And why R is before D, making the driver behind you think you are backing up?
In: 18
You want the drive gear to be as far away from the reverse and park gears as possible. On not-very-much-older automatics it was entirely possible to shift from drive to reverse or park while in forward motion, which annihilated the transmission and was very dangerous to the passengers. A lot of modern cars have a locking mechanism that engages to prevent this but that can fail of course. Having the order like this is not only safe, but it is consistent across vehicles, making driving a new vehicle east to pick up.
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