In Morocco, we have buses in which men and women mix when they sit next to each other on seats, likewise, when it is crowded [mixing occurs] when people are descending from it. So is it permissible to board these buses?

Shaykh Zayd al-Madkhalī: 

If the Muslims find an alternative – especially the students of knowledge, if they find alternative means [of transportation], then it is better for them, since men are weak with regards to looking at women. However, if you cannot find an alternative and you are compelled to board [these buses], then there is no sin upon you to board them, even if there are women passengers. Nonetheless, you are obligated to lower your gaze and to not sit directly next to a woman. And he is also obligated to provide advice as much as necessary.