You’re assuming that it’s visual distraction which is the problem.
It ain’t necessarily so. As we know, your eyes can be fixed straight ahead, but something directly in your line of vision can fail to “register” if you are preoccupied. And, by the same token, your eyes can be fixed straight ahead but a movement in the very periphery of your vision – a child moving to dart out from behind a parked car to your side, for instance – can register if your are alert and your attention is focussed on the task at hand (driving your car).
The problem with a mobile phone, I think, is not that it distracts your eyes – it need not, as you point out - but that it distracts your mind. Operating two different and relatively complex machines simultaneously is difficult for most people; most of us can’t even stroke our stomach and pat our heads simultaneously. Add to that the fact that a phone conversation demands your cognitive attention; you have to focus on what is said, and offer at least minimally intelligent responses, while simultaneously focussing on the wholly unrelated question of what you see in front of you. The speed limit sign, by contrast, is a small and simple part of the visual environment in which you are driving. All things considered, then, participating a phone call is a much greater distraction from driving than noticing the “60 km/h” sign as you pass it.
What about the sign in Samuel’s post? Well, it’s part of the overall visual environment in which you are driving, and therefore doesn’t call your attention away from that environment. But I still find this particular poster very distracting. It may be just a trick of the particular angle from which the picture is taken, but the horizon in the poster seems to line up with the actual horizon behind it, and the trees in the poster likewise seem to line up with the actual trees. It would make me do a double-take; am I actually looking at a printed poster, or am I looking at a large transparency, with the word “focus!” and one or two other things superimposed upon it? And a double-take, of course, is distracting.
Admittedly, that effect is a function of the precise angle from which the poster is seen. Perhaps, if seen from a moving car, it wouldn’t have this effect; I don’t know.