In the early days, when the Metaverse was a featureless black ball, this was a trivial job. Later on, when the Street went up and people started building real estate, it became more complicated. On the Street, you can pass through other people's avatars. But you can't pass through walls. You can't enter private property. And you can't pass through other vehicles, or through permanent Street fixtures such as the Ports and the stanchions that support the monorail line. If you try to collide with any of these things, you don't die or get kicked out of the Metaverse; you just come to a complete stop, like a cartoon character running spang into a concrete wall.