Wendell Cox at New Geography takes a look at the Brookings data in Transit: The 4 Percent Solution
He says:

Brookings did not examine a 30 minute transit work trip time. However, a bit of triangulation (Note 1) suggests that the 30 minute access figure would be in the range of 3 to 4 percent, at most about 4,000,000 jobs out of the more than 100 million in these metropolitan areas. At least 96 percent of jobs in the largest metropolitan areas would be inaccessible by transit in 30 minutes for the average resident.

He concludes:

More money cannot significantly increase transit access to jobs. Since 1980, transit spending (inflation adjusted) has risen five times as fast as transit ridership. A modest goal of doubling 30 minute job access to between 6 and 8 percent would require much more than double the $50 billion being spent on transit today.
Moreover, there is no point to pretending that traffic will get so bad that people will abandon their cars for transit (they haven’t anywhere) or that high gas prices will force people to switch to transit. No one switches to transit for trips to places transit doesn’t go or where it takes too long.
Nonetheless, transit performs an important niche role for commuters to some of the nation’s largest downtown areas, such in New York, Chicago, Boston, San Francisco, and Philadelphia. Approximately half or more of commuters to these downtowns travel there by transit and they account for nearly 40 percent of all transit commuters in the 50 largest urban areas.
Yet for 90 percent of employment outside downtown areas, transit is generally not the answer, and it cannot be made to be for any conceivable amount of money. If it were otherwise, comprehensive visions would already have been advanced to make transit competitive with cars across most of, not just a small part of metropolitan areas.
All of this is particularly important in light of the connection between economic growth and minimizing the time required to travel to jobs throughout the metropolitan area.
The new transit job access is important information for a Congress, elected officials, and a political system seeking ways out of an unprecedented fiscal crisis.
A four percent solution may solve 4 percent of the problem, but is incapable of solving the much larger 96 percent.