It depends really in the Time Place Circumstances really. For example a traffic fella will struggle to spot someone on the phone in the open road at open road speeds (plus isnt it all about speed).
Front line popo in marked cars do it, problem is it depends on Time Place Circumstances once again. If said popo had an offender in the back travelling back to the Police station then it poses a problem trying to conduct a traffic stop with a possible angry, drunk, drugged, violent offender.
Then it depends on calls for service (I fucking hate that term). Front linein cities are often going from job to job and may not be free to conduct the traffic stop. Police are criticised for either taking to long or for activating red and blues for what some public believe are not priority jobs eg shoplifter, so they must drive at the speed limit, but wait there is more. If said shoplifter becomes violent Police are then criticised for not responding fast enough. Not whinging just saying how it is, I have way better things to whinge about
Then there are the unmarked cars, well the feds cant have their coffee going cold (sorry in house joke). To be fair they do write a few out because as you point out peeps don't recognise the vehicle.
Lastly is the traffic itself. hell peeps on here moan about u-turning cars (yes they were at fault, nuff said (hopefully)). Sometimes it is deemed unsafe or unable to safely stop the vehicle. The old dammed if you do dammed if you don't .
Obviously my perspective only.
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