I was doing a drop today and the building next to me had this height warning sign. I was sat 30ft away and couldn’t make out what it said, or do I need my eyes testing 
Chocolate and fire guard springs to mind.
We’ve had bigger than that in here Drive!
Can’t see the problem, a dinky yellow sign against an olive green background! Bet they insist on hi-viz jackets though…
Little sign on the left tells ya…5 metres innit 
The fact it specifies Royal Mail drivers only would suggest it’s only for drivers familiar with the site and as most of the red Royal Mail fleet are the same height the clearance wouldn’t be an issue.