They're generally not well-positioned, in that you don't always notice them until you've already committed to walking in one of the directions (which may be the wrong one). Barriers prevent people from switching to the correct lane, so if you do spot one pointing against you then you have to do a U-turn and go back the way you came.
I think the idea of having them looking like blue footprints from size 12 boots is to associate them with law enforcement in the minds of pedestrians. Blue is the colour of the police (even though these days their uniforms are black) and the size and heavy tread match the popular conception of police boot footprints. Having seen the footprint a police boot left when an officer broke into the house of my father-in-law to rescue him after he'd fallen and fractured his hip, this popular conception seems largely correct, too.
What I don't get is why the footprints are standing still rather than showing movement. It may be to do with stencil size, but you only need a reversible stencil of one boot to spray walking footprints.
This shows why interface designers are in demand.
I took this photo a couple of weeks ago, by thw way, since when I haven't been into Colchester town centre. It's entirely possible that a different system is now in place, ready to deal with the mass of people expected to descend on bars, restaurants and hairdressers following today's relaxation of the lockdown rules.
Given what's likely to happen if a mass of people does indeed descend on bars, restaurants and hairdressers, we're probably going to need similar symbols painted outside the intensive care units of hospitals two weeks from now.