It seems like it would calm traffic, because people would travel the speed limit instead of speeding and then abruptly dropping from 90mph to 65 when they see a cop hidden behind a bridge. I've seen demonstrations that traffic waves cause jams even when there is no problem with throughput, just because someone slammed on their brakes. I'm actually not sure what it'd do to ticket revenue. If there was no way around it because your average speed gets measured, I guess people would stop speeding. Maybe the gambling aspect of it is desirable to governments because if there's a small chance of getting a ticket, lots of people will speed and thus get tickets. If tickets are a certainty, no one speeds and revenue dries up.
Do police unions lobby against using this kind of enforcement because it'd mean less cops lurking under bridges for an easy payday?