When the Affordable Care Act’s insurance mandate takes effect in 2014, some 30 million newly covered patients—people generally treated in emergency rooms now—will be shopping for doctors. Yet teaching hospitals aren't rushing to fill the void. The federal government foots most of the bill for residency programs—and Congress has capped enrollment at about 85,000 students for the last 15 years. Medical schools are pressing lawmakers to pass legislation introduced in August that would add 3,000 residencies a year through 2017. The cost to taxpayers: about $9 billion.