Was taking a exit on to a major three lane highway pulled out behind a semi. The car kept accelerating I had to slam on the brake and put the car in neutral when I drifted off to the side of the highway. The engine was still racing and continued to this for about 25 seconds. After the engine slowed down I put the car in drive and was able to drive home. This is the second time this has happened, the first time it was so brief I thought I must be mistaken.