You're making a generalization here based upon your sense of smell and your sensitivity (or lack thereof) to cigarette smoke. Someone smoking in front of me on a sidewalk at walking speed will cause my breath to catch and I'll end up holding my breath until I can pass them. I can easily smell cigarette smoke coming from a car 100 yards ahead while in my car at cruising speed although, at that dilution, it is a minor annoyance. Luckily, I've never had to ride behind someone smoking, but I have no doubt that it would "bother" me if not interfere with my breathing until I could pass them.