“There are miles and then there are miles,” Dmitri Dolgov, the software lead on Google’s car program, said in an interview. “An even better way to think about it is not just in terms of miles or time, rather it’s the number of interactions that you have with the world and richness and complexity of those interactions.” It has taken a little more than a year for the program to go from one million to 2 million miles. In that time, the field of autonomous driving has changed dramatically, suggesting a new era for transportation is about to emerge.