Geogrids are widely used for unbound aggregate base stabilization in flexible pavements. A new field evaluation methodology for quantifying the effects of geogrids on stiffness enhancement of unbound aggregate layers is demonstrated in this paper. The method utilizes a bender element field sensor, which measures the shear wave velocity transmitted between a source and receiver pair of bender element (BE) transducers and relates that to the localized stiffness. The paper introduces the new methodology and discusses the recent measurements obtained from a geogrid–aggregate system stiffened zone in the full-scale Construction Cycle 9 (CC9) pavement test sections constructed at the Federal Aviation Administration’s (FAA’s) National Airport Pavement Test Facility (NAPTF) in New Jersey. The aggregate materials used in the construction of the base and subbase layers were in accordance with the FAA-designated P-209 and P-154 specifications, respectively. A biaxial geogrid was placed at the interface between the subbase and base in the “North” test section. Two BE field sensors were installed in this “North” geogrid-stabilized section at 25 mm (1-in.) and 102 mm (4 in.) above the geogrid location. A third sensor was installed in the “South” control section with no geogrid at 25 mm (1 in.) above the bottom of the base layer. Shear wave signals were collected before and after base layer compaction, and shear wave velocities were calculated. The BE field sensor measurements clearly indicated that stiffness enhancement was achieved in the proximity of the geogrid, and that such effect of a stiffness enhancement diminished at about 102 mm (4 in.) away from the geogrid. The findings of this study are illustrated in this paper to quantify the magnitude and extent of the local stiffened zone near a geogrid.