A group of people are protesting more cars in San Francisco by disabling self-driving cars with a cone on their hood. It is likely that the cone disables some sensors and won't let the cars move. For truly autonomous cars, this could be an issue if people could prevent them from working with small hacks. I could imagine a strip of duct tape or some other smaller obstacle that might also work and be less noticeable to anyone looking at the vehicle.

Ignoring the humor here, or the more-cars-in-cities debate, this is an interesting place where computing systems can fall down. People do find ways to hack sensors or other ways in which our software gets data from the real world, which can force behaviors we didn't expect. We might allow for these, or we might not. In this case, I don't know that the developers could do anything different, other than perhaps detect and alert the owners that the vehicle can't move until someone clears the obstacle. They could add more sensors, but I suspect hackers would just block those.

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