Nonsense. Roads change very rarely compared to the number of trips taken on them. This precompute needs to be done just once per change, just like building an index that will be used for many searches. The road changes themselves almost always happen in less inclement weather.<p>This is pretty much exactly the same thing that humans do. I drive much better on a road with missing lane markings if I have driven on it before, and just like me, these systems will have degraded performance without the precompute.