[mikeash]

It does not take carelessness to park in a no-parking zone where the signage is illegible, yet people still get fined for it, and there is typically no recourse.

It does not take carelessness to drive at the speed of traffic rather than turning yourself into a hazardous obstacle by driving 20MPH slower than everybody else, yet in many places this is a speeding violation.

It does not take carelessness to drive through a red light when the yellow is set so short that you don't get enough warning to stop for it.

Inconsistent enforcement may not render the law invalid in terms of whether it can be enforced on those who get unlucky, but it does have major effects on the law's legitimacy in a moral and public opinion sense.

It is basically impossible to live life without violating traffic laws. So "you choose to violate them" is at best meaningless and really is just plain wrong. If we're all going to violate them then we need to at least make sure the punishment isn't completely crazy.