The lead vehicle
benefits from being drafted.
"In car drafting, the lead car is also getting a benefit. Trailing cars fill in the lead car's low-pressure wake, thereby cutting down pressure drag."
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The reason close drafting is dangerous by bicycle is because if you bump, the front wheel is the first thing to touch, and with no steering damper, the wheel makes a sudden turn you weren't expecting. Same for a motorcycle, but since a car has a bumper, that's really not a fair comparison.
Considering 1) there is a minimal speed differential in drafting (and its is differential between vehicles, not absolute speed, that determines impact force), 2) a semi has about 4 times the stopping distance of a car, 3)they tend to drive more steady than your average car driver, and 4) Mythbusters showed there is a benefit as far back as 100ft;
it really doesn't make sense to make a blanket statement that "drafting is dangerous".
Just like with some people claiming it is dangerous to drive slow on the highway (when in fact the opposite is true), danger isn't just about the risk of an accident, but the product of the risk of accident AND how severe that accident would be if it occurred.
However, I never draft anymore. All the semi's around here speed, all of the time. They are supposedly limited to 10mph under the limit for everyone else, but speed limits are kind of a joke.