Vehicle that may catch a speeding motorist (4)
I believe the answer is:
trap
I believe this is a double definition.
'vehicle' is the first definition.
(trap is a kind of vehicle)
'that may catch a speeding motorist' is the second definition.
(I've seen this before)
(Other definitions for trap that I've seen before include "that may be sprung?" , "Golfing hazard" , "Confine" , "Cakehole (slang)" , "Driver's obstacle" .)