I believe I remember seeing a thread a few months back where someone put some sort of a stick with a flag sticking out past their left-edge with the same idea in mind (I couldn't find the thread).
I've considered that, but it would need to be carefully designed to break away easily if struck or grabbed by a passing vehicle, while holding just securely enough to hold on through the usual wind and jolts.
Alternatively, a leftward-aimed distance based discharger for Silly String, orange paint or an exploding dye pack could be amusing.