Companies such as General Motors Co's (GM.N) Cruise and startup Aurora have said the metric, called disengagements, is not an accurate or relevant way to measure their technical progress, even though it is widely used to do just that. The debate is taking on more importance amid delays in the rollout of self-driving vehicles and concerns over a lack of regulation and the prospects for profitability for the companies that make such vehicles. The focus on disengagements -- when a human driver must take manual control from a self-driving system -- and the backlash from self-driving companies have been growing since the California Department of Motor Vehicles began releasing annual disengagement reports five years ago. California requires all companies testing self-driving vehicles on public roads to submit an annual report on disengagements and what caused them, "written in plain language." In 2018, the companies with the most miles between disengagements were Alphabet Inc's (GOOGL.O) Waymo and Cruise.