The number of autonomous vehicles on California’s roads has more than doubled as carmakers and big tech companies race against secretive start-ups to test their self-driving cars. Two new permits granted for autonomous testing on Wednesday by California’s Department of Motor Vehicles has taken the total number of companies licensed to drive their prototype vehicles in Silicon Valley’s home state to 27 — more than twice as many as a year ago and up from just seven in early 2015, according to the state’s automotive regulator. The latest entrants to the increasingly crowded market are Uber, whose testing vehicles were forced off the roads in San Francisco in December after it initially refused to apply for the DMV’s approval, and PlusAI, a 15-person start-up based in California and Beijing whose existence has until now been little known beyond a select group of automotive executives and tech investors. Earlier this month Renovo Motors, a Silicon Valley start-up developing “automated mobility on demand”, and Navya, the French creator of an autonomous electric shuttle, were both added to the DMV’s approved list. The group looks set to increase further still as companies from all over the world flock to Silicon Valley to develop their technology and tap its sought-after talent pool. Didi, the Chinese ride-hailing service, opened a new R&D centre in Mountain View on Wednesday to work on intelligent driving, cyber security and artificial intelligence.