This study aims to investigate the lithology of rocks along the Cot Iri Darussalam route to identify the causes of road instability to reduce risks and ensure public safety. The research employed geoelectric resistivity methods, using an ARES resistivity meter for 1D soundings and Geotitis for 2D profiling with a Wenner–Schlumberger configuration. Data were processed with Res2Dinv for 2D inversion and Ipi2win for 1D modeling to derive subsurface resistivity distributions. Results show that Line L1 has resistivity values ranging from 0 to 508.36 Ωm and Line L2 from 0 to 173.56 Ωm, while the 1D soundings (SP1–SP5) range between 1 and 299 Ωm. The lithology consists of wet clay, clay, sandy clay, and sand, with clay-rich layers dominating and interpreted as the main weak zones due to their impermeable and water-saturated nature. These layers, combined with high rainfall, poor drainage, uneven traffic loading, and micro-vibrations from vehicles, are considered the primary causes of road instability in the study area. The study is limited by the number of profiles and the absence of borehole validation, which restricts the precision of lithological correlation, but it highlights the necessity of integrating geophysical and geotechnical approaches for more accurate interpretation. The practical value of this research lies in its contribution to road maintenance and rehabilitation planning in alluvial environments, offering a scientific basis for identifying subsurface weak zones before infrastructure construction. The originality and significance of this work stem from applying resistivity methods to characterize lithological variability and weak layers in an alluvial setting, thereby filling a knowledge gap in understanding road instability mechanisms and providing insights for more resilient transportation infrastructure development in Indonesia.