I don't know. Ask a cop. 
A few years back I saw a cop doing speed monitoring and tickets on 5th street. I got off my bike and said that I think there are a lot of people on cell phone and asked if I could help him catch some.  He was not too interested but did not stop me. 
In 15 minutes I saw someone and said "cell phone!". He saw that, and started his motorcycle and gave a ticket. In another 15 minutes another woman (I think 75% of the time it's women) was on her phone, looked at us and just kept talking. The cop said to me, "she didn't even try to hide it". He rode off and gave her a ticket.
I had to go but could have done this all day much like your proposal.