I have just been on my Speed Awareness Course after missing a speed sign last April. It was speeding 4 am on a motorway where the speed limit went from 70mph to 40mph without warning and the temporary sign was at road level where the other signs were all on a bridge over the road. So, naturally, because I was concerned about where I was going I was looking up there. I worked out that to read 5 unlit signs of that size at 70mph I had less than half a second, let alone moving my sight line from about 4m down to 1m. I imagine if they had put the speed sign a few metres further down the road, away from other signs I might well have seen it.
Mike.X