Same thing that happens to humans. You see parts of the signage. Or none of it, and drive on memory (and follow the behavior of other cars).
The idea behind adding special markings for computers is that you can waste a lot of time and effort trying to perfect algorithms interpreting signage optimized for human consumption, or... you can spend much less time and effort, to much better effect, setting up additional signage that's easy for computers to consume and provides information relevant to computers (which is not necessarily the same as what humans need).
Note that roads are controlled, artificial environments. There's always someone responsible for maintaining a road and signage on it. The infrastructure to deploy additional markings dedicated to self-driving cars already exist.
The idea behind adding special markings for computers is that you can waste a lot of time and effort trying to perfect algorithms interpreting signage optimized for human consumption, or... you can spend much less time and effort, to much better effect, setting up additional signage that's easy for computers to consume and provides information relevant to computers (which is not necessarily the same as what humans need).
Note that roads are controlled, artificial environments. There's always someone responsible for maintaining a road and signage on it. The infrastructure to deploy additional markings dedicated to self-driving cars already exist.