Just create a big old test suite, procedurally generated. It would make for a nice game and driver training tool too. Verify against average and expert drivers to set grading. Additionally get statistics about prevalence of bad driving, which will allow you to check if the driverless system is safe enough using plain old cost-benefit. Moreover, include assisted driving systems (emergency takeover) in the evaluation.
No need to require perfection, failing gracefully is also acceptable. For example, in a weird situation, the car could slow down and engage notifications at drivers.