Can I do voluntary fasts while traveling?

The choice is yours

A Companion called Hamzah ibn Amr al-Aslami used to fast a lot and asked the Prophet ﷺ whether he should fast while traveling. He replied, "Fast if you wish, and do not fast if you wish" (Sahih al-Bukhari 1943). The Prophet ﷺ himself sometimes fasted on journeys. Abu ad-Darda said they once traveled on a very hot day, and nobody was fasting except the Prophet ﷺ and Abdullah ibn Rawahah (Sahih al-Bukhari 1945).

So fasting on a trip is allowed, whether it's Ramadan or a voluntary fast. Voluntary fasts are even more relaxed, because you don't have to make them up. See What are the voluntary fasts in Islam?

When it's better not to fast

The same Prophet ﷺ who fasted on journeys also saw a man fainting from the heat, with people shading him, and said, "It is not righteousness to fast while traveling" (Sahih al-Bukhari 1946). Scholars explain that this is about someone for whom the fast has become a real hardship. Islam's rule for travel is ease:

Long-haul flights, dehydration on a plane, driving for hours, or a packed sightseeing day in summer are all good reasons to skip a sunnah fast. You lose nothing by leaving it.

Practical points for fasting on the road

• Intention: for a voluntary fast, most scholars allow you to make the intention in the morning, as long as you haven't eaten anything since Fajr.
• Breaking it: you may break a voluntary fast at any time, and the majority say no make-up is needed (the Malikis and Hanafis prefer one if you broke it without a reason).
• Time zones: you break your fast at sunset wherever you are at that moment, even if that day ends up longer or shorter than usual. See Is it okay to fly while fasting?
• Mondays and Thursdays: if you have a regular habit, keeping it on an easy trip is lovely, and dropping it on a hard trip is fine. See Why do Muslims fast on Mondays and Thursdays?

What about make-up fasts?

If you owe days from Ramadan, you may also make them up while traveling. Many people find a quiet travel day, like a train ride, is actually a good day to fast. Just remember that a voluntary fast should never get in the way of the obligatory prayers on your journey.