What do you say when a person/an idea/... is crazy to your mind?
You're asking about three things: a gesture, an idiom, and showing someone the bird.
One gesture sometimes used in the U.S. to indicate "you're crazy" is pointing a finger at your temple, and turning it in a small circle a few times. I found one blogger who wrote, while this is rather benign in the U.S., it's regarded as highly offensive in some parts of Europe.
We express the meaning of that gesture by using terms like "you're crazy," "you're nuts," or even "you're loco," borrowing from the Spanish. "You're cuckoo" is also well-known (I guess a lot of cultures associate birds of some sort with a loss of sanity or judgement). The idiom "bats in the belfry" is well-understood, but not all that commonly heard; a few more popular idioms include, "has lost a few marbles," and "has a few loose screws;" I think the latter might be more likely to be applied to a single rash act (such as passing recklessly on the highway).
In the U.S., to "flip the bird" is to point your hand at someone with a raised middle finger. You might do this when they pass you unsafely on a roadway, but it hardly means "you're crazy!" (actually, it means "*F**** you!"). That gesture is considered vulgar and offensive; I'm don't know if someone would or could be prosecuted on the roadway, but it evidently does violate obscenity laws when broadcast on television.
The corresponding gesture in the English-speaking regions is pointing a finger at the temple while turning the closed palm: Screw loose!
It suggests that a screw (a threaded nail) seems to have come loose in the person's head and tells him to tighten it thus: by turning it clockwise!
You'll have to excuse my friend; he:
- is not playing with a full deck
- has lost his marbles
- is a few apples short of a bushel