I think the wording could be clearer too. Does it mean 'as much room as if they were a car parked at the side of the road' or 'as much space as a car driving along' (which to most people means getting on the other side of the road) or 'give them the width of a small car between them and you' (which would be fine) - all of which I think could be implied. Assuming they even know of the rule, it seems many think it's the first, and that the small car in question is an old style mini, possibly one made specially thinner.
To get round the issue of narrow roads, you could word it "give x metres, or as much as possible on narrower roads".
Still assumes anyone reads the Highway Code and remembers/cares about it for more than a minute after passing their test though.