90+% of the near altercations (and the only real altercation) I've had with cars were because they seemingly had no concept that I could be moving more than 5 mph. The driver of the car I actually hit (came from behind and turned in front of me) actually said he didn't realize how fast I was moving--luckily I was on the binders early enough that only a slightly out-of-true front wheel was the damage.
This gadget wouldn't of helped in most of my close calls. Most of my close calls were the result of a car pulling out in front of me (even when we've made eye contact)...folks figuring I wasn't moving 20-something mph. These days I grab a handful of brake regardless.