The most common machine learning algorithms found in self driving cars involve object tracking based technologies used in order to pinpoint and distinguish between different objects in order to better analyse a digital landscape.
Algorithms are designed to become more efficient at this by modifying internal parameters and testing these changes.
I hope that provides a general overview of the subject.
Since Google's cars are in development and are proprietary, they will probably not share their specific algorithm, however you can take a look at similar technologies to learn more.
To find out more, take a look at an Oxford-based initiative in self driving cars and how they work.