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Waymo's autonomous cars have steadily rolled through test routes in multiple states over the past few years, and now the company claims it has passed a new milestone: 4 million self-driven miles logged on public roads. That makes the Waymo fleet the most experienced autonomous car platform currently on the road, according to the company, which says the average American driver would take 300 years to hit the same mark. While the number is arbitrary to a degree, the progress it represents is essential to Waymo's mission to create truly driverless cars. The AI behind the platform needs to be trained in real-world situations to understand how to react to every potential condition it might face, so the more test miles it logs, the better. The Google spinoff says its fleet of test vehicles drove the last million miles in just six months, a rapid improvement from the 18 months it took to accumulate the first million (from the first public test).