The cause of the crash is not yet clear, but California Highway Patrol spokesman Mario Lopez was quoted by the Associated Press as saying that the bus driver reported brake problems as he descended a road leading from the Big Bear ski resort area.

BBC: California bus crash kills eight and injures dozens

He said he didn't notice whether the driver tried to brake before hitting the other train.

CNN: Investigator: D.C. Metro crash 'a scene of real devastation'

In motor racing, you can see how fast a car is going, whether the driver is using the brake or the throttle and a host of other data.

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It said "the driver applied the emergency brake as soon as he realised this, but there was insufficient distance in which to stop and avoid trapping the guard who had moved back between the coaches to complete the work associated with the uncoupling".

BBC: Robert Lund

Police said the driver had pulled the emergency brake but was unable to stop in time.

BBC: Yarnton level crossing death 'unexplained'

All the driver has to do is brake.

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Cars on the roads can already park themselves, brake automatically and alert a driver if they are slipping out of a lane.

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In addition to the aforementioned traction-control system, standard features include ABS brakes, electronic brake assistance, child safety locks and driver, passenger and side-head air bags.

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That reduced clearance may mean the driver's foot could brush the side of the brake pedal when going from the accelerator to the brake, thus increasing stopping distances and the risk of a crash.

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It takes an average driver a fifth of a second to move his foot from accelerator to brake.

ECONOMIST: Drive by wire