In Jordan, men always sit in the passenger's seat in a taxi :) A taxi driver would assume that a man

is a foreigner if he sat in the back seat, because as I mentioned we are accustomed that a man sit

in the passenger's seat always.

If a woman took a taxi, she would sit in the backseat always, unless as Sarah said; if we are sharing

a taxi with friends and we don't all fit in the backseat, so one of us would sit in the passenger's seat.

I have no idea Lang.Learner! But it must be a custom and people got used to it this way.

Knowing my society from the inside though, I think they believe that a man who sit in the

backseat must be snobbish or something like that, and we are no better than a taxi driver,

so why not sitting next to him in the passenger's seat. In a woman's case, she's supposed

to sit in the backseat because we are still living in a conservative society, and if she sat in

the passenger's seat people would assume that she has a relationship with the driver, or 

that he's a family member.

I agree with Sarah that it's not a snobbery thing.

When I was growing up, in Kuwait, my dad used to tell me off when he'd drive me and I decide to sit in the backseat. He would tell me that he's my dad, not my chauffeur. This one time, he refused to move the car till I moved to the front. Because of that, when I was young, I developed a habit of sitting in the passenger seat out of respect.

However, when I moved to North America (Canada), I noticed that most taxi drivers didn't expect people to sit in the passenger seat. Many even had personal belongings on the seat.

Nowadays, I sit depending on how I'm feeling. If I'm feeling social, I'd probably sit in the passenger seat. If I just want to look out the window or stare at my phone, I'd probably sit in the back.

@Sarah We sit in the passengers seat if there is not enough space for all of the passengers. The senior/important  passenger would sit in the most comfortable seat, the opposite side to the exit door (seat of honor, sorta..) and the passenger seat is considered the least comfortable. I guess Japan is very hierarchical. Among friends, we don't care though. 

@Mumtaz Is there any reason why a man is expected in the passenger seat in your country?

@Андрей So, you mean some women prefer to sit in the passenger seat in your country?