Ad hoc wireless networks are expected to have many useful applications, such as emergency communications after accidents and natural disasters, distributed sensing in buildings and natural environments, versatile wireless monitoring in health care and support system, and to make our daily lives easier, more productive and more enjoyable. In order to fully realize the potential of wireless ad hoc networks, we need to be able to realize large-scale networks, which connect large numbers of ad hoc terminals over wide areas. Previous research on ad hoc networks has been based mainly on computer simulations and small-scale implementations, and there have been few successful implementations of large-scale ad hoc networks. We have made significant breakthroughs toward realizing real-world operation of large-scale ad hoc networks, by the testbed based approach. In this paper, we discuss the breakthroughs toward realizing large-scale ad hoc networks and inter-vehicle communications for vehicle safety, which is one of the promising applications of ad hoc networks.