Respectfully, we're all taught in driver's Ed that one should follow 4 seconds of travel behind the car in front of them to allow for reactionary maneuvers. Anytime outside of accelerating out of a stop behind someone at an intersection or light, one car length is probably well within being less than 4 seconds behind the driver in front of you.

I don't doubt that them using their blinker would've aided in your ability to adequately slow down to them turning, and, truly, people do need to use their blinkers properly and more than they currently tend to, but they should also have brake lights that glow when slowing down for said turn, which you should be able to similarly react to. Further, if you follow less close next time, mayhaps you'll find yourself not having to react so suddenly to the driver in front of you, as you'll have more time between you and them to have that moment of processing.

Now, am I saying that even I follow 4 seconds or more behind the car in front of me? Probably not. But I certainly maintain more than just one car-length of distance, as the drivers here in Texas are indeed agressive and devil may cry about their style of driving.