Well no they discourage fast driving

Not really, or at least not with the fixed cameras. They encourage speeding as normal then harsh braking just before the camera then speeding again. Just watch traffic on the motorways when a temporary speed limit is imposed. People slow down for the gantries where there might be cameras mounted then floor it to the next gantry.

Average speed cameras are a better bet if you want to control speeding, but perhaps don’t make as much money.