It has likely happened to all of us: we're casually opening the door of a car when another car or bike comes whizzing past, nearly hitting the door because they didn't see it opening. Instructables user milesfromnelhu recognised the problem and decided to fix it by spray painting a warning strip on the inside of the door.
As the above picture shows, milesfromnelhu suggests painting the inside of the door that oncoming traffic is going to see first so they're warned of the door opening. It's true you should be looking in your side mirror before popping open the door, but it doesn't always happen. Smaller vehicles like motorcycles or bicycles might still be invisible when you look in the mirror. This is a great way to protect both your car and oncoming traffic. If you're not keen on painting the inside of your door with a bright coat of paint, a strip of reflective tape will also do the trick. You can find the guide to painting your door over on Instructables.
Car Door Warning Color [Instructables]