Sea-level rise poses a well-documented threat to coastal infrastructure, yet its most immediate impact, the upward displacement of shallow groundwater, remains largely absent from transportation planning frameworks. This study examines how sea-level rise-induced groundwater inundation threatens the transportation network of San Mateo County, California, one of the most vulnerable coastal counties in the United States. Using four sea-level rise scenarios drawn from the San Francisco Bay Conservation and Development Commission's Adapting to Rising Tides program, the study applies spatial overlay analysis, an Origin-Destination Cost Matrix, and service area analysis to assess impacts on roads, BART and Caltrain stations, and San Francisco International Airport. Results show that groundwater emergence progressively severs road connectivity, reduces transit catchment areas, and compromises airport accessibility well before permanent surface inundation occurs. The case-study OD Matrix analysis shows that one selected socially vulnerable census tract could experience reduced or blocked access to hospitals, schools, emergency services, food, and regional transit under higher groundwater scenarios. The findings demonstrate that subsurface flooding represents an urgent and underestimated equity and infrastructure crisis requiring immediate integration into regional coastal adaptation planning.