When combined, 旅途 (lǚtú) forms the noun “journey,” and 愉快 (yúkuài) forms the adjective “pleasant.” So, “旅途愉快” is a concise wish for a “pleasant journey.”
In Chinese culture, showing care and concern for others, especially during moments of transition like departures, is highly valued. “旅途愉快” is a perfect embodiment of this social norm. While similar to the English “Have a nice trip,” “旅途愉快” carries a slightly more formal and complete feeling due to its balanced, four-character structure, which echoes the elegance of classical idioms (成语, chéngyǔ). However, its usage is extremely broad and not considered stuffy. The key difference lies in its focus. While an American might say “Have fun in Paris!”, focusing on the destination, “旅途愉快” wishes for the entire process of travel to be smooth and pleasant. This reflects a subtle cultural appreciation for the journey itself, not just the destination. It's a wish for safety, comfort, and enjoyment from the moment a person leaves until they arrive.
“旅途愉快” is a versatile and widely used phrase appropriate in almost any situation involving travel.