The reason this private company uses police is to intimidate the public into volunteering. It’s clear that the police are intentionally violating people’s constitutional rights and should be liable.
I think that is true. The reason goes beyond just a desire to have subjects for their study. The study will have no scientific validity whatever if people are able to "self-select" with regard to participation.
The reason is obvious; people who have any reason whatever to believe that they may be guilty of driving while impaired would, if sufficiently sober to do so, decline to participate. This would cause a dramatic under-reporting of the situation that is supposedly being studied.
There is no way to run such an experiment while relying on volunteers.
If those who designed and operated the study are unaware of this, then they are scientifically incompetent.
If they were aware, then they are scientifically unethical. If they were aware and failed to explain this factor to the enforcement agencies whose authority they abused, then they are guilty of a fraud.
If they were aware and DID explain this factor to the authorities, then the authorities are dangerously incompetent.