Shaykh Saalih al-Fawzaan (may Allaah have mercy upon him) said:
“It is not permissible for other than the Mahram (non-marriageable relative) to enter upon the woman, shake her hand, or be secluded alone with her, except that there’s one in the home who is present to where the seclusion will take place. As for him entering upon her in a home where there’s no one there except her, and he is not from her Mahaarim (pl. of Mahram), then this a prohibited seclusion alone with her and it’s dangerous. Or even if he enters upon her in an empty room and there’s no one there except for the two of them, this is also not permissible. This is due to it being a reason for fitnah to take place even if the man who is in the place is a doctor. Just as the Prophet (sallallaahu alayhi wa sallam) said:
“Never does a man seclude himself with a woman alone, except that the Shaytaan is the third of them”
(Musnad of Ahmad and Sunan of At-Tirmidhee)
This means that the Shaytaan is present and he beautifies for the both of them falling into that which is indecent. That is because he calls to Fitnah and seeks to capitalize off of the opportunity to bring about corruption between the two of them. In order to cut off the path to the Shaytaan and his supporters, in bringing about the matter of corruption, the legislator has prohibited the man from being alone with the woman.
From that which is present in our time from the prohibited seclusion is the woman riding alone with the driver who is not from her Mahaarim. He takes her to school, the market place, and even to the Masjid. This is not permissible. It is not permissible for the woman to ride in the car alone with the driver of that car who is not Mahram for her. That is because this is from the prohibited seclusion. This type of seclusion (between the man and the woman) is prohibited whether it is in the home, the car or any other place. This is from that which is obligatory upon the Muslim woman to be aware of, especially in our day and time where there is an abundance of leaving the home by the woman for their occupations, going to the market places, visiting relatives or other than that.”
Source: Naseehah wa Fataawa Khaassah bi Al-Mar’ah Al-Muslimah by Shaykh Saalih Al-Fawzaan prepared by Abu Abdir Rahmaan Aadil bin Alee Al-Furaydaan pg. 18