To the millions of Americans without health insurance, preventive care is a luxury, not a necessity. In addition to costs, long hours spent working and taking care of children leave little time to visit a doctor’s office.

But health-care organizations want to come to the rescue by taking their services on the road, and delivering them to people who need them most.

More than two-thousand mobile clinics are in operation in the United States.

The mobile units typically visit low-income neighborhoods, where access to health care may be limited. Most are outfitted with the equipment needed for general medical services, although some offer specialized treatment such as dentistry or mammography. They are often run by nonprofit organizations and staffed by health-care educators or medical students, which helps control costs.

Advocates say the vans help offset the rising cost of health care by treating people with chronic diseases, such as diabetes, who would otherwise seek help at an emergency room. Many mobile units also provide free checks of blood, blood sugar and cholesterol, which help promote better health.

One organization that operates out of Boston estimates every dollar it spends saves the country thirty-six dollars in nonemergency visits to hospital emergency rooms.

The vans may become even more popular. With health-care reform, more people will get access to insurance. But without a corresponding rise in the number of doctors, it may be hard to get treated. Advocates hope the mobile units will help offset that shortage.

Of course, the units will never fully replace the need for the doctor’s office. But advocates say anything that rolls back the cost of health care while rolling around town is a good thing.