No, you cannot impede manned vehicles in the same way. If you put a traffic cone on the hood of a manned vehicle at a stop light, the driver will simply get out (when safe), remove the cone, and continue. It takes all of a minute to recover. Whereas the unmanned vehicle requires the responsible person to be notified and to arrive from who knows how far away to resolve the situation.

The fact that this prank can be done quickly, cheaply, safely, and without property damage means the car software needs to be prepared to deal with it without becoming a public nuisance. If the software is not prepared, the car is not fit for widespread deployment on public roads.

The equal possibility, which you raise, of dangerous impediments on both manned and unmanned vehicles is not relevant here, because the argument has not been made that the cars inadequately deal with dangerous impediments.

Finally, our society is largely comfortable with the current level of risk that manned vehicles entail, together with the ongoing progress of safety improvements. Not many would agree that the concept is a big mistake.