Recent advances in micro-electromechanical systems (MEMS) technology, mobile computing, and wireless communications have enabled rapid deployment of a self-organizing, infrastructure-less, multi-hop, and distributed inter-vehicle communication (IVC) network based on pre-existing road layouts. Vehicular traffic scenarios pose great challenges to physical topology connectivity, which is a prerequisite to providing reliable applications to the users of a vehicular ad hoc network (VANET). This paper provides a probability analysis algorithm to calculate the necessary condition of 1-connected VANETs in highway scenarios. Extensive experiments were undertaken to verify the derived analytic expression via realistic mobility traces. Results demonstrate that the radio communication range of each node must be subject to Θ(|log(1-p^1/n)|/n) in order to ensure that there is no isolated nodes within the entire network.