In September 2013, leading experts in intervehicle communication from all over the world met at the renowned Dagstuhl Castle for a seminar discussing the question "Inter-Vehicular Communication - Quo Vadis?" The objective was to identify the current state of the art and, more important, the open challenges in R&D from both a scientific and an industrial point of view. After more than a decade of research on vehicular networks, the experts very seriously asked whether additional research in this field is necessary and, if so, which will be the most intriguing and innovative research directions. It turned out that the overall perspective has changed in the last few years, mainly as a result of the ongoing field operational tests in the United States and Europe. In this article, we report the key outcomes and results from the discussions, pointing to new research directions and new challenges that need to be met for a second generation of vehicular networking applications and protocols. In particular, we present the reports and findings from the four working groups on scientific foundations of vehicular networking, field ... mehroperational tests, IVC applications, and heterogeneous vehicular networks.