In the first example, the idea is that the primary function of blinkers is to convey information. The law makes it mandatory to transmit that information with presumption that the information will be processed by human beings you are sharing the road with. If there are no humans around, the transmission of this information is meaningless and therefore should not be mandatory. If the road is used by autonomous vehicles which transmit information about their paths via radiowaves, signalling could be obsolete as well. The most important part in law is the underlying idea of the social agreement which is being enforced by it and reasonabless of the law (and enforcement).
My point is that by automating fine dispensaries, you are subversing the underlying idea itself (automatron can't know what or why is it enforcing) and reasonability.
Your second example (speeding) doesn't incur a fine because in this hypothetical example the automated enforcement is only used for signalling violations.
For your third example (self defence), maybe you are forced to change lanes without signalling. How often? Once per year? What if the fine were $2, and people like you had to pay it once per year, but people who persistently refuse to signal end up paying 1000s of $2 fines per year?
Seems like a reasonable trade-off.