Vehicle did not slow down when in assistant driving mode. The vehicle was coming up on an intersection with a stoplight with two cars ahead of it. I had to apply breaks manually with the failure of the system slowing down fast enough. When I manually applied the brakes I pressed them hard after disengaging the driving assistant the car did not send any notifications following disengagement. The automatic emergency braking system also was not activated given that the vehicle had at minimum a cars length distance to stop. I did not receive an alert for the forward collision warning system even before applying brakes. I rear ended another car without any nonfictions to the car making impact as well. I commute on this road often as it is on the way to my job so I am familiar with the lights and intersections but was going further than usual which is why I decided to engage driving assistant.