I like this idea but with a twist. If you manually activate the turn signal before turning, you're good to go. If you fail to activate, a big lightboard on the back of your car lights up saying "I forgot to signal" and then they automatically turn on anyway. Now we have shame in play. Some people are immune to shame though, so you get, let's say, 2 of these a day, or 4 a week, or whatever, for free to account for the unexpected. If you exceed that you have to take an online remedial training class. If you get dinged again within one year you have to take an in-person class. 3 times in a year and every time thereafter, and you get points on your license. After 5 or 6 times you'd lose your license. This way normal people in abnormal circumstances aren't impacted, and the bad drivers slowly get escalated into either doing it right or losing the privilege to drive.