When is it obligatory to recite the traveling supplication? And at what distance does its recitation become obligatory?

The supplication for travel is not obligatory; rather, it is sunnah to recite it at the beginning of a journey or immediately upon boarding, regardless of the distance. The Prophet (peace be upon him) used to say, when he mounted his camel, "Allahu Akbar" three times, then he would say: "Glory be to Him Who has subjected this to us, and we were not ourselves able to subdue it. And verily, to Our Lord we are returning. O Allah, we ask You in this journey of ours for righteousness and piety, and for deeds that please You. O Allah, lighten this journey for us and make its distance easy for us. O Allah, You are the Companion in the journey and the Successor over the family. O Allah, I seek refuge in You from the hardships of travel, the sorrows of sights, and the evil return concerning wealth and family." And upon returning, one adds: "Returning, repenting, worshipping, and praising our Lord." The supplication for riding is considered desirable by some only for travel, while others consider it desirable for any kind of riding.