The worst accident I ever had was when I collided with another bike, not a car - I flew over my handlebars and hit my head directly on the edge of a concrete slab. There was a loud crack, and in half split my....helmet. I was pretty ok(apart from severe bruises all over).

My point is that whether you cross your path with cars or not is irrelevant - you're still travelling anywhere between 15-30km/h and your head is 1.5-2m above the road - it's enough of a speed and enough of a distance to kill you just from a fall, vehicles notwithstanding.