly) a road isn't classed as a structure but a bridge (for instance) is.