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I always thought it meant the road was part of the highway that can be used for a emergency runway.
As a private pilot, I can assure you that if I have an emergency which requires me to put the airplane down anywhere other than an airport, then I'm not going to waste my time looking for little airplanes painted on the highway before I set it down. I'm going to make the landing anywhere convenient/appropriate (flat, firm, and unobstructed) in the immediate vicinity.
The symbols on the highway are indeed timing markers for airborne troopers. A diamond shaped yellow highway sign with an airplane symbol is a warning that there may be low flying airplanes OR (rarely) that airplanes may taxi across the highway/street at the intersection. The sign will indicate which.
JB
"Long live the Duck Force!"
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