From The Philadelphia Inquirer :
Philadelphia police directives on plainclothes officers , which are posted on the department’s website but are heavily redacted, do not appear to include guidance on pedestrian stops. The directive does advise that unmarked cars should not “routinely” make traffic stops  and should not initiate a pursuit “barring exigent circumstances.” 
Jillian Snider, an adjunct lecturer at John Jay College, policy director for criminal justice at the free market think tank R Street and a retired New York police officer who spent a decade in plainclothes duty, said the work assigned to these units is inherently risky.
But, she added, officers should be highly trained in how to engage members of the public for the safety of everyone, including the officers.
“I never one time had anyone say ‘I thought this person was trying to rob me or carjack me.’ I think it’s because we are trained to immediately identify ourselves as police. I also wore my bulletproof vest as my outermost garment and my badge around my neck. My first word would be ‘Police!’”
- “The Philadelphia Inquirer”: https://www.inquirer.com/news/philadelphia-police-south-task-force-tj-siderio-shooting-20220427.html
- “directives on plainclothes officers”: https://www.phillypolice.com/assets/directives/D10.8-PlainclothesOfficers.pdf
- “should not “routinely” make traffic stops”: https://www.phillypolice.com/assets/directives/D12.8-VehicleOrPedestrianInvestigations.pdf
- ““barring exigent circumstances.””: https://www.phillypolice.com/assets/directives/D9.4-VehicularPursuits.pdf