Um,cars and computers are human inventions. How is combining the two into one machine make it less prone to error than are the humans behind it?
Machines are human inventions and human error can certainly occur in the design, and building of the machine. But once the machine is tested and most flaws identified and removed, the machine is capable of performing the exact same acts repetitively without error.
Everytime a human drives there is a potential for multiple human errors. Everything from driver distraction, road rage, visual perception failure, medical emergency, or simply error in judgement.
When a machine drives you have those same potentials, but they are far less likely. Machines don't get distracted easily, and they don't suffer road rage. They can suffer from a perception error, a sensor or mechanical failure, or a failure in logic embedded in the program. But those sort of errors can be managed with testing and redundant systems.