Simply put, the reason that the rear windows in most cars don’t roll down all the way is that the window storage compartment in the rear doors is not large enough. When you roll down the windows, the glass needs a place to go.

Car doors have a small cavity that holds the window glass when it’s rolled down. The cavity is larger in the front doors because they are typically bigger. They have enough space to accommodate the window cavity and other mechanical parts of the door.

On the other hand, the rear doors are usually smaller and do not extend all the way down to the bottom of the car’s body. Because of their smaller size, there is less room for a window cavity. Typically, there is only enough space in a rear car door to lower the window about three-fourths of the way. The remaining space is needed for the mechanics of the door.

Newer, larger cars have windows that can roll down all the way because the passenger doors are bigger. Having bigger doors means there is more space for a window storage cavity. This allows the rear windows to safely roll down without affecting the door’s functions.

Any safety benefits that come from the window not rolling down all the way are simply a result of design or may be an unintentional consequence of how passenger doors are made.