Well, why?

More finely, why not? Is there anything intrinsically wrong with making a left (or right) turn when the way is clear to do so? Is it not ridiculous – in the sense that it is mindless (as well as wasteful of time and fuel) to sit waiting for a light to change color when the turn could be safely made?

Implicit in this is the assumption of driver competence. That he is able to see and competently judge time-distance relationships. If he is and can then he is capable of safely making a left or right turn without mindlessly waiting for a light to change color. The same goes for stopping completely – or not – at a stop sign. If the driver can see that there is no cross traffic then there is no good reason to come to a complete stop. It wastes time and fuel and also increases wear and tear on a vehicle's brakes (and clutch, if it is equipped with a manual transmission). Mindless obedience is the only reason – and that's never a good reason, because it encourages mindlessness rather than judiciousness; i.e., the exercise of judgment based on circumstance. Encouraging peopleto exercise judgment fosters competence and the more competent a society is, the less it stands in need of mindless obedience and equally mindless enforcement.

But there will be more accidents if people are left free to decide for themselves when it is safe to make a left or right turn (and so on)  . . .  .

Well, let's examine this assertion.

In the first place it's an assumption rather than a fact that there will be more accidents. Another example of a similar assumption is that there would be more accidents on the highways if the National Maximum Speed Limit that made it illegal to drive faster than 55 MPH for almost 20 years (from1974 to 1994) were to be repealed. It was repealed – and there weren't more accidents; just fewer tickets issued for driving at speeds that had previously been legal.

It seems wrong in the sene that it demeans people to assume incompetence in the absence of evidence thereof – and to punish people not because they acted incompetently but because they chose to not act mindlessly (i.e., they did not just sit and wait for a light to change color and decided to make the turn when it was safe to do so). Just as it was wrong to punish people for driving 70 on highways designed for much faster where it had been legal to drive 70 until the federal government arbitrarily decreed no one could legally drive faster than 55. This was dirty pool and everyone knew it, including the cops who enforced it. The ones with consciences probably felt dirty at the end of each shift, knowing that while it was "the law," it certainly wasn't right.