MIT's John Leonard was saying years ago that unprotected lefts are one of the hard problems. They're so situationally dependent and can depend a lot on social signaling. I have to make one to get into my driveway probably daily on a fairly busy road. I've had drivers flash their lights to let me know that, yes, they see me and are going to let me turn. I've also (rarely) been honked at by someone who was not about to slow and didn't think I was giving them enough space/turning quickly enough.

(Many other hard problems of course but unprotected lefts are something that drivers face multiple times a day in often ambiguous situations.)