Mich. Comp. Laws § 256.531

Sec. 1. It shall be unlawful for any person, firm or corporation engaged in the business of operating any motor vehicle on the highways of this state for carrying passengers for hire, to lock the doors of any such motor vehicle or permit the same to be locked at any time while any passenger or passengers are aboard any such motor vehicle in such a manner as to prevent such doors being readily opened by such passengers.