The court has considered that for decades, it was a key point in Florida v. Bostick which relied on jurisprudence going back to Terry. Unfortunately the court ruled that it has to be explicit that you are not free to leave an encounter with police. Coercion and trickery is OK, which is why ACLU and similar videos tell you to repeatedly ask "Am I free to leave?", as you note. In this case, being on a bus with the aisle blocked by police officers didn't count, the Court insanely ruled that a reasonable person would have felt free to leave the encounter. Justice Marshall dissented (with 2 others), one of the last decisions he took part in. Only Scalia and Kennedy are still on the Court and sided with the majority.