I will try to provide the sunni view that a woman is allowed to travel alone. (Of course there is a sunni view that it isn't allowed, but here i will focus on that it is allowed.)
A woman is allowed to travel alone
A woman is free to travel alone if it generally is expected to be safe.
As other stated, we should follow Sharia. But in the end, the interpretation of what true Sharia is, and who is following the "true" sharia and who isn't will most likely be debated until the day of judgment.
The ones who interpret that a woman can travel alone, interprets the prohibition to be based on reason, where the reason is the womens security and safety. I don't know anybody who doesn't say that the prohibition of a woman to travel alone isn't based on security. Therefore, scholars draw the conclusion, that if a woman has nothing to fear, there is no reason left to apply the prohibition which was based on a situation, which now is changed (i.e the safety).
Here is a quote from a fatwa by dar-alifta.org: "Can I travel alone with no mahram?":
It is permissible for a woman to travel without a mahram provided the way, destination and return journey are safe and provided she does not meet with any harassment jeopardizing both her safety and religion. It has been narrated through 'Adiy Ibn Hatem (may Allah be pleased with him) that the Prophet told him, "And if you live a long life, you will surely see women traveling from Hira till they circumambulate the Ka'ba, fearing no one except Allah"
[Bukhari and others].
Imam Ahmed's report of the hadith includes: "By He in whose hands is my soul, verily Allah will bring this matter [Islam] into completion till women travel from Hira and circumambulate the Ka'ba without being accompanied by anyone."
From this hadith with its different chains of narration, some scholars have derived the permissibility of a woman traveling alone if her safety is guaranteed. Moreover, they have used this hadith to restrict other prohibiting hadiths which, according to them, referred to the lack of security associated with travel in the distant past.
The majority of scholars have permitted a woman to travel for obligatory hajj without a mahram if accompanied by trustworthy females or company. They based their opinion on the precedence of the Mothers of Believers (may Allah be pleased with them) who went on hajj after the Prophet's death and during the caliphate of 'Umar (may Allah be pleased with him) accompanied by 'Uthman ibn 'Affan (may Allah be pleased with him)
The conclusion of the fatwa:
The hadiths forbidding a woman to travel without a mahram pertain to lack of security which was the case in previous times. Based on this, if a woman's safety is ensured, the prohibition is lifted.
According to this, a woman is free to travel alone as she is safe and no harm is expected.
Shortly about the other opinion
It is known that there is another opinion that it is forbidden for a woman to travel alone. I also like to point out that most of the scholars (of today), that follow the other opinion, does not mention the reason why it is forbidden, instead they try to describe it to be an absolute rule in Sharia given by Allah, that we have to follow, without questioning it, in order to be faithful to the command of God. Never do they mention that Sharia is a matter of different of opinions and ijtihad. This is a matter of ikhtilaf (difference of opinion), and we should respect each others opinions without looking down on each other. Both opinions are from the Quran and Sunnah!
Somehow off topic; about the authority of dar-alifta.org:
Dar al-Ifta al Misriyyah is among the pillars of the religious foundations in Egypt which include Al-Azhar Al-Sharif, Al-Azhar University, Ministry of Religious Endowments, and Dar al-Ifta al-Misryyah.It plays a significant role in giving rulings to the masses and consultation for the judiciary in Egypt.