They do slow people down, the problem is they only slow people down at the point of the speed camera, then people just hit the gas again.
But honestly as much as we hate them, can you imagine our roads with no speed or red light cameras at intersections and freeways? would be a mess
Having said that i believe that the correlation between road toll dropping and more cameras going up is not correct. The way see it the drop in the road toll is due to people upgrading into safer cars and not the rise in cameras going up.