I flirted with the idea of a flag early on, but found my 'bent already quite terrifying to the local horse-riding school. From what I've read, a pennant dancing around in the breeze only makes it much much worse. There's a lot that can be done with defensive riding, so I tend to rely on that instead. Around 70% of motor vehicles on my regular commute are high enough to block visibility of the flag to the car behind anyway. Mostly its 4x4s, vans, people carriers, luton trucks, lorries, buses, taxis, with the very occasional saloon dotted between.