traffic sunburn
A condition which typically affects the arm which sits on the driver's side window when the windows are rolled down. The driver will notice that one arm is more dark than the other after sitting for long periods of time in heavy traffic. It's most often found in metropolitan areas during the late-Spring/Summer months.
I was driving home from work but I got stuck behind a parade maker who was driving by a road work site. I sat in traffic for over an hour and got a traffic sunburn.