A planned trajectory is analyzed by an L4 emergency braking module, before the trajectory is navigated by the autonomous driving vehicle (ADV). An L4 emergency braking signal is generated in response to an autonomous driving vehicle (ADV) determining that any portion of a planned trajectory of the ADV has a deceleration that is less than −2 m/sec2 and a time to collision with the ADV, by a vehicle following the ADV at a speed Z and a distance Y