As per your explanation, it is feeder vehicle.


Feeder Vehicle - A police vehicle not directly involved in the pursuit which has been strategically placed in order to maintain or direct a subject vehicle in a preferred direction, either to deploy a particular tactic or prevent access to an area whereby the level of risk would be increased. The positioning of any feeder vehicle must always be carefully considered in the light of a risk assessment by the vehicle driver.

http://library.college.police.uk/docs/NPDSC-Police-Driving-M...

and
https://www.cheshire.police.uk/media/1570/police-pursuits.pd...



As for decoy car or bait car is طعم but in order to
capture car thieves or thieves who steal items from cars.

https://en.wikipedia.org/wiki/Bait_car

Best


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Note added at 18 hrs (2016-06-29 07:51:37 GMT)
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According to National Police Driving Schools Conference
Police Driving 2009 Feeder vehicle means:

A police vehicle not directly involved in the pursuit which has been strategically
placed in order to maintain or direct a subject vehicle in a preferred direction,
either to deploy a particular tactic or prevent access to an area whereby the
level of risk would be increased.