A fairing like this one can protect the rider against the dreaded "Right Hook". It can crush the cars right rear quarter panel in, which will help in identifying the driver.
This is a frame mounted fairing, and will prevent the bike from doing an "Endo" on impact.

A fairing is a protective shield. On the downside , the fairing must be heavily built to provide crash protection. I can't discuss aerodynamics without blowing a lot of smoke, but crash protection can be measured.

I posted another thread just on this topic:
http://www.bikeforums.net/showthread...tective-Shield

Please feel free to join the discussion. This topic is a lot like the helmet discussion, which is always lively. Unfortunately, the state of the art in protective bicycle fairings is where bicycle helmets were in the 1960's.- There are none commercially manufactured .
The few fairings currently available are for aerodynamics only, and have no crash protection that I know of.