The problem with "ask first" is that it is a self-defeating behaviour in current culture: Touching is an integral part of flirting and also common in interactions that don't involve sexual or romantic interest. And the current social norm is to start with light touching at a non-intimate place, and see if the other person reciprocates. Doing so is associated with confidence. "Can I touch the outside of your arm?" would be perceived as creepy for two reasons: It will be considered a sign of insecurity, and people often aren't explicitly aware of the implicit rules on touching, and would thus find the request strange.

Asking first already works well in some sexual subcultures as well as some professional settings: For example, in some armies, a superior will ask a soldier whether they may touch them before fixing something that's wrong about the way they are wearing their uniform. That's a very professional way of dealing with having to touch somebody who is reporting to you. The hard question is how one can engineer this cultural change in society in general. I would love to see that happening in my lifetime