Google Inc., BMW AG and other car makers are running demonstrations of their driverless technologies. But before such vehicles are commonly found on the road, experts say the size and cost of the laser-powered sensors that acts as the cars' eyes will have to shrink.

Today, an advanced lidar (for light detection and ranging) system for self-driving cars costs between $30,000 and $85,000 apiece, manufacturers said, and some prototypes include as many as four of the smaller, cheaper lidars.

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