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xkcd #2932: Driving PSA
(imgs.xkcd.com)
Alt text:
This PSA brought to you by several would-be assassins who tried to wave me in front of speeding cars in the last month and who will have to try harder next time.
Right, but its not your responsibility to slow down at all. Its kind of you to do so when the merging vehicle picks up on your intention, but when they don't, it makes a miscommunication like you describe.
Basically, i'd just describe it as being predictable. And bending the rules (even to be kind) is not predictable, usually.