I've found for me to be successful, I have to be moving at at least 18 mph. I dunno - it's my threshold I've discovered. Anything less than that, and things can get kind of.....messy.
I fare better with stop signs and traffic lights. Just lean and blow. I'm sure the folks in cars love it.
Rode last winter with a guy who had NO luck at all, and he had quite a mess on his shoulder. GAG! I was actually wretching in the back, and had to go up in front and lead for the rest of the ride. It was NASTY.