Wednesday is a big day for Obamacare. Because it's the day that at least 6 million Americans gain health coverage under the Affordable Care Act.

Except, health policy experts don't expect it to feel like a big day, at least not for the country's millions of health-care providers. They don't expect huddled masses showing up at their local emergency department when the clock strikes midnight and their insurance coverage kicks in. Nor do they think most Americans will find their doctors suddenly unavailable, swamped with new patients they need to see -- not on New Year's Day, and maybe not ever.

"People are going to be surprised by how little happens," says Ashish Jha, a Harvard professor who has studied Massachusetts's insurance expansion. "We're all thinking there will be this new flood of people. And there will be some people with pent up demand, but I think there's a lot more slack in the system than we give it credit for."

Researchers know that people do tend to use more health care when they have insurance. That point, proved most famously in the RAND Health Insurance Experiment of the 1970s, is pretty much undisputed.

At the same time, many doubt that these patients, who account for less than 2 percent of the population, will cause sudden capacity issues. For the 6 million people gaining insurance coverage Wednesday, their new plan is certainly a big deal. At the same time, for a health system that sees hundreds of millions of patients already, this increase could amount to a drop in the bucket. (And, one caveat: Some of those 6 million people already have insurance right now, and are renewing their policies under Obamacare, so we're not necessarily taking about 6 million newly-insured Americans.)

Even before the Affordable Care Act kicked in, some patients had difficulty finding a doctor who would accept their coverage. This is especially true with Medicaid, where 30 percent of doctors are not currently accepting new patients. The problem is particularly acute in states that pay Medicaid doctors much lower rates than the Medicare program.

"There is a shortage of primary-care physicians," says Reid Blackwelder, president of the American Association of Family Physicians. "It's a crunch."

Leighton Ku, a researcher at George Washington University who has spent decades studying insurance expansions, remembers when he had to switch health plans a few years ago.

"When I came over to GW, I had to go find a primary-care doctor," he says. "The first five primary-care doctors said they were not taking new patients. And this is something that will happen in the exchanges too, where providers listed in directories will say that. This has been a traditional problem." 

In other words, we're starting from a baseline where patients already face challenges finding doctors. The question about health reform, then, is whether it makes those challenges greater.

More broadly, Ku does expect a lot of confusion -- about which doctors patients can see, and who is covered by what insurance plan -- in the first days and weeks of 2014. That's true of any insurance expansion, he says, and has more to do with a really big change to the health-care system than an actual capacity issue.

"I remember many years ago being in Hawaii for a big Medicaid expansion," Ku says. "I was wandering around their Medicaid agency, and every telephone was ringing constantly because there were so many questions people had. Doctors had questions, insurers had questions, pharmacists had questions. Then it calmed down."

Massachusetts may offer the best proxy for understanding how the insurance expansion will go. Annual surveys by the Massachusetts Medical Society don't show an especially clear pattern on access to care. There has definitely been a big drop in internists accepting patients, from 66 percent in 2005 to 45 percent this year. The wait time to see an internist has bounced around a lot, showing no clear pattern related to the state's insurance expansion.

But at the same time, there's been an increase in ob-gyns taking on new cases (from 71 percent in 2005 up to 84 percent today.)

Jha, the Harvard professor, has explored the Massachusetts expansion from a different angle. His research, published in the journal Health Affairs, looked at Medicare patients' access to primary care. He and his colleagues were trying to see whether the insurance expansion, bringing millions of new patients coverage, made it more difficult for the elderly to access their primary-care doctor.

They found no change; seniors had just as easy of a time getting in to see their doctors after the expansion as they did before. And they didn't experience any declines in their quality of health care, either.

"The big picture is that we really see no impact whatsoever on access to care or quality of care for everyone else," Jha says. "That is certainly the case for Medicare beneficiaries."

Jha thinks that part of the explanation is the health-care system having more capacity than we give it credit for, meaning that doctors have the space to fit more patients into their schedules. "One of the things that's a fallacy is that there's a fixed amount of care the population needs," Jha says. "That turns out not to be true. If doctors have lots of open slots, they'll see patients more. But if you don't see them, they'll probably wait six months and be fine."

There are other factors at play, too. AAFP president Blackwelder points out that the uninsured are already using the health-care system. Surveys of his organization's 110,000 members, all primary-care doctors, show that they see an average of nine uninsured patients each week right now. His expectation isn't that more will turn up at their offices -- but rather that emergency departments will refer the newly-insured to these doctors for preventive care.

"We expect things will pick up over the next little bit as people test out the new insurance system," Blackwelder says. "But a lot of patients aren't waiting to join the system. They're showing up at hospitals. I hope to get patients away from emergency rooms and into our offices."

Any really big policy change is uncertain, so it's hard to know exactly what will happen Wednesday -- that's true now, and it was true back in 1965, when Medicare was launching.

"What will happen then, on that summer day when the federally insured system of paying hospital bills becomes reality?" Nona Brown, a New York Times reporter, wondered in a story published April 23, 1966. "Will there be lines of old folks at hospital doors, with no rooms to put them in, too few doctors and nurses and technicians to care for them?"

What happened was, well, not much. "Medicare takes over easily," was the headline in the Washington Post on July 2, 1966. "It was a smooth transition," Dan Morgan and Martin Weil wrote, describing the local scene as "undramatic."

Whether the past will be prologue will become a lot clearer Wednesday, when the Affordable Care Act's biggest provisions come into effect.