A mobile speed bump is a derogatory term for a driver or vehicle travelling exactly at or below the speed limit, effectively blocking the flow of faster traffic. This usually happens on single-lane roads or when a slow vehicle camps in the passing lane. Enthusiasts and commuters use the term out of frustration when their momentum is ruined by someone strictly adhering to the rules.
Heard on the job
I was making great time until I got stuck behind a mobile speed bump in a Prius for twenty miles.