Developing autonomous vehicles has turned out to be slower, more difficult and expensive than what industry experts thought, and it may be some time before we see a fleet of driverless cars on public roads. 

Pony.ai has become one of the most valuable autonomous driving startups in the world, as it raised another $267 million in its latest funding round. The California-based company – which is also backed by Toyota – is now valued at a whopping $5.3 billion, according to a Reuters report.