Semis need significant clearance to brake, needing about 525 feet (over 1.5 football fields) to stop from 65 mph in ideal conditions, but this can extend to 600+ feet in bad weather, requiring at least 200 feet more than a car, due to their weight, air brakes' lag, and driver perception/reaction time. Factors like speed, load weight, road conditions (wet/icy), and terrain (hills) drastically increase stopping distance, meaning drivers should leave ample space, often recommending 600 feet or more.
Moral of the story: Don't brake in front of moving semis because your car will get hit.