Good points. As it is, there are lots of prescription medications which preclude driving, and rightly so. I don’t know if there is much research on how marijuana affects driving, because that would be hard to test. Anecdotally most people don’t seem to think it’s as dangerous as tired driving, distracted driving, drunk driving, if dangerous at all…but research probably also shows that being stoned lowers executive function and reflex time, which responsible operation certainly call on.
We could spend all day making jokes about roadside tests might be administered, like asking a driver to correctly identify a Captain Beefheart riff (to see if he can confidently lie), or saying" poop" really slowly to crack his composure. In reality, there is probably no possibility a court-admissible test of general wherewithal that every sober driver could pass and that every stoned driver would fail. Without a reliable ad hoc exam to establish a driver’s capacity, and acknowledging that in general marijuana does negatively affect some of the capacities called on for driving, all stoned driving – at any level – will probably remain illegal for good.
That said, since it will be difficult to prove either way, cops will likely take it easy on people most of the time if they are caught – like they used to with drunk driving before the portable breathalyzer test.