It is very easy to know. You slow down while slightly pumping your brakes, which both warns people behind you and allows you to see just how slick the road is. It shouldn’t look icy, or you already know without pumping the brakes that you have to let off on the accelerator.
For semi drivers, I can’t fathom how stupid they have to be to go full speed into a cloud, knowing they have the greatest death machines on the road with the highest cost.
You don’t know they were going as fast as you stated. And if they’re suddenly caught in it….the best they can do is ease up and let it slow by friction. Trying to brake could cause big swerving which could take out more than just 1 car (not just the front car) in an instant.
The point is if this happens suddenly you can’t blame them for “going at a high rate”, since they didn’t know this was coming.