I have been doing brevets now for a few months, a few that involved night riding. The rules say that "During hours of darkness or other low-light conditions, all riders must wear a reflective vest, sash, Sam Browne belt, or some other device that clearly places reflective material on the front and back of the rider." I have used some cheap running vests, with velcro and mesh, but they flop around and get caught on stuff, and they are not aerodynamic at all. I also don't like velcro with other stuff in the laundry, or catching other stuff in my bag or jersey pocket. On my last 200Km brevet, that started in the dark, I tried one of these: Nathan Flash L.E.D. Reflective Vest (Small / Medium, Yellow). Its Lycra!! It stays put, and its aerodynamic. It also takes very little space once you take it off. I bet its the lightest solution too. A note on sizing: I am 185 pounds and wear mostly size L, but I got a S/M in this vest and its perfect. If your chest is over about 46", get the bigger one. Its very elastic though, and the S/M was only slightly loose on my 100 pound GF. I think it comes in white too, but the yellow went well with my stylish PCH Randos Kit.