Law and evidence.
Ok, just as a note to all those that keep saying "You're breaking the law, full stop, end of story.".
There is a general rule of thumb that any law that criminalises a significant section of your population is a BAD law. Laws, as any lawyer will happily explain to you (for the commensurate fee, of course) are intended to be grey. Life is not black and white.
When the speed limits were introduced, they were put there knowing that they couldn't be enforced on a fine scale, but could be used as a big hitting stick if you caught someone being truly stupid.
Noticed those stickers on bins that say "No littering, fine up to £2000"? Well, what if they put cameras up in shopping centres to completely enforce that. Something comes out of your pocket? £2k please. You need to be more careful with your clothing. Someone may trip on discarded rubbish and hurt themselves.
Silly example, I know, but there are plenty of others you can think up which are closer to the bone.
For those that say they've been on the wrong end of a collision with a car, so have I. In fact, I'm the only one in my family that's not ended up a hair's breadth from dead because of it (yes, I've seen them all on life support thanks). That doesn't make me agree with the cameras.
Basically, like much of politics, it all comes down to lack of scrutiny and evidence. For a society that's grown up and prospered by embracing scientific principle, I find it odd that the one group that steadfastly refuses to use scientific principle is Politics. Here, have a law. We have no evidence for it. We don't know how it'll affect society. We have no idea what the consequences are, or how it'll be interpreted, but just do it anyway, otherwise we'll punish you.
Scientifically, the evidence for the "Safety Cameras" is shaky. It also says that speed isn't the problem, stupidity and selfishness and lack of attention are.
Now, from this, you get two possible approaches. Try to fix the problem, which will cost money by implementing the software to pick up the dangerous drivers (there is behavioural analysis software out there that'll do the basics, but it's costly),or go for something that doesn't fix the problem but micro enforces an arbitrary law but raises a lot of money.
We all know which avenue has been chosen.
Honestly, carping on about "It's the law. It's black and white. Live with it." makes me wonder what kind of country we're living in, and what kind of country we'll be leaving to our descendants if we keep saying "If it's written in a law book, it is good and must be slavishly adhered to."
Wrong. The law is a very good guide to what you should and shouldn't be doing. It's a really crap way to live a life by the word (as your interpretation will vary. Anything that's truly explicit is usually largely hard to enforce, but you can see where they can use it as a stick when necessary).
If they're going to use the flexibility of technology to enforce a law, they should leverage the same technology to adapt the law such that it is fitting. People on the roadside? Sure, 20mph if it's near a school, or largely kids/old folks, otherwise 30 in the day. At night, nobody around? Open it up to 40! Sure, it's more expensive with timing, and if you're really into having fun, a detector to see if there's pedestrian activity too, but it's safe, and fair (warning that there are pedestrians around is a useful thing to a driver). There are all kinds of ways to re-evaluate this, but nobody is interested because the current 'Do nothing and slavishly obey' makes too much money (though they do have variable speed limits on some motorways. Always down though, alas).