Conventional street-lighting systems commonly operate on fixed schedules and remain at full power throughout the night, resulting in unnecessary energy consumption. Although sensor-activated streetlights can adjust their output in response to pedestrians or vehicles, most operate as independent luminaires and therefore do not illuminate the road ahead in advance. This study proposes a segment-synchronous smart street-lighting control system in which streetlights are divided into multiple segments and all luminaires within a segment operate synchronously. When no pedestrian or vehicle is detected, the streetlights remain in dim-light mode. Once a road user is detected, Wi-Fi communication switches the entire segment synchronously to strong-light mode; after the road user leaves and the hold interval expires, the luminaires return to dim-light mode. The system integrates Internet of Things (IoT) technology with a cloud platform for parameter configuration and remote monitoring. For a nominal 100 W streetlight, the model-based energy-saving rate reaches 90% under no-traffic conditions and is approximately 81.00% under the deterministic 12-trigger/h case and 81.44% under a homogeneous Poisson vehicle-arrival model with a mean rate of 12 vehicles/h. In the four-luminaire laboratory prototype, the measured synchronization latency was 120 ms and 199 of 200 trigger tests achieved segment-wide synchronization. These analytical and prototype-scale results support the feasibility of the segment-synchronous concept for low-traffic-road applications; larger multi-segment field validation remains necessary.