Flags
Nobody likes them. They look dorky. Geek alert.
But honestly, when I did a big organized ride and had a honkin' orange flag on my bike (required by the ride) cars gave me wide berth. If only so they wouldn't scratch their car on the flagpole.
I kept using that flag for a while on my bike commute. It was amazing.
If I get into commuting on the busier streets or if I start hauling a trailer, you can bet I'll be putting a flag on my bike again.
I don't mind looking stupid if it helps drivers see me.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson