Final answer:
The high population density in Japan has significantly led to the development of efficient public transportation, facilitating mobility and helping to manage urbanization and environmental impact.
Step-by-step explanation:
The high population density of modern Japan has potentially led to various issues, but regarding the provided options, it has most notably led to efficient public transport. High population density typically necessitates improved public transportation systems, as a high number of people need to move around the cities without the space for everyone to use personal vehicles. This efficiency in public transport is pivotal in managing urbanization and reducing traffic congestion. Moreover, efficient public transport is an important aspect of Japan's approach to handling the environmental impact of high population density.