Keep quiet, as those in car should (4,2)
I believe the answer is:
belt up
I believe this is a double definition.
'keep quiet' is the first definition.
(both are used to tell someone to be quiet)
'as those in car should' is the second definition.
(put a seatbelt on)
(Other definitions for belt up that I've seen before include "Be quiet (colloq.)" , "Wear car safety device - be quiet" , "put a sock in it!" , "To secure safety strap" , "Strap yourself in - keep quiet!" .)