slantsixdan
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Federal Motor Vehicle Safety Standards (FMVSS) 111 (1993) sets performance standards requiring the ability to see both sides of the vehicle from the driver's seat. This means that dual outside mirrors have been required since 1993.
That's not quite correct. Passenger vehicles, covered by Section 5 of FMVSS 111, still today do not have to have a passenger-side mirror unless the field of view provided by the inside rearview mirror does not meet the requirements of paragraph 5.1. This could be the case if the vehicle's backglass is small, or if the inside mirror's view of the outside is blocked by an opaque panel of some kind inside the car (like a limousine partition) or an exterior feature or design aspect of the body.
Although the inside mirror provides the legally required field of view on most vehicles, a passenger side mirror is provided anyhow because people want them.