If you are in the left turn lane at a standstill waiting for oncoming traffic to clear, the turn signal is doing very little for cars behind you. It is to indicate to oncoming traffic that you might pull out in front of them.

If you come up behind someone and are frustrated that they are not turning as quickly as you would like, the main resolution to this problem is for you to take a deep breath. At most, they are increasing your commute time by a matter of seconds.

If someone not turning when you want them to is almost causing accidents, then you probably need to slow down. Other people in this thread are talking about how the U turner is at fault for being unpredictable - and sure, it is very nice when all drivers are predictable all the time. But that doesn’t really matter if you get in a collision. Sure, a judge might say the other driver was in the wrong - but it will still suck to hear it with a totalled car, endless insurance headaches, and a broken arm. Assuming you don’t want that outcome, it is your job to drive such that you are prepared for people behaving in ways you don’t expect. And that means leaving enough time distance between yourself and a stopped car so that you don’t rear end them if they continue being stationary.