Waymo’s self-driving cars have crossed the 4 million mile mark on public roads. However, this also marks a push the company has been making to expand its self-driving operations in the past year. The first million miles took Waymo a year and a half to accumulate, while the last million miles only took six months. Waymo is able to speed up and scale its tests through a combination of public road driving experience and testing on the company’s private track. Although public road tests are important, the private test facility also plays a critical role as 20,000 unique scenarios have been modeled at the facility. — THE VERGE