Your taillight must produce hundreds of lumens of red light to be plainly visible to motorists on sunny days in traffic. No regular taillight will cut it. My old Dinotte 140L (~120 lumens) ran off 4AAs and was more than adequate at night, but did little to make me conspicuous in the daytime.

There are only 2 truly daytime-capable taillights AFAIK, linked below. The DS-500 is superior in that it's output in flash mode can be reduced for responsible nighttime use, whereas the DDR output in flash mode is fixed and will be blindingly bright to motorists behind you at night (DDR output can be reduced to 10% in constant/steady mode only). The DS-500 is about twice as bright (800 vs 400 lm) and has a wide range of flash and steady output modes, adjustable in pulse length and intensity.

http://store.designshinelighting.com...-p/ds500v4.htm

http://store.dinottelighting.com/day...ount-p188.aspx

Short of spending hundreds on one of the lights above, OSHA lime yellow high visibility shirts/jackets are pretty good at capturing attention in the daytime - at least as good as "regular" taillights but require no recharging:

http://alertshirt.com/poloslmtanco.html

http://alertshirt.com/wirajaancl3.html