Absolutely no offence intended; but simply saying "any bar should be considered rough" doesn't help a lot; there are folks who by reason of their job - such as cops or bouncers - have no choice but to be in those areas - cops and bouncers get headbutted a lot
. Ma-ai is also in these situations wishful thinking; if a guy's attacking; he's going to be all over you like a dirty shirt; you'll have neither time nor space to establish distance. Bar fights are always
quick, dirty and alcohol-driven; it's important to remember that. If the guy knows what he's doing, a headbutt is generally too quick to avoid. The arc of movement is only about 6 inches. Those experienced with this form of confrontation have a tendancy to protect the head by keeping it hunched low and cocked to the side in a 'bull' position - there's less of a chance of a guy headbutting you if he thinks he's going to hit your forehead instead of the nose. Also in this sort of conflict it's important to keep your hands in front of you - up in a guard position whenever possible both for protection and to keep him from rushing into a clinch; the most likely time for a headbutt.
Hope that helps.
I am sorry; those that are advocating a knee to the head as a defense against a head butt are being unrealistic about the range at which a head butt takes place and how quick it is. Those who advocate tenkan movement as a response would be correct in that this would PRECLUDE a head butt but once again it is unrealistic at the range that one is most likely to encounter a head butt.
Dave is correct, the best defense against a head butt is position. Hold you head so that it doesn't present a good target and keep your hands up and your elbows down and close so that the subject who takes you to grappling range will encounter your forearms against his chest or shoulders. This will allow you to head butt him but he won't be able to head butt you.