To improve highway transport security, VANET (Vehicular Ad Hoc Network) is used which is a developing technology incorporating ad hoc network, cellular technology and wireless LAN. VANETs are different from other type ad hoc networks by their cross network constructions, node association features and new application setups. The approach of an effective routing protocol for VANETs is vital as VANETs show various distinctive networking research challenges. In this paper, we discuss the research challenges of routing in VANETs and compare recent routing protocols of VANETs.