A disengage should be understood to probably be an accident, given that it's a failure of the self-driving system without warning, and given that Google's argument is that the steering wheel should be removed. Contact with a traffic cone means the car could just as easily make contact with a baby in the street, you can not just say "since it hit something cheap, it doesn't count".

282 failures of their system across 434,000 miles is 1,539 miles between incidents. That's a system that fails more often than even old school cars need to change their oil.

Suffice to say, Google's claim that they're ready to remove the steering wheel and it's only the law holding them up, which is the argument they made on the floor of Congress... they're liars.

EDIT: Since I'm rate limited, my response to Retric below: False. The system disengages when the system has an error and has conflicting data about the state of the world around it. It isn't overabundance of caution, it's overabundance of confusion.