Geogrids have been widely used to reinforce and strengthen paved and unpaved roads constructed on soft subgrade. They may enhance the performance and design life of flexible pavement systems. In general, geogrids reinforce subgrade soil and subbase/base layers. Although significant testing has been conducted in the laboratories and several theoretical models have been developed, the effectiveness of geogrid in low and moderate volume flexible pavement roads has neither been adequately proven or validated. One of the reliable methods to quantify the effectiveness of geogrids in flexible pavements is by testing full scale pavement sections and measuring the pavement response to loading utilizing pavement instrumentation. In this study, nine low-volume flexible pavement sections were constructed over a weak subgrade having a California Bearing Ratio (CBR) of 4% in order to measure the response to accelerated loading. The sections were built at University of Illinois' Advanced Transportation Research and Engineering Laboratories and were instrumented with more than 170 sensors to measure stress, strain, and deflection due to axle loading as well as temperature and moisture content at various depths within the pavement. Response testing was conducted at three wheel loads (26, 35, and 44 kN) and two speeds (8 and 16 km/h). Traffic loading at 44 kN and 8 km/h was applied until the pavement sections failed. From the responses of the instrumented test sections, the effectiveness of using geogrid for flexible pavement reinforcements was evident. Most importantly, instrument response showed that the control sections had significantly greater lateral deformation in the aggregate base layer; especially in the direction of traffic, compared to reinforced sections. In addition, this study showed that for base layers greater than 300-mm-thick, the optimum location of the geogrid would be at the upper third for the granular layer, given no construction constrains.