There are source out there for accidents, traffic information and construction where the system could pretty reliably predict when it's going to encounter an odd traffic pattern that would be better handled by a human than the automated systems. In cases where there's a decision needed extremely quickly we're generally talking about an accident or near accident that relatively simple existing assistance systems like autobraking and obstacle avoidance would handle.

Trucks are already really slow compared to traffic inside cities and in the rare cases where they're in this mode on a highway they're probably already in a slow traffic area because they're dealing with construction or accident areas.