Conventional measures of primary care physician (PCP) shortages rely on fixed geographic boundaries, which ignore key factors such as traveling for care across boundaries, within-area heterogeneity in access, and patient travel costs. We develop a novel framework to overcome these limitations, and estimate that 11.6 million individuals in the U.S. lack sufficient PCP access, substantially lower than the widely cited 30 to 100 million based on government-designated shortage areas. Using Medicare fee-for-service claims, we estimate a gravity model of primary care visit flows between ZCTAs, which allows us to predict the equilibrium total demand that shares PCP resources in each area. We propose a more efficient targeting plan that requires adding no more than 3,800 PCPs to eliminate shortages nationwide. The Medicare Health Professional Shortage Area (HPSA) bonus program spent $150 million in 2019, which could alternatively fund subsidies exceeding $40,000 annually per physician added in our proposed allocation. These results underscore the importance of accounting for network dynamics in shortage assessments and suggest actionable strategies to improve primary care access.