I just finished writing a paper on Air Cargo Security and as a result I've been doing a lot of research on the topic. Although I didn't include this in the paper, I remember a story that was told to me by an employee at Air Canada Cargo a number of years ago.
They received a call from a well to do woman who was wondering if they would be able to ship her sailboat's sail from Toronto to Florida. Altough the sail would be rolled up, it was still quite long. The cargo employees checked out the specs for the different planes, at the time, the largest that Air Canada flew to Florida was the 767. They tried different angles, but this sail just wasn't going to fit in the cargo hold.
The cargo employees called her back to tell her the bad news. They couldn't take the sail. Now this wasn't quite good enough for this woman, she paused for a moment and then with an air of authority asked "Well, why can't you just use a roof rack?"
Now in her defence, I suppose it makes sense from her point of view, they use a roof rack for the space shuttle, why not for cargo?