Logan Smith, 18, was being taken to hospital in an ambulance at about 11pm on Sunday when the vehicle stopped on the hard shoulder of the M5 in Somerset.
The Independent Office for Police Conduct (IOPC) said the vehicle stopped on the northbound carriageway “due to the concerns of ambulance staff”.
Mr Smith got out of the ambulance near the junction for Weston-super-Mare and “entered the live lanes of the motorway”, the watchdog added.
Police were called and an officer arrived at the scene, with bodycam footage showing the officer discharging their Taser, causing the teenager to fall to the ground.
