What were the circumstances of Myrtle Wilson's death?

The important detail concerning the accident that kills Myrtle in The Great Gatsby is that Myrtle is hailing the car, because she thinks Tom, the person she is having the affair with, is driving the car.

Earlier in the day when Tom stops for gas at the Wilson's business he is driving Gatsby's car.  From the room in which she is locked, Myrtle sees Tom and the car. 

When she sees the car coming her way that night, she runs out to flag it down, assuming Tom is inside.  That's how she gets hit. 

Of course, Daisy--Tom's wife, which is convenient--is driving and Gatsby is the passenger.  Tom is not in the car. 

Apparently, Myrtle assumes Tom is driving the car and that he will stop for her.  She is trying to escape "captivity" at the hands of her husband, Wilson.  Daisy, though, an inexperienced driver, runs her over instead, presumably by mistake.