Face masks must be worn when riding the elevator.

[sitifan]

Is the sentence grammatically correct?

 

[Rover_KE]

It's a notice—it makes its point without needing to be grammatical.

 

[emsr2d2]

It's a notice—it makes its point without needing to be grammatical.

That's true but, in this case, it's perfectly grammatical. (Note: BrE doesn't use "ride the elevator". For us, it would be "in the lift".)

 

[sitifan]

Isn't it an example of a dangling participle?
https://www.quickanddirtytips.com/education/grammar/dangling-participles

 

[probus]

Isn't it an example of a dangling participle?
https://www.quickanddirtytips.com/education/grammar/dangling-participles

No it isn't. Compare for example: breathing must be controlled while singing.