[scrumper]

It's a bigger issue than one of legality. The high seat in a standard truck cab offsets two of the biggest challenges with driving a truck: it takes ages to get up to speed, and a very long distance to stop. High visibility gives better sight lines for further so the driver can plan. That helps to improve fuel efficiency by better anticipating traffic and planning acceleration accordingly. It's absolutely vital for safety because the driver needs to brake for what's happening a huge distance up the road. If you don't have that line of sight, they'd have to drive so defensively ("stop in the distance you can see to be clear") that any efficiency gains from the aero would be completely swallowed by all the accelerating and braking.

A sports car of course can stop in a heartbeat and the excess power means it can easily manage its flow in traffic. They'd perhaps not even be legal if they took as much road to stop as a loaded truck. It's four times the distance - 150ft for a decent sports car at 70mph; 600+ for a semi.