Two of my students have been struck by cars while crossing 19th street on bicycles this semester. It does happen. Both suffered injuries bad enough to send them to the emergency room, though thankfully no head injuries. Only one of them was wearing a helmet, however, which to my mind, is one too few.
Get a bike helmet. Wear it. Please.
In fact, if you are a student of mine, I'll even make you a deal: If the reason you're putting it off is because money is a little tight right now, I'll buy it for you. Just say the word.
For a closing graphic reminder, here's a picture of Walu, following a bike accident on campus (his own fault--no cars were involved*; notice that he is still wearing his hospital wrist band from his trip to the ER). His helmet was dented, but thankfully, his considerably brilliant brain was not:
*His iPhone got caught in his spokes. Yes, you read that correctly. And no, he was not texting and riding.