Speed and alcohol ruled out as bus crash causes Swiss investigators have ruled out excessive speed and alcohol as the cause of the school bus crash. They are probing other possibilites, including human error and a technical problem with the coach. The local police chief told reporters that officers had checked the coach's tachograph - a system which records speed. They found the driver was doing less than the permitted speed limit when the vehicle crashed inside a tunnel. They also found no traces of alcohol in his blood. Some media reports have suggested he was trying to change a DVD and became distracted, causing him to crash as the bus returned from a school ski trip in the Swiss Alps. But that has been denied by the bus operator and dismissed as pure speculation by police.