From an analysis of brain function data, it is confirmed that auditory information improves the cognitive effect and based on this a voice warning system using speakers has been developed for use in tunnels to mitigate traffic congestion. This was the first time in Japan that speakers were used to provide voice guidance on speed recovery information during traffic congestion, which is traditionally carried out with visual information. This study will present an outline of this system and will show how it was applied in the Kobotoke Tunnel and will verify using data from vehicle detectors and ETC 2.0 probe data whether the voice information was effective. For example, results showed that the breakdown flow rate increased by about 8% with the system, and the average speed in the queue increased by about 10%, showing the possibility of reduced congestion. An upstream effect could also be inferred, with the average speed near bottlenecks increasing due to voice guidance.