Japan is currently grappling with a shortage and aging workforce of truck drivers, alongside the pressing need for improved fuel efficiency. In response, the Ministry of Economy, Trade and Industry (METI) and the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) have set a goal to achieve autonomous driving technology, facilitating truck platooning with no drivers in following trucks on expressways. METI and MLIT have initiated demonstration tests for an autonomous driving system featuring truck platoons traversing expressways, with no drivers in following trucks. When multiple trucks form a platoon on a viaduct, both the trucks and the viaduct may generate coupled vibrations, leading to significant viaduct vibrations. Nonetheless, the authors consider that it is possible to reduce the vibration of the viaduct by specifying the platooning conditions. The aim of this study was to examine the impact of truck platooning on viaduct vibrations. Vibration simulations and truck platooning experiments were conducted on an existing viaduct. The findings confirmed that, under the conditions of present experiments, there exists a distance between trucks that can reduce vibration when two trucks are travelling in a platoon compared to travelling alone.