The people are not in the taxi, they are in the street. Picture this. The people are walking down the street. They are chatting to each other and as such are distracted and not completely aware of their surroundings. Consequently they are at risk of being hit by a vehicle, such as a taxi, in a busy city like NYC. The final sentence is said sort of ironically. The grad student is so engrossed in what they are talking about that they are at risk of being hit by the taxi (the taxi will hit the people, throwing them over the bonnet and the roof of the car.)

I would have to guess that the writer means that the car will make a very sudden stop, forcing the passengers to be thrown forwards in their seats.