Originally posted by deliriou5
if only the tailwind blew 25% of the time!!! 
This is not even a point, solar powered systems don't expect the sun to shine at night!
murphy's law of cycling - whatever direction you pedal in, the wind will blow the opposite way.
Superstitious approach. If the wind doesn't blow in the favorable direction, just take the sail down, it will weight not more than 3 lbs...
joking aside.... any wind other than directly behind you will compromise the steering handling of the bike - this is a HUGE safety issue. and don't forget about the hairiness of wind gusts!!
You can still roll if the wind blows from the angle less than 90º perpendicular. Remember, all sail ships need is SOME KIND of wind, they will adjust the sails. Loaded bike is heavy, and you can adjust the tail size to come to a compromise between handling and wind power assistance.
Safety is important, that's why the sail would have to be clear, in order to be able to see what's coming from behind, but we are talking about reasonable conditions on where to use it, something like lonely highways of Saskatchevan...