Thinking about the speed differential part --

This makes it a very narrow market indeed.

Motor traffic rarely trundles along at 12-15 mph. They're either stopped, crawling in first gear at 5 mph, or hustling at 25 mph or faster. If you're going to be fast enough to need to be signaling with lights, you're saying that you can cruise for a while -- on city streets, with starts and stops -- at 25-30 mph or more.

If traffic is jammed, you can pass everyone else without going too fast yourself. You don't need to signal then. If they're all going faster than you, you can try signaling, but they're going to be bearing down on you pretty quickly -- and you'd better hope that they see you, AND hope that they don't cause another wreck while trying to avoid you.

Turn signals, then, and IMO, are only useful when you're going the same speed as motor traffic when they're cruising, and that range starts at 20 mph. Think of how many cyclists you know who can do that, and then think of how many more you know who can't ride that fast.