This technology is still at its very early stages and 300,000 miles is not all that big of a sample. According to a "cursory" analysis by Bryant Walker Smith of Stanford Law School, "Google's cars would need to drive themselves (by themselves) more than 725,000 representative miles without incident for us to say with 99 percent confidence that they crash less frequently than conventional cars. If we look only at fatal crashes, this minimum skyrockets to 300 million miles." We're still a long way away from there.
The Center for Internet and Society at Stanford Law School is a leader in the study of the law and policy around the Internet and other emerging technologies.