If children learn that strangers are all people who they don't know, then they have no one to turn to if they're lost or need help. Beyond police officers, children should know that "safe strangers" are also security guards, firefighters and teachers. To teach her the difference, show your daughter these safe people when you're at stores and in the community. Allow her to interact with them.
Warn your child to never enter a stranger's vehicle without a parent's permission. A good way to control this is to provide your child with a password. If the adult doesn't know the password, then he knows not to enter the vehicle. Keep in mind that some child abductors and abusers are someone he may very well know and trust. Don't make the mistake of teaching him to trust easily or take gifts from strangers.
Teach children to make a scene if a stranger makes them feel uncomfortable. Let her know that it's OK to scream, "He's kidnapping me!" or "Call the police!" She should know to run toward people and run toward the back of a car, because driving in reverse can be much more difficult. Most importantly, she should know to keep screaming the entire time. This may scare your daughter, but it's important to teach her this strategy that can deter an abductor. Practice abduction scenarios with your child until you feel secure that she'd be able to do it in case an abduction attempt occurs.