Who do you think the sign refers to? Cars sharing with bikers? Or bikers sharing with cars? I have my doubts for each.
I was first surprised to hear about driver animosity with bikers in Lance Armstrong's book where he mentioned truckers treating him on a bike with little respect, but since then I have gotten my license, and spent a year running trails with bikers yelling every time they passed me, and I have to say, most of them are buttheads.