She was probably just cautious first time when you stopped and s
thats OK, you could`v been some serial murderor
She is smiling when she sees you? Great, she`s open minded and polite person. Next time stop, say something like: "I won`t offer you a drive again, cause you obviously are thinking who is this guy, but let`s have a coffe some day and we`ll have a chance to get to know each other. Then maybe later you can save some of your taxi money!"
Say it with a smile and correct body language and you are in the game my friend!
Marco