Software Defined Networking (SDN) has been already used in recent literature to add flexibility and programmability to Vehicular Ad hoc Networks (VANETs). However, there are numerous open issues in implementing SDN for central control and management of VANETs. One open problem is how to adapt SDN for routing data from a source to a destination in VANETs. The main limitation of recent literature is that they don't consider the dynamic topology of VANETs when designing an SDN-enabled routing protocol. This limitation results in inefficient resource usage and congestion in VANETs. Moreover, there exist challenges on how a central SDN controller can contribute in efficient resource sharing and maintaining QoS in VANETs. In this paper, we use SDN controller to mitigate congestion of Vehicle-to-Vehicle communications while routing data on road segments. This is achieved by efficient utilization of VANET bandwidth on road segments. In contrast to recent contributions, the proposed SDN controller provides a novel routing mechanism that takes into account other existing routing paths which are already relaying data in VANET. New routing requests are addressed such that no road segment gets overloaded by multiple crossing routing paths. This approach incorporates load balancing and congestion prevention in the routing mechanism. We model the problem as a Weight Constrained Shortest Path Problem (WCSPP) and provide an efficient algorithm for a practical solution. Our simulations show QoS improvement, in terms of channel busy ratio, achieved by our proposal in comparison with recent related contributions.