Why do drivers enter the car via the passenger door in old movies?

Question

In old black and white movies as well as several episodes of the Brady Bunch the driver of a car enters the car through the passenger door and then slides across to the drivers side. Why?  This happens in the movies Psycho,  In the Heat of the Night and tv series the Brady Bunch and Leave it to Beaver just to name a few.

Answer 1

My guess would be for safety reasons, or maybe it's what looked best on camera from the angles...could be a number of things.