Is It Legal To Drive While Pulling A Flag On Your Vehicle? Yes
Over the weekend while taking our exchange student to Burnsville for indoor soccer I ended up passing several vehicles with large flags being pulled behind them. As they went by both my wife and our student asked if someone could legally drive around with a flag attached to their vehicle. I didn't see what made those flags any different than one of those window flags people have of their favorite sports team, but I decided to ask a member of law enforcement to make sure.
I emailed Faribault Police Chief Andy Bohlen about the question and his answer was short and sweet about the legality of driving around with a flag attached to your vehicle. That answer was yes, it is legal to do so as long as do not "obstruct the driver’s vision or create obstructions for other motorists."
So for those of you who had the same question as my wife and exchange student had, there is your answer. It is legal for people to drive around with a flag flapping in the wind behind them as long as it doesn't obstruct the driver or other motorists.
As far as to flag flying etiquette, according to military.com when flying a flag on your vehicle "the staff shall be fixed firmly to the chassis or clamped to the right fender." However you shouldn't use the flag as decoration the same website states that you should "use bunting with the blue on top, then white, then red" rather than using a flag for decoration.
As far as obstructing other drivers, the answer to that my friends, to quote Bob Dylan, is blowing in the wind.