Vehicle was involved in a rear end collision. Vehicle was setup to have Emergency Braking enabled and used the Rivian High Regen setting (one pedal driving). In freeway traffic, the vehicle in front slowed quickly. The vehicle initiated emergency braking, but when the vehicle in front sped away, the vehicle failed to release emergency braking thus resulting in the car behind hitting my Rivian. There was no vehicle to the right of my car. I should have been able to accelerate out of the way and move to the right. It would appear the Rivian system requires a FULL STOP. I was unable to accelerate to mitigate the rear collision damage. Secondly, the vehicle acted like it did not recognize HIGH REGEN setting as a brake input. I have had multiple time where it flashed at me to brake and the HIGH REGEN setting was stopping the vehicle. This is not acceptable. Rivian was contacted and the vehicle taken for service and repair. I was informed that they reviewed the accident and that all systems checked okay. Rivian noted that I did request acceleration from the car, but I did not hold the accelerator long enough for the vehicle to allow me to regain control. I have asked what the threshold is to regain control and they were unable to answer. Minimally I would hope they can describe how their system has been designed so that I can properly drive their vehicle. Ideally, the system should display emergency information on how to override in the event it forces the car to stop