I always wear a helmet when riding my tri bike or mountain bike. But I don’t always wear a helmet on short CaBi trips. My speed is very slow on CaBi. I don’t ride on any steep hills. Most of the time, my average speed is around 6-8 mph on CaBi, according to my Garmin. Sometimes even slower.
When I plan to do some faster riding on CaBi (which is rare) or I’m taking a longer trip, say from Arlington to D.C., I will wear a helmet. The upright riding position, the heavy weight of the bike and the slow speeds make CaBi pretty safe. The accident rate for CaBi as a whole has been very low, even with all of the inexperienced riders on the bikes. I don’t know the exact rate but I believe it’s something like 1 accident for every 90,000 bike trips or so. And even with those accidents, most of those did not involve serious injuries (I think).