&gt; difference is cars have a clear, visual grid of where they can travel and where they cannot travel.<p>Visual grid for people to look at. All those cues and signs were not designed to be read and processed by robots. And there are crazy pedestrians, crazy drivers, maybe cattle crossing the street, an accident, ice, fog, ice and fog and cattle, unpaved roads, etc.<p>I would think an airplane&#x27;s environment when in the air seems more predictable and stable, than an environment for car on the streets. Same thing for underwater vehicles. There might be crazy pranksters playing games with the vehicle 100 feet under water, but probably not a high probability.