Abstract Continuous evaluation in Vehicular Ad-hoc Networks (VANET) will contribute to spectrum scarcity problem in near future. Cognitive radio (CR) system aims to provide opportunistic access and adapt the available frequency resources instead of conventional static spectrum allocation. Spectrum sensing is one of the most demanding aspects in CR design and implementation. Low signal to noise ratio (SNR) and fading effects posed limitations to deploying CR in realistic propagation scenario with fast and fine sensing features. Energy detection(ED) based spectrum sensing is a viable choice for many vehicle-to-vehicle (V2V) and vehicle-to-roadside infrastructure (V2I) communications. In this paper, we consider the performance of ED over composite Generalized-K (KG) fading to deal with both small and large scale fading in VANET. Diversity combining using Maximal ratio Combining (MRC) and Selective Combining (SC) over KG fading channel are investigated. A novel tractable expressio for Energy detection based average detection probability for optimal MRC diversity combining scheme is derived and closed form possibility is analyzed for SC. Both numerical and simulation models are examined for practical low to moderate shadow fading conditions. The results highlight the notable impact of shadowing spread and fading severity on detection performance and meliorating effects of employing combining techniques.