Bruce Schneier points out in his blog New Risks of Automatic Speedtraps that fixed speed cameras can be more dangerous than no speed camera at all because all that happens is that drivers slow down at the speed trap, and bunch up.
Now, we've had fixed speed cameras here in Australia for several years now, and I gotta agree. The only thing that happens is you're travelling along at 110kph, then at the speed camera you slow down to 90, then when you're out of range, you just speed up again. This slow-down is actually quite dangerous, because the driver behind you doesn't really know how quickly you'll slow down, or he may not even notice the camera and therefore isn't expecting you to slow down at all (though that's rare now that most of Sydney knows where all the fixed cameras are anyway).
One thing that's being thought of here, though, is instead of a single fixed camera that measures your instantaneous speed, you place a number of cameras along a section of road. The cameras note the times you pass each point, and some software works out your average speed over a long distance. If your average speed is above the limit, bam, you're nicked.
Now, this has a few advantages. First, it allows for small bursts of speed if you're over taking someone for instance. Second, it eliminates the sudden braking at the speed camera, because you gotta stay below the limit for longer. It should also be cheaper to install and run, because even though you need more cameras, cameras are cheap - the expensive bit is the maintenance on the radar which measures your speed. Software is cheap (to reproduce anyway, initial development may be expensive)
Of course, many people believe that fixed speed cameras are little more than revenue-raising machines for the police, and I tend to agree: most fixed speed cameras in Sydney generate millions of dollars a year in fines, with almost no effort.
Anyway, time will tell if this scheme will more effective than the instantaneous speed traps...