Information dissemination is an important application in VANETs for traffic safety and efficiency. In urban area, roadside infrastructure nodes can be deployed for information dissemination. However, it is inefficient and uneconomical to cover the whole urban area. How to find the optimal locations to place DPs is a research problem. Some works on this issue have to collect accurate trajectories of all the vehicles, which is not practical in the real environment. In this paper, we propose a novel approach for DPs placement in grid road networks without knowing trajectories. Based on the analysis of path number between two intersections, a probabilistic model is proposed to get the trajectories estimation of vehicles. The theoretical optimal algorithm (OA) and two heuristic algorithms (called KP-G and GA) are developed for the problem. Simulation results reveal that GA is scalable and has the highest coverage ratio on average.