Via Change in Speed

When you've had to make a decision as to who is passing whom, an appropriate change in speed is a good way to communicate that decision. (An appropriate lane change also works.)

In several contexts, a change in speed can serve as a rebuke. Why a rebuke? My guess is that by virtue of being sudden and decisive, a change in speed becomes a definite action, and so indicates that it has a cause, a dissatisfaction that provoked it. It's possible that's not the reason; it's even possible that what I see as a rebuke others see only as erratic driving. That's communication for you. In any case, here are the particular contexts I had in mind.

When someone's hovering in your blind spot, you can either speed up or slow down, as described in Relative Speed.

When an oblivious driver has caught up with you, and should be passing, but isn't, you can—again—either speed up or slow down. A small change in speed probably won't work, though, because the other driver isn't paying attention, and will probably just match the change. What I like to do is apply a combination. First I speed up by perhaps 5–10 mph, opening a gap, then I slow down to about the same amount below my original speed, creating a nice contrast in speeds. This usually makes even an oblivious driver recategorize the situation as passing rather than following.

You have to be on a multilane road, of course; on a road with only one lane in each direction, catching up and not immediately passing is correct. Also, if you're in the rightmost lane, you have to be aware of the possibility that the other person intends to exit rather than pass, in which case slowing down is rude.

You also have to be sure to distinguish oblivious drivers from tailgaters—the built-in meaning of tailgating alters the meaning of any change in speed you might make. Here are some rough translations from car-language to English.

tailgating	“You are driving too slowly.”
speeding up	“You're right. I'll speed up.”
slowing down	“You're wrong. Please stop tailgating me.”

In most cases, I think the right and wrong of it is pretty clear cut. On a multilane road, according to the principle of relative speed, those who want to drive more slowly should stay more to the right; therefore, tailgating makes sense in any lane but the rightmost. On a single-lane road, I'd say tailgating makes sense only if the car ahead is driving at an abnormally low speed and there's no opportunity for passing in the opposite lane. (Now all I need to do is define “abnormal”.)

I had in mind one other context in which a change in speed can serve as a rebuke. When a barrier has just fallen, you can speed up substantially once your way is clear, to try and indicate that the barrier was frustrating, that other people really do want to go significantly faster.