The poster child for unsustainable urban growth in recent decades has been none other than Atlanta, Georgia, my home city. You either love living in Atlanta or absolutely loathe it based on these simple facts: 1.) whether you live inside or outside of the perimeter (I-285), 2.) whether you have access to MARTA, Atlanta's sorry excuse for a public transportation system (but nevertheless a transportation system; never mind if you live in Cobb County) and 3.) whether or not you actually have some extra income to spend on the fine amenities that Atlanta has to offer. If not, still be prepared to spend any extra cash on gas.
Since 1959, Atlanta's population in the metropolitan area has rose from about 1 million to a whopping 5.36 million, although the inner city itself only has about 432,000 inhabitants. That means about 4.9 million people reside within Atlanta's over-stretched suburban areas, ranging from Pickens county up at the north to Lamar county in the south, which is closer to Macon than to my house in Cobb.

(Image of Metro Atlanta)
So, with so many inhabitants spread out among a conglomeration of counties, with a large portion of them driving to downtown Atlanta every day for work, with only a few main interstate roads connecting the main areas, Atlanta has indeed created a disastrous traffic problem. In fact, people in Atlanta spend more time commuting in cars than in any other city (even L.A. or New York City!). At least New York City has developed a sophisticated public transit system and has made the city perfect for walking. It's almost as if suburban Atlanta was designed to prevent people from walking from one area to another; roads are incredibly wide, and sidewalks can end suddenly without warning and are so scarce in general that almost all of us are forced to drive everywhere, even if the location is only half a mile away.
And with heavy traffic and long commutes comes severe environmental concerns: air pollution, storm runoff polluting rivers, psychological stress, acid rain, increased incidence of asthma and asthmatic attacks, etc. There have been some mediocre attempts to lessen Atlanta's traffic problem; one example is the HOV lanes on I-75. Most cars in Atlanta, however, only have one occupant anyway, and cannot use the HOV lanes. Suburbs are especially guilty of promoting the trend of single-occupancy driving; as parents are often at work (working at different locations, so they need their own cars), and teenagers desire their own independence, families who have more than one child may end up with five different cars in the driveway. This puts an unnecessary amount of cars on the road, which then leads to more traffic. I don't know if Atlanta still continues their radio campaign for carpooling (played during hours of heavy traffic), but I doubt it was very successful.
To sum things up, Atlanta has handled its population explosion in the worst possible way. Its negligence to proactively invest in public infrastructure, including efficient transit systems, during its previous years of rapid growth have complicated any attempts to install them now. Years of unchecked urban growth have led to the sprawl and over-extention of cookie-cutter neighborhoods that are built further and further away from the inner city every year, even though downtown Atlanta or other large edge-cities (ex: Marietta) still remain central business hubs. All of this has cumulated into a traffic nightmare from which Atlanta seems to be unable to wake up.