By driving long haul distances at 40 mph instead of 60 mph, a truck can save at least 50% on fuel, tires, wear and tear, and could get by with lighter construction and a smaller motor. The truck would arrive at accidents with much less kinetic energy, and would have more thinking and reaction time for better outcomes. Who knows what the actual optimum speed point is for cost efficiency and lower insurance costs, but AI truck drivers will almost certainly save big money.
The Tesla trucks that could easily be fitted with the FSB ability modified for size and conditions are electric
It appears you have not done much driving on two lane highways behind big trucks that do go slow. They are a huge safety hazard.
Back in the day when big trucks were under powered it was common for them to shift down to speeds of 40 mph on a steep incline. This made passing very difficult, especially if there was plenty of oncoming traffic.
Several years ago I talked to a state trooper about speeding tickets, and was told they let truckers exceed the speed limit going down a hill so they can make it up the next. The reason was safety, just like I described above.
And many trucks are hauling time sensitive loads, like UPS, FedEx, and USPS where the shipments are to be on time. Your huge reduction in speed would greatly affect this.
AI is not yet perfected to be safe for any self driving vehicle. The time may come, but it’s not now.