In general, people prefer to face the direction they are travelling. A lot of people have issues with moving backwards, especially with the frequent start/stops in a motor vehicle (as opposed to the smoother train rides).
Besides that, the rear passengers usually aren’t the ones at risk in motor vehicle accidents. They typically don’t have any protection other than their seat belt. If you’re referring to the front passenger, that would mean turning the seat around, which prevents the passenger from seeing in front – they might not need to, but they would certainly want to, and often is required to assist the driver in navigating. Plus, the reverse seat would block the sight lines of the driver.
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