What is the ruling on a woman traveling alone without a mahram from one town to another without staying overnight, and is it permissible out of necessity if it is impossible for the mahram to accompany her?

A woman should not travel from one country to another unless accompanied by a mahram, due to the hadith: "No woman should travel except with a dhī mahram." However, if the travel is necessary and permissible by Islamic law, and she cannot find a mahram, many scholars have permitted her to travel with trustworthy company.