An interesting graph from Greater Greater Washington showing the correlation between density and transit use:
Atlanta isn't on the graph, but I think it would fit in as a pretty normal city. The metro area has a population density of just 671 people per square mile, while the City itself has a density of 3,162 people per square mile. The 3.7% of metro residents who take public transit falls right in line.*
Interestingly, 15% of City of Atlanta residents take public transit, according to 2000 Census data. That would seem to be an outlier, although if this graph is based on Metro areas some of the inner cities on it could exhibit a similar pattern of high use in the city and very low use in the suburbs and exurbs.
The take-away for me is that we can blame MARTA management, lack of service, race relations, or elected officials, but at the end of the day the name of the game is density. Obviously increased density for the region in the level needed to get transit ridership up is out of the question, so getting density up for the city should be the goal.
Short term, the real estate market and the economy will prevent a certain amount of this, but the long term growth numbers for Atlanta are still good. Generally speaking, the city has recognized the need for increased density, so I'm really advocating for something that is already occurring. It's just more data supporting the premise.
*Combined Census data for the city and metro area here.