for some values of plenty < ε. As cyclist I can tell you that human drivers often don't even use their turn signals at intersections where it is important for me to know where they're going. And it's not like I'm sneaking up on them, it's when they're facing me at the other side of the intersection.
Sometimes, not always though. Anybody who has been cut off by the driver crossing 4 lanes of traffic without looking 20 feet before the end of the exit lane can attest.
We give plenty of indications. It's just that they're hard to define objectively.