You can automate it for much less than what traditional cop labour costs, and speeding ticket revenue would pay for the automation. If there was more enforcement, people would keep to the speed limits more frequently and there would be less stops. It's a control system with a feedback loop.
They really ramped up automated radar deployment in 2003-2007, I spent 10 minutes trying to find data that would let me see at a glance if accidents went down more than usual starting in 2003 but didn't find anything straightforward.
Anecdotally, most people who speed just slow down at the traps, then accelerate back. I'm not sure that's much safer than going steady at the higher speed.
https://hal.archives-ouvertes.fr/hal-00430965/document
> This public policy has yielded remarkable road toll statistics. The number of fatalities has diminished by over 40% since 2002, and the authorities are speaking of 12,000 lives saved since the new policy was implemented. The number of injuries avoided is also considerable, even if the fatality figures are those that draw most attention from official decision makers and the media.
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The "slowing down for camera" behaviour might be less of aproblem, if camera locations are biased towards safety critical places. But it also has a fairly simple technical solution: average speed cameras. These work by timing vehicle sightings between two cameras.
But according to the article, there is little or no correlation between the posted speed limit and the actual speeds at which people drive. So lowering the speed limit does not actually reduce fatality rates. But it does, as others have commented, creaet a misperception of what the actual expected speeds are on a road, which increases risk.
> Why not enforce speed limits more frequently so people obey them?
Because that would be way too expensive, not just in the increased cost of cops but in the lost time of all the people getting stopped even though they are driving at about the average speed of other drivers on the road--which, as the article notes, is the biggest factor in traffic safety.
The enforcement algorithm that most cops appear to actually use is to go after people who are going significantly faster than average, i.e., who are increasing the variance in speeds and therefore the risk of an accident.
> instead of punishing more people for driving irresponsibly?
This assumes that driving over the posted speed limit must equate to "driving irresponsibly". I strongly disagree with this assumption, and so, I suspect, do many others.