There are flashing bike lights that attach to helmets. I use the Blackburn Flea 2.0 combo set: white facing foward, red facing back. They recharge via USB. Rated 5 hours for the front, 12 hours for the back, in flashing mode. Charging the front unit is a bit inconvenient as the mounting strap has to be removed for the magnetic charger-dongle to click onto the light. For me, the set of features is an acceptable tradeoff for the small form factor and negligible weight. There are larger, brighter lights with longer runtime and/or removeable batteries that will also mount on helmets.
BTW, a UK crowd-funded bike safety light enterprise quotes studies that say the vast majority of bike collisions result from oncoming motorists not seeing the cyclist regardless of time of day; i.e., forward lights matter for visibility, even in daytime.
Last edited by dorkypants; 02-28-14 at 06:12 PM.
Reason: add word to clarify