Honda Civic idle mystery

We have a 2012 Honda Civic E and when started every morning, the motor runs at a normal faster than slow idle, but when backing out of the garage, the car will accelerate on it’s own and without holding down the brake pedal, would travel at a speed of 20 mph once on the road and moving forward, all without touching the accelerator. What’s wrong with this Civic?

What does your independent mechanic say? High idle usually turns out to be something quite simple but the reasons are too many to list.