"Buen viaje" is the correct way of saying "good trip" and the phrase alone can be used to say "have a good trip". "Buen" is used when the noun can be described by "bueno" (i.e. un viaje bueno), but when you place the adjective before the noun you use "buen". Un hombre bueno --> Un buen hombre.
"Bien viaje" would be gramatically incorrect. You usually hear bien before verbs like "bien hecho", "bien dicho", when describing a verb, not a noun.
That's how I see it at least!
"Bien" is an adverb, so can't go with "viaje" that is a noun.
It has to be "buen viaje".
"Bueno/a" is the adjective, before the noun loses the a/o that indicates gender.
It is an expression, it can't be "viaje bueno".
But we can say
Thanks everyone! I guess "bien viaje" is a cultural expression... I didn't think it was right, but I hear it all the time in real life. I'm from Georgia, so I guess it's like us saying, "Hey, y'all!" all the time. :o)
I'm from Spain and I have never heard the expression "bien viaje", as I said it is grammatically incorrect.
Yeah, I too doubt that any native Spanish speaker is saying "Bien Viaje."
Probably, they are saying "Buen Viaje"..
but that can sound similar to "Bien Viaje"..
Next time you hear it.. listen closely.. and I'll bet you'll realize they're saying "Buen".. not "Bien"