Transitional effect

Disney’s Haunted Mansion the Movie website…

There is a transitional motion blur effect that occurs when you enter a new room when exploring the mansion. I’m supposing this blur is done in photoshop but being that the blur areas are so large I’m wondering how that would be done without making the movie size huge. Is there any other way of creating this illusion other than using an additional blurred image?

Check it out here:
Click on “visit the masion” right at center.