She left the biggest sign she could use. Her car. So much could have been done to get rescued.
That aside, her "HELP" would be difficult, at best, to see from the air. Contrast, contrast, contrast!
Notice the exclamation mark? It actually says HELP!
And, if you must leave your car, leave a note and make an arrow saying which direction you are heading and why.
Could have saved her 5-6 days in the wilderness. From the article -
Rodgers went missing March 31 as she headed to visit her grandkids in Phoenix. She got lost, and her hybrid vehicle ran out of gas and electric power, authorities said. Her car was discovered three days after a search began, but rescue crews struggled to find her.