Its hard to argue against the logic that people travelling slower would reduce injury and harm. It's sure annoying to crawl along slowly, but it's also pretty hard to hurt yourself doing it.
Well, there's a limit, right? The point of a highway is to move people from point A to point B as quickly and safely as possible. In other words, we need to optimize for both. If we optimize only for safety we'd crawl at 5 mph.