What the last post by Hotbike points out is that it is not just the intensity of the light but also the size of the lighted area that makes for an effective tail light at night. It seems to be exactly the opposite during the day when a compact but intense light works better. I've seen the "arms race" for more powerful lights get to the point where you almost need to take out a bank loan to afford some of them. Just being brighter isn't always the answer.

I've been using a pair of DIY lightsticks on a recumbent trike mounted vertically on the frame behind my head. There are 78 individual red LEDs wound around each 11" long plastic tube. It's visible from every direction but since it is behind me, the strobe mode isn't distracting. Depending on how you power it with three 18650 Li-ion batteries or lithium polymer battery pack, the cost can be from less than $15 to about $25. The run time between charges is more than 8 hours on strobe mode.

Some roads are just too dangerous to ride day or night. If it is not the distracted motorist it could be somebody who doesn't have enough space to get around you but tries to do so anyway.