1. Head so far up your own ass that you can't pay attention.
2. You're doing bills, while riding, and therefore not paying attention.
3. You've got headphones on, while talking on a cell phone and checking your hair while not paying attention.
4. God struck you with blindness moments before impact, as part of some Old Testament revival, so you weren't paying attention.
5. A squirrel was discussing Aristotle with you, and you weren't paying attention.
6. You saw a frisbee, and started thinking about frisbees, so you weren't paying attention.
7. You had some hot dish waiting at home, and so you weren't paying attention.
8. You were thinking about how far you could ride to begin a new hip trend, and so you weren't paying attention.
9. You were in tune with your touring rig, feeling the delicate shifts in weight and metric changes as the wheels broke across cracked ground, and so you weren't paying attention.
10. You were reviewing things to do that night, and weren't paying attention.
11. You were blogging, and weren't paying attention.
12. You closed your eyes to rest them for a few moments, and weren't paying attention.
13. Small children angered you earlier, and so you were meditating while riding, and weren't paying attention.
14. Right at that moment, you ran out of ADD meds, and therefore, you weren't paying attention.
15. You were munching on a granola bar while sipping green tea, and therefore weren't paying attention.