The not-on-a-wall paradox

The statutory utilities place signs describing the location of their apparatus in a highway on adjacent walls. The fire brigade does the same to show the location of their hydrants.  However local authorities as a matter of course erect poles in the footpath to carry information about waiting restrictions.  The question is "Why should there be a difference?"  It cannot be that signs on the wall are difficult to see.  After all, the most urgent and important sign of this type must be the fire hydrant sign. I think that it is a matter of habit and convenience for the highway authority.  It is easier to erect a post on 'highway authority land' than ask the frontager for permission to fix the sign on their wall or fence. Here are four examples of what I mean.

Councillors may need to convince their officers that a scheme is necessary and some points that they should bear in mind are here.

I would like to develop here the argument to place signs on frontages where possible.