A speed trap may be what you're looking for.
From that Wikipedia link:
Methods used include roadside speed traps set up and operated by the police and automated roadside 'speed camera' systems
Hence a mobile operation is a speed trap, as opposed to a fixed speed camera.
The article also states that:
Groups such as the National Motorists Association define speed trap more narrowly as a place where "traffic enforcement is focused on extracting revenue from drivers instead of improving safety".
This appears to be the exact use you want, although it possibly only applies in American English (a US group is used as the example). Wikipedia states that it is only "some groups" who use this definition, but it does seem to have fairly widespread usage in the US, as shown by this thread from the officer.com forums (thanks to @H.R.Rambler for the link).
For British English there are plenty of UK google hits for cash cow speed trap, where a cash cow is (via Wikipedia):
a business venture that generates a steady return of profits that far exceed the outlay of cash required to acquire or start it
I would say that the meaning of this would be well understood, but it's not exactly a common phrase. Plus it's a bit clunky. Plus lots of the google hits are about fixed speed cameras, rather than mobile speed traps.