Other suggest revenue source; but I think it's simply because it's easiest to enforce and least prone to personal judgement given it's based on strict numerical

Other suggest revenue source; but I think it's simply because it's easiest to enforce and least prone to personal judgement given it's based on strict numerical limits. Unsafe driving (braking distance, crossing lines, holding a cell phone) should be as good or even better revenue sources, but they are more difficult to enforce automatically. While nowadays there would be tech for implementing that, historically it has not been so, so the momentum of the history is perhaps making advancements slow? There were some new cameras introduced here some time back which were advertised to be able to have quality sufficient to detect that seat belts and cell phones: but still, pictures were taken only when exceeding the speed limit, so those factors could not be used as the primary source.