I understand your urge to yell at him. I am "impressed" by the carelessness of many cyclists and other users of the road around me every day as I commute to work. (No-helmets are the tip of the iceberg.)

This being said... I think there is a significant risk of startling a cyclist when yelling at him or her from a car. Especially if the conditions of the road are not so good and there are potholes, pylons, etc.

How would you have felt if he swerved as you yelled to him and he turned his head to you to figure out what you said, hit a deep pothole and effectively fell from his bicycle, hitting his (unprotected) head? (Not to say how he would have felt.)

There are millions of reasons why people don't wear helmets, or wear them properly. My own pet peeve is people who wear a helmet with the best of intentions, but the helmet is too small, too big, or crooked on their head, effectively putting them in as much danger as if they were not wearing a helmet at all. Whenever I have a chance to discuss the matter with various people, I do (and they get bored of it pretty quick). Who knows, perhaps the helmet he had strapped to his bike had been pooped in by his dog? I'm making this up, but for me the bottom line is that it's their problem - and their loved one's. If a cyclist (or pedestrian or motorist) is not directly endangering my life by her actions, I leave it up to them... and, more importantly, avoid putting their life in danger more than it already is...

I just got a bunch of "You'd look hotter with a helmet" stickers. They would work great on a bumper. If you'd like one I'll send you one right away (PM me your mailing address!).