Google says that a recent crash between one of its self-driving Lexus RX 450h test cars and a city bus happened due to an obstruction in the road and a subsequent miscalculation.

The admission came this week at the SXSW festival in Austin, Texas, where Google's autonomous driving boss Chris Urmson told a gathered crowd what happened in the car's computer system. You can fast forward to the 21:40 mark for the start of the discussion.

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Urmson says the SUV picked up on a sand bag on the road and came to a stop to avoid driving directly over it. Once a light ahead turned green and the road ahead cleared up, the SUV began moving—but not without noticing a bus behind. However, the SUV's autonomous system assumed that the bus driver would slow down, a mistake that would ultimately lead to the crash. The bus driver, for his part, figured that the SUV would come to a stop.

As history now tells us, the two made impact when the SUV continued moving and hit the side of the bus.

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Urmson stopped short of admitting to any problem with the autonomous SUV's sensors, but he did say that Google will analyze what happened and that the company plans to implement 3,500 new tests to reduce the risk of an accident occurring again. 


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