Accelerator Coasters with the top hat element are known to perform an occasional rollback – when the train is unable to complete the top hat element and rolls back to the starting point – which can occur due to a number of different factors. Despite this, Accelerator Coasters have a great track record for safety and are extremely power efficient, especially compared to older technologies such as the classic chain lift found on most roller coasters. Formula Rossa, the world's fastest Accelerator Coaster, and Kingda Ka, the world's tallest, are some of the most well-known installations around the world.