being of individuals and the issue of _backwash_ vs. _spread_ is particularly relevant since access to social services relies on human capital and resources that are often only available in urban centers and to a different degree, depending on their size.

# _Addressing rural–urban disparities in health_

Examining health care service availability and use along the rural-urban continuum is critical to understanding and addressing inequities in health outcomes. Consideration of connectedness and CPT can help policy makers effectively target interventions to the specific health needs of populations.

Populations farther away from urban centers, both in rural areas as well as in small towns and cities, often need to travel long distances for specialized or high-quality services, for example to large national or regional hospitals often located in capital cities, thus incurring substantial time and financial opportunity costs (Kruk, Mbaruku, Rockers, & Galea, 2008). This can result in foregone care and, ultimately, avertible death and disability. Inequities in health care access tend to be particularly stark in low- and middle-income countries, where health facilities in rural areas, and even peri-urban areas and small towns, often lack the basic infrastructure for proper health service provision. Facilities in rural areas and small towns are often equipped and designed to provide only basic services, many endure low stocks of drugs and limited equipment, and are staffed by lower qualified health workers, for example nurses and community health workers rather than doctors. Both absenteeism and unfilled posts tend to be more acute problems outside of large cities, as it is difficult to recruit and retain health staff to work in these areas (Daniels et al., 2007). Demand-side barriers, such as disparities in access to insurance, employment and income that enable households to cover the costs of care also differ across the rural-urban continuum and depend heavily on proximity to large population centers.

Improving access to health care for rural and urban populations, particularly when resources are limited, is essentially an optimization challenge, complicated by spatial contextual factors that preclude a “one size fits all” approach. The contextual factors that affect optimal resource utilization and should inform a research agenda include: (1) location-specific health needs defined using deaths and illnesses reported sub-nationally to national ministries of health; (2) geographic distributions of health care resources, including the number of staff employed, beds, functioning equipment, and other supplies; (3) constraints on the supply and demand for health care resources; and (4) potential indirect benefits associated with different resource allocation scenarios, as for example investing in roads to speed up transportation thereby making existing health care facilities more accessible. Addressing the challenge of reducing inequity in health care access along rural–urban gradients necessitates datadriven approaches that take into account where population in an URCA or FEA is located, while also incorporating local expertise on health care needs. In many cases, rural populations are well connected to an urban center, but access to services and opportunities can vary widely with the size of the urban center.

Recent developments that can shape a research agenda for defining health care priorities include global initiatives like the Global Burden of Disease project, which estimate morbidity and mortality for all causes of death in all countries (Vos et al., 2020); open-source initiatives such as OpenStreetMap provide a rich resource of health care facility information; and detailed information on population distributions and characteristics contained within national censuses and mapped at high-spatial resolutions by initiatives such as the WorldPop project (Tatem, 2017). By incorporating such 22