Vehicle candidate generation is important for vehicle detection. Existing vehicle detection studies usually employ general-purpose region proposal methods to generate vehicle candidates, which do not consider the specificity of on-road vehicles in traffic scenes. In this paper, we propose a model to re-rank the candidates that are generated by general-purpose region proposal methods. Our model considers the specificity of on-road vehicle candidate generation in traffic scenes by encoding global-local semantic context and location-size geometric compatibility. In the experiments, we test our model on three art-of-the-state region proposal methods using two public datasets. The results show the significant performance improvement is gained after applying our model.