For a long time, cars had no passenger side mirrors.
I have seen vintage cars from the 60s, 70s and 80s with plane mirrors on the passenger side.
My first car, a Honda Civic bought in 1993 had no passenger side mirror. It was an option at extra cost.
Our first family cars were station wagons bought in the 1970s (from AMC) and mid 80s (from Ford). They had no passenger side mirrors.

When were convex mirrors first used on the passenger side?

Obviously there had to be an adaptation period with automobile manufacturers and dealerships, government highway traffic acts and laws, etc... Drivers also had to get used to these new convex mirrors which could cause a lot of accidents if people didn't realize that objects in the mirror are closer than they appear.
When did convex mirrors become legal and when were the words "Objects in mirror may be closer than they appear" first written on passenger side mirrors.
Also, why did the words use to say "Objects in mirror may be closer than they appear" .
Why the "may be"? Convex mirrors always make objects look further anyway. Plane mirrors give you the real distance.

Now it says "Objects in mirror are closer than they appear" which makes a lot more sense since convex mirrors always make objects look smaller and therefore they look like they are further away.

Thank you.