Mr. Meehan has summarized it very well, and I concur with everything that he has said.
And, as Mr. Meehan implied, if your friend is worried about running out of gas while stuck in traffic, she is allowing her gas tank to get too low prior to her urban expedition. I believe that the old expression was “plan ahead” or something like that.
Simply having a sufficient amount of gas in the tank beforehand can prevent her from having these worries and will prevent the problems inherent in hauling a couple of gallons of gas around for months or years at a time.