Yes, there are many US citizens who have cars, but there are many US citizens (like those who are children, those who can't afford cars, etc.) who do not have cars. There are over 250 million registered motor vehicles in the US.
In 2009, there were 246,283,000 licensed drivers and 254,212,610 registered cars and trucks in the US. According to the US Department of Transportation, 92% if US households have a vehicle. The average US household had 1.8 drivers and 1.9 vehicles.