Originally Posted by David Bierbaum

I'd offer the practical tip of, instead of using your arm, get one of those "I'm here!" safety flags on their long fiberglass poles, and mount it sideways on your bike. This way you can keep both hands on the handlebars while keeping some distance between you and any traffic afraid of marring their vehicle's paint job, and if someone comes too close, it's better to break a fiberglass pole than your arm...