It's not only speed signs being obscured which don't help. It's taxi drivers. I had one in front of me doing about 30 in a 40. When the single carriageway became a dual carriageway, I pulled out to overtake. He then thought it would be mildly amusing to accelerate to 40, thus making me exceed the limit in order to complete my overtake. It was when I saw the scamera van parked at the end of the road, eager to relieve members of the public of their hard earned readies, I realised what the Son of Satan had been planning all along.
EDIT: it's ironic. I've only ever had two speeding tickets. The first was on a downward stretch of the A43 near Silverstone when those bloody roadworks were everywhere. It was late at night, nobody was working and I let my concentration slip momentarily. I was flashed at 48mph near that Green Man pub. When I let friends and colleagues know about this, the number of "oh yeah, we were done there too" replies almost made me form a "Friends of the Green Man on the A43 Speed Camera Society". I'd even designed the window sticker. Anyway I digress. Both tickets have been earned in black Lexi. So far, my silver Lexi have been luckier.