There's a reason that highway department and other public safety employees wear high-vis vests on the job... Hell, even the guys who round up shopping carts in the Wal-Mart parking lot wear these vests. It makes them stand out from the background, and also in our benefit, it accustoms drivers to looking out for these sort of vests.
A high-vis vest is a good thing to have if you're riding during the daytime. They're pretty light, can be folded and stuffed in a seat bag or jersey pocket, and most will help at night by adding reflectors, and showing a human shape. For the most part, it's a good, cheap invest ment in your safety. Be Visible...
"Let's Get Visible"
http://www.youtube.com/watch?v=r7DBf_Uxfeg
LCI Night Ride – How Visible are You?
http://www.youtube.com/watch?v=epZdXfvrRmU