I bet this has been asked before, but I haven't found a definitive answer. 
How visible should official No or restricted parkin notices be? 
I as as my daughter received a PCN in Salford yesterday. 
There were no yellow lines etc., but after searching she found the sign, not facing out onto the road, but looking firmly at a blank wall! 
It was not visible from the road - or at least the front wasn't and with the back being grey it was hard to see.
                            
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                        In areas where there is a Controlled Parking Zone, I don't think there have to be any notices at all [except for at the entry and exit from the zone, which could be miles away].
                            
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