Police officers who stop drivers for traffic violations will always be in a marked police car and will be in uniform. Although it is rare, some criminals steal cars and valuables by bumping a target car. When the driver gets out to assess the damage and exchange information, the thief and his companions rob the victim. If you are bumped by another car, pull into a well-lit area with other people around before getting out. If you are uncomfortable with the situation, call 911 on your mobile phone.
Carjacking tends to be a crime of opportunity.
Don't give criminals a chance. Keep your car doors locked and don't drive in bad neighborhoods late at night. Check your maps before setting out. Carjackers look for cars which slow down or stop to ask directions. It is best to park in well-lit, heavily traveled areas.