I'm sure it's because I read and commented on this thread yesterday, but I got 2 cars on my short (12km) commute to work today. First car was parked, and I head the ping of the rock hitting it as I passed (I did feel a little bd about that one, but didn't hang around to see if it left a mark). Second car was 5 blocks later - they were over taking me as we rounded a corner. Rock was really loud when it hit, but the driver didn't seem to even notice.