Oil tankers optimize for both fuel consumption and timely (= at the right moment for the spot market) arrival. If prices are low, tankers can get 'parked' for months. If that's the case at one's destination (or rather: possible destinations), there is no need to rush there.

I also think truck drivers already adjust their speed depending on economic reality. If, say, you are transporting fresh flowers towards an airport, arriving in time is more important than when you are transporting furniture to a furniture shop.

That certainly is true at the extreme ends. Drivers who drive to be transplanted organs between hospitals get special training to drive fast, while drivers driving garbage to a landfill likely will not be pressed for a timely delivery (although I do not rule out that someone who knows the latter business will tell me that missing one's reserved spot for delivery at a landfill can cost a lot of money)