There also was pressure to live up to a goal to offer a driverless car service by the end of the year and to impress top executives. Dara Khosrowshahi, Uber's chief executive, was expected to visit Arizona in April, and leaders of the company's development group in the Phoenix area wanted to give him a glitch-free ride in an autonomous car. Mr. Khosrowshahi's trip was called "Milestone 1: Confidence" in the company documents. …. When Uber moved to a single operator, some employees expressed safety concerns to managers, according to the two people familiar with Uber's operations. They were worried that going solo would make it harder to remain alert during hours of monotonous driving.