The Pre-Collision System (PCS) failed to engage the automatic emergency braking during a stop-and-go traffic, thereby, resulting in rear-ending the vehicle ahead of our 2018 Highlander Limited vehicle. Our Highlander was at a complete stop, then when the traffic moved slowly forward, we too started moving forward but didn't realize the vehicle ahead of us had suddenly stopped again. Our Highlander alerted us with high-pitched beep-beep-beep sound, but it didn't stop before colliding with the rear of the vehicle ahead of us. The speed couldn't have been more than 10 MPH since it started out from a complete stop and the distance traveled before the collision was about 40 feet. There were no warning lamps, messages or other symptoms of the problem prior to the collision. It always beeped, like it should, when there were nearby obstacles, pedestrians, and when we got closer to the end of our garage parking space, etc. PCS sensitivity was set at the highest level (3 bars).