I can tell you why from experience.

I am a retired, high-time helicopter pilot that from 1986 to 1990 was lead pilot for Life Flight in Boise, Idaho. One of my callouts was about 20 minutes away, east of Boise at around 1:00 AM. The report was a single car rollover (pretty common) but the site was on a narrow, winding highway a few thousand feet higher than Boise.

When I arrived with our nurse and paramedic, we were directed by the state police to land on the highway approximately 150 feet away from the rollover. I called the state policeman over and told him that I thought staying on the road was a bad idea and there was a suitable landing area very near that I could move to.

He told me not to worry as the highway was closed off in both directions by state police. Well….., about 15 minutes later, while the team was working to save the driver of the accident car, a semi with trailer came screaming by after running through the roadblock.

The only thing that saved the helicopter, and me, was I had parked the helicopter on the eastbound lane and the semi was in the westbound lane. I had been careful to align the main rotor blades fore and after so that they did not extend over the westbound land.