ANPR cameras are cheap enough and technology is advanced enough that entire areas of 20mph could be enforced relatively easily. It would end motorists thinking that speed limits in urban areas are optional.
However, police forces are generally weak willed, and of course if you install them in rural areas it's more ker-ching for them. Hence why you have massive amounts of enforcement on motorways and major roads, and sod all in town.
I pointed out to the dullards at Thames Valley Police that there are scores of places they park camera vans around Oxfordshire including plenty on the M40 where there are no vulnerable road users. But they only have one within the city boundaries itself, despite there are lots of vulnerable cyclists in the city and relatively fewer outside. Got some reply with idiotic excuses along the lines of "well we have all those BRIGHT ORANGE boxes on poles you can see from ages away instead" as you would expect from plod.
