But in North America, it would be your left arm.
It seems to be becoming more prevalent for young men to drive around with their car window down and the arm hanging out. They should talk to trauma surgeons at a major hospital about it and ask for some statistics.
People generally divide into two categories when driving in warmer weather. There are those who have windows open on all but the hottest days and often have bits of the bodies hanging out the car window. They clearly like the warmth in their car and the warm to hot breeze.
Then there is the likes of me who so rarely opens the car windows, that the motors struggle to get them down. It is a bit of a bother at pay car park booms, but so far I can get the driver's window down enough. I do open the rear windows a bit when the car needs hot air blasting out of it.
Windows up people like to be insulated from the exterior life. We like our temperature on the cool side. We don't want to hear the musical taste of a car driver next to us. We don't want to hear people yelling on the street. We don't want to give any indication that we wish to interact with any other motorist. We don't want critters flying in, or leaves or pollen or dust. We tell ourselves wrongly, that if we cocoon ourselves like this, we are safer.
I will never change but your windows open people need to keep their limbs inside at least.
PS My father was pulled up for having his elbow outside the car in the late 1960s. He was talking the policeman out of giving him a ticket when my mother muttered mongrel copper. Ticket immediately issued.
PPS An Australian woman in the US was killed recently. She looked the wrong way on a busy road and stepped straight into the path of large vehicle. Aussies take care where they drive on the wrong side of the road.