If you are driving when you see bad driving behavior, pull over, and bring your car to a full stop. Remain calm, and do not take the bad driver's actions personally.
Write down the license plate number. Make a note of the color, make and model of the car and any other distinguishing characteristics. Write down the description of the offending driver.
Take out your cell phone, and dial 9-1-1. Inform the desk agent of the incident and bad driving observed. Answer all questions the agent asks. Give the agent the address or street that the driving is taking place and the direction the bad driver is going. If you do not have a cell phone, look around to locate any available police officers or patrol cars. Flag them down, and provide them with the notated information.
Avoid confrontation with the bad driver. Pull over or walk away, and rely on police to find the bad driver.