Forget all the Home Depot or Walmart stuff. It's not bad, but you can do much better for bike-specific purposes.

Terralux Lightstar 220, on Amazon.com. 2 x AA, 180-220 lumens, legitimately, x 1 hr runtime. Mount with a velcro lockblock to bars, or use a rubber band to loop it to your helmet vents. It's bulletproof, definitely bright enough to see with, even at 18-20 mph, and has been used extensively by other bike folks. It's well worth the $28ish dollars.

You will not be able to get a brighter light than this one for 2 x AA regardless of price today. The FENIX is also excellent, and has better flash & mid/low settings, but costs twice as much.

P7 flashlights generally use 18650 batts. Not a big deal, but not AA, and these flashlights are noteably bigger than the AA ones. You also tend to have to get them on DealXtreme from China if you want them under $50; quality control has definitely been spotty in the long term with P7s from this company. Mine already is on the fritz and I haven't used it much.