Ive encountered them in two places.
1.) Masjid chowk, lahore, DHA , near Y block. During rush hours there is a covered van on the left side of the road ( assume you are driving toward masjid chowk, and the masjid will be on your right side. ) That van has a mounted speed gun on the back.
Very frequently i have seen that they give everyone challans, from prado drivers to mehran drivers. I saw a fortuner get stopped just the other day. Motorcycles also get flagged down.
Masjid chowk has a few traffic policemen on site while the trap is set up, both next to the van and the chowk itself. Dont try to run, you will have your licence plate noted down by any one of the police on standby even if you get away.
These traffic policemen also flag down misbehaving motorists, in the chowk and around it, so be sure to behave.
The traffic policeare present during rush hours and same with the speed trap. They are not regularly present, but they show up often enough that its worth behaving yourself in and around the area in general.
Rush hours ( Mon-sat 7 to 9am , 1 to 230pm and 4 to 6 pm. ) They are also present during those times on the weekend when everyone decides to shop all of a sudden. ( evening, of Friday and Saturday. )
2.) ring road. They pick particularly busy stretches of ring road and then set up the speed trap ( still in a van ) in the ENTRANCE / MERGING LANE, i.e where you get on ring road.
They also follow rush hour timings.
Slow down while passing highway exits on ring road.
Lastly some , not all of the ring road police corollas have handheld speed guns. Slow down as soon as you spot one.