I do not think it would make the car accelerate on its own, just flood the engine and or waste gas. If it were me I would let them keep it until it is fixed. Though it is more expensive to get fixed at a Lexus dealership, it would usually be the safest place for it to be. AND you get to drive around a new car while they work on it. Try doing that at your local shop. And hopefully you still have powertrain warranty and it should not cost you a thing.