Most people around me, colleagues, friends etc. Even some relatives will do 25-30 in a 20, over 35 in 30s and so on. They just learn where the cameras are, and most days I get tailgated on 20 mph streets, I just let them pass where I can as I can't be bothered with someone right behind me.
I don't go over the limit, don't have cruise control or anything as it's an older car but if i accidentally creep up to 1/2mph over I go right back down. I just think that it only takes someone running out into the road, a child or someone and if you're doing 25 and hit them, there's not going to be an excuse for that.
I agree 20 can feel very slow, but I just think it could take 1 time, and i don't think it's wise to just rely on where cameras are.