In English, one can say "open road," which connotes freedom, lack of obstruction, and things like that.

You should go on a road-trip to feel the freedom of the open road.

One day he quit his job and took to the open road.

There´s no independence like that of the open road.

I´ve heard Turks say "yolun açık olsun," wishing someone well, kind of like "good luck," or "I hope it works out for you."

But what about the simple phrase "açık yolu"? How would this be understood in Turkish?

*açık yol.

"açık" is adjective, "yol" is noun. All them aren´t nouns, just one of them is noun.  I think you thought it as a possessive determiner and thence you add -u as possessive suffix.

yol+n(possessive)--------------------->yolun (your road)

ol+sin(imperative)--------------------->olsun (let it be)

Yolun açık olsun. (Good luck./I wish successess for you. (direct: Let your road be open.))

Your question... For example:

açık yol (an open road)

yol+i(accusative)---------------------->yolu (the road (acc.))

açık yolu (the open road (acc.))

Açık yolu kapattılar. (They closed the open road.)

(The road was open, then it was closed by them. (explanatory translative))

Thank you! I understand perfectly.