‘Abdullāh ibn ‘Amr Ibn al-‘Ās (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: “The lonely rider is a devil, two riders are two devils and three are a traveling team.”
This Hadīth discourages traveling alone, or even that two people travel alone, in empty places where no people pass by. A person alone may need a companion in case he needed help or if died the devil may not target him. If there are only two people and one gets hurt then he will be left alone. The Hadīth encourages traveling in a group so that the devil would not target the travelers with his schemes. Nowadays with fast cars and highways a person traveling in the car by himself would not be considered alone because there are many other people traveling on the highway, whereas if he was away from main roads it could be considered alone.