Former employees say staff should have held back Smiler carriage until empty car returned, but also that collision should have been mechanically impossible Human error is likely to have played a major part in the rollercoaster crash at Alton Towers that saw four riders airlifted to hospital with serious injuries, former staff at the theme park have claimed.A carriage full of thrill-seekers collided at speed with a stationary test car on the £18m Smiler ride at the Staffordshire attraction on Tuesday, leaving a total of 16 people hurt and suspended 25ft in the air at a 45-degree angle.