Set the limits right for more respect
As has been pointed out, speed limits would be respected more if they showed more evidence of having been set carefully and with the aim of increasing road safety rather than, for example, satisfying some political objective or of saving effort for the person setting them.
Here is a fine example of illogic: http://maps.google.co.uk/?ll=52.997323,-1.524943
The main road - the B5023 - has a 50mph limit, as do all the main roads within a radius of several miles. I'm not necessarily disagreeing with this, incidentally: it is a lovely road but does have a number of field entrances and agricultural traffic, so perhaps it makes sense (at least for some stretches). The limit was reduced a few years ago (5ish).
But look at the short stretch of road that resembles a layby. It's about 150m long and serves three houses and a pub. No traffic will use that road unless to access one of those four destinations. Yet the limit there is 60mph (you can confirm that with Streetview)! Admittedly I'm not sure how fast a car would need to be to reach that speed in the space, but you may still ask why. Presumably it made the paperwork easier (only one road for which the limit was being changed), although it did mean that more signs had to be installed.
While we're at it, look at side the road opposite. Again, this carefully tells drivers that they may now drive at 60mph. But Streetview will also confirm that (a) this is a much smaller road than the main road, with a poorer surface; and (b) a few feet after the national speed limit sign are two warning signs indicating that the road narrows and is bumpy, because there are bridges over the river and railway.
Some limits should be higher. Some should be lower. But either way they should be properly thought out and relevant, so that 'my judgement was that I could safely drive above the limit' ceases to become a valid statement. Then, and only then, we can talk about proper enforcement (e.g. using GPS).
Final point: much though I hate them, I respect average speed cameras much more than spot ones. You still get numpties braking for them, but at least they mean that if, for instance, it becomes necessary to exceed the speed limit temporarily (e.g. because it is the quickest, and therefore safest, way of overtaking a slow-moving vehicle) then it can be done without necessarily falling foul of the machine.