You could wear one in the car, since traumatic brain injuries are almost as likely in a car on car crash as with a car-bike collision. (.65 times as likely). And, given that slip and falls are the most common cause of TBI, wearing one while walking on a sidewalk -again, the most likely of the slip and fall scenarios - might be a good idea.

That's if you're really worried about that level of risk. It's not enough for me to wear one driving, or walking, and not enough to worry about while riding if I'm not wearing one. But, in fast traffic, like the other guy says, preventing any injury in a high-speed interaction with cars is a good thing