Wear your helmet. Even on a bicycle.
“If you can afford nothing else, protect the head. This is so important that we often see soldiers acquiring rigid metal head protection (meaning a helmet) before acquiring even textile body armor (it should be noted any kind of metal body protection is much more expensive than textile equivalents). Very light infantry – Roman velites, unarmored medieval bowmen and the like will often still wear a helmet. Likewise, as gunpowder increasingly made armor obsolete on the battlefield, metal head protection was the last element to drop away (and the first to return in the modern period).”
https://acoup.blog/2019/05/03/collections-armor-in-order-part-i/