Millions of U.S. adults went without necessary health care services in the last 12 months due to a lack of transportation.

According to a new study sponsored by the Robert Wood Johnson Foundation, 21% of U.S. adults who do not have access to a vehicle or public transportation went without appropriate health care.

While 91% of adults have access to a vehicle, a significant number of at-risk individuals do not, and public transportation is not meeting their needs. In total, 5% of all non-elderly adults in the U.S. reported missing health care appointments or declined to seek urgent care due to a lack of transportation.

According to the American Hospital Association, 3.6 million Americans are prevented from receiving appropriate medical care due to transportation issues every year. Some researchers suggest that the number could be as high as 5.8 million.

While public transportation is helpful in getting patients to regular doctor visits and checkups, it is far less effective in getting sick or injured patients to the clinic or to rehab appointments. If a patient does not have a car and lives a half a mile from the bus stop, they are unlikely to make the trip due to sickness or pain.

Individuals who receive Medicare or Medicaid, as well as low-income families and adults with a disability, are two-to-three times more likely to skip out on needed health care visits due to lack of transportation.

Last year, the University of North Carolina Health system discovered through a survey of 323 adults on Medicare or Medicaid that one in three experienced transportation barriers to health care.

In short, it is not enough to have high quality health care services and a panel of government programs to help pay for those services if the patient cannot get to the provider.

In today’s health care marketplace, successful providers are becoming more and more centralized. Every major U.S. city has clusters of large hospitals surrounded by clinics of every major medical discipline.

Modern medicine has moved away from the idea of the community-based hospital. Now, it is commonplace for patients from rural areas to be forced to travel an hour or more to a larger city to meet with a medical specialist.

The same phenomena plagues big cities when public transportation is not available for sick individuals from at-risk neighborhoods to get to a hospital or clinic. Even when public transportation is available, a lengthy bus or train ride might be required. In addition, the nature of the person’s health need might expose others to illness as a result of using public transportation.

Unfortunately, people facing these challenges often will put off seeking care until they are forced to call for an ambulance. This causes a longer and more expensive visit to the hospital.

In like manner, some put off getting needed physical therapy which could assist them in returning to their place of work within a short period of time, but the subway station or bus stop is too far, so their recovery takes longer.

Transportation even impacts how patients pick up prescription refills, causing many to go without because the neighborhood lacks a pharmacy. All of this places patients at risk and increases everyone’s overall health care costs.

While genetics has a significant impact upon one’s health, it may not be the biggest factor. Research suggests that 20% of a person’s overall health is attributed to clinical care, 30% is linked to behavioral factors like diet and exercise, and 10% is associated with physical environment.

Surprisingly, the remaining 40% is directly connected to socioeconomic factors like safety, education, employment, income and, yes, transportation. These become what the World Health Organization calls the social determinates of health, or SDH.

Transportation has become an increasingly significant SDH. How one gets to the grocery store, school or their employer is linked to access to transportation. Just because a city has a bus or subway system does not mean that all its citizens can adequately access these services.

Both political parties have championed the idea that basic health care is a fundamental human right. While they disagree about what that means and the extent of the right, Democrats and Republicans alike rarely address the issue of access.

We like to create programs but forget to think about how people will access them. Therefore, we need to change how we view both health care and health care delivery.

Throwing more money at a problem typically does not improve outcomes, so what we need is to change how we define the scope of health care. It is about more than what happens at the clinic.

Addressing health care inequities must be multifaceted, factoring in where the patient lives and their access to transportation. Only then can we ensure that everyone can access basic services.

Public health professionals need to generate creative solutions via transportation programs or special clinics that target at-risk neighborhoods. Houses of worship and civic groups need to look at their neighborhoods and consider how they can help individuals who lack transportation get to the doctor’s office or the pharmacy to pick up refills.

One reason our hospitals are so crowded is because they are filled with many patients who could have gone to the doctor’s office before their condition worsened, but they didn’t have a way to get there.

Part of the solution might be simpler than we think: returning to a sense of community and developing a desire to care for our neighbors by giving them a ride.