Next to the helmet, being seen is about the most important defense against injury. For the darker seasons, equip yourself with a simple, ultra-light safety light from Nashbar. The Lumo Combo weighs just 30g for the headlight, 27g for the taillight...a.k.a. borderline weightless, and simple handlebar/seatpost mounts will have you on the road in just a couple of minutes.
Super light and compact...won't interfere with performance