Following a suggestion from user Gerben, I looked for a way to isolate the camera from vibrations while riding.
A quick search DIY camera vibration isolator seemed to find many were building wire rope isolators:
DIY: Build Your Own Anti-Vibration Camera Drone Mount for Only $10!
DIY Wire Rope Vibration Isolator for RC CAR Camera Mount ( Part 1)
I built an isolator with some wire and plywood, and mounted it along with the Pi to my bike:
I didn't tune it, I'll have to look into how to matching the springiness of the wire by tweaking the length against the weight of the Pi. But it wasn't enough - I think a faster camera is still required. Here are some sample photos from my bike, with the Pi mounted on the isolator:
Photo isn't clear enough to make the the writing on the green sign, or the plates on that car:
Photo is clear enough to make out objects that aren't in motion - Finch Ave., Soban cafe. But not the cars in motion.
I may try to "tune" the isolator, but I think either way, I need a better camera sensor.