The promise of driverless vehicle technology to reduce road fatalities hangs in the balance now as never before. Two recent deaths involving Uber and Tesla vehicles using driverless systems have raised the debate on safety to levels that threaten to significantly delay or derail adoption of the technology.   Uber has temporarily halted tests of self-driving cars after the latest crash, and so have Toyota and graphic chips manufacturer Nvidia , whose artificial intelligence technology helps power driverless cars. Arizona, where the Uber vehicle had its crash, has banned the company from testing its driverless cars in the state. And even before the latest crashes, California had introduced a permit process for autonomous vehicles with elaborate requirements . The publicly available information on the two accidents does not appear to predominantly place the blame on either human error or technology.…