I think it depends on the context. In some cases it may not matter which one do you use, but I think there is a slight difference between them and you shouldn't use them interchangebly. For example you can say: Útközben találkoztam vele. (I met her on the way.) Here on the way doesn't mean that she was standing on the road, but you met her during you were travelling. But if you say "Az úton találkoztam vele" (I met her on the road.) it suggests primarily that she was physically on the road/street etc.

Another example when it would mean almost the same thing: Útközben találkoztam vele. Az odaúton találkoztam vele. It would mean almost the same. Odaúton means that on my way there, so it was not on the way when I was coming back. Útközben can refer to the way there and also to the way back as well. If you met her on the way back you could say: A visdzaúton találkoztam vele.

That's clear now, köszi szépen :)