Depends on what the model believes about the prevalence of self-driving / autonomous-agent-driven cars at the time the last human on Earth remains (and how much these agents would care about a "closed" highway status, and who exactly it's closed by and for). This estimate can differ very widely. I'd be curious if the results would change if the scenario explicitly specified that this is specifically an alternative history scenario where the last human remains after the rest of humanity was wiped out in some nuclear apocalypse back in the 20th century, before any possibility of all the autonomous stuff.