Du`a To Protect Travellers
Question

What is the Du`a that if a person says it when he is travelling, he will come back to his family safe and sound by virtue of this Du`a?
Praise be to Allah.Du`a for Travelling
In the Sunnah, there are several Adhkar which it is recommended for the one who wants to travel to say. These include: 
It was narrated from `Abdullah ibn `Umar (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings of Allah be upon him) mounted his camel to set out on a journey, he would say Takbir three times, then he would say: “Subhan alladhi sakhkhara lana hadha wa ma kunna lahu muqrinin wa inna ila rabbina la munqalibun. Allahumma inna nas’aluka fi safarina hadha Al-birra wat-taqwa wa min Al-`aml ma tarda, Allahumma hawwin `alayna safarana hadha watwi `anna bu`dahu. Allahumma anta as-sahib fis-safar wal-khalifah fil-ahl. Allahumma inni a`udhu bika min wa`tha’ as-safar wa ka`abat Al-mandhar wa su’ Al-munqalib fil-mali wal-ahl (Glory be to the One Who has placed this (transport) at our service and we ourselves would not have been capable of that, and to our Lord is our final destiny. O Allah, we ask You for righteousness and piety in this journey of ours, and we ask You for deeds which please You. O Allah, facilitate our journey and let us cover its distance quickly. O Allah, You are the Companion on the journey and the Successor (the One Who guards them in a person’s absence) over the family. O Allah, I seek refuge with You from the difficulties of travel, from having a change of heart and from being in a bad predicament, and I seek refuge with You from an ill-fated outcome with regard to wealth and family).” And when he returned he would say the same words, with the following addition: “Ayibuna ta’ibuna `abiduna li rabbina hamidun (Returning, repenting, worshipping and praising our Lord).” (Narrated by Muslim, 1342) 
The phrase “wa ma kunna lahu muqrinin (and we ourselves would not have been capable of that)” means: we would not have been able to subdue it and make use of it, had not Allah placed it at our service.    
“The difficulties of travel” mean hardships; “from having a change of heart” means changes caused by grief and the like: “ill-fated outcome” means, upon one’s return. (Sharh An-Nawawi `ala Muslim, 9/111) 
Is There a Specific Du`a for Protection during Travel?
We do not know whether there is any specific Du`a in the Sunnah for protecting a traveller until he returns to his family safe and sound but if the traveller regularly recites the Adhkar for morning and evening, and asks Allah to keep him safe and sound, and says the Du`a for travelling that is quoted above, then we hope that Allah will answer his prayer and protect him and bring him back safely to his family, unless Allah wills in His wisdom to test His slave, in which case nothing can put back the decree of Allah. 
Du`a when leaving the house
What he should remember, when he wants to go out of his house – in order to travel or for any other purpose – and he hopes that Allah will protect him, is the report narrated from Anas ibn Malik (may Allah be pleased with him), according to which the Messenger of Allah (peace and blessings of Allah be upon him) said:  
“Whoever says – i.e., when he leaves his house - Bismillah, tawakkaltu `ala Allah, wa la hawla wa la quwwata illa Billah (In the name of Allah, I put my trust in Allah and there is no power and no strength except with Allah)” , it will be said to him: You are taken care of and you are protected and guided, and the devils will move away from him, and one devil says: What can you do with a man who has been guided, taken care of and protected? (Narrated by Abu Dawud, 5095 and At-Tirmidhi, 3462; classed as authentic by Al-Albani in Sahih Abu Dawud)
It says in `Awn Al-Ma`ubud (13/297): 
“Then it is said” means, an angel called out to him: O slave of Allah. “You have been guided” i.e., to the path of truth , “you have been taken care of” means your worries are taken care of.  
For more details, please see the following answers: 12173 , 21341 ,  204955 , 41734 , 105280 , 171173 .
And Allah knows best.