You know that rule about driving distance, roughly 1 car distance per 10mph?
Its the same for semi's, except a semi is 60ft long. So when you see a semi with a big nice tempting clear spot in front it, he doesnt leave that there just so you have a place to drop on into. Its because at 60mph, it takes almost 400ft to stop.
So when you get the high beams, or the horns from a semi because you squeezed right into that space in front of him, its not a friendly greeting, nor an invite to hit your damn'd brakes either.
It might be because you are about to get an 80,000 pound enema up your ass.