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In recent years, Vehicle Ad Hoc Networks (VANETs) have become very popular. A typical VANET defines an intelligent way of using vehicular networking by integrating vehicular connections via a number of short-distance ad-hoc networking links, such as the IEEE802.11p. This type of system could provide effective communication between moving vehicles and fixed Road Side Units (RSUs), which are usually very expensive to install and may, in some cases, undergo heavy demand. To improve the efficiency of communication between vehicles and RSUs (Vehicle to Infrastructure, V2I), most existing work ignores the following facts: the uneven distribution of vehicles and unbalanced working loads on different RSUs. By considering the periodic and the unchanging features of public transportation, we propose a novel V2I hybrid optimisation scheme named Dynamic Resources Allocation Scheme (DRAS). In this scheme, all resources on the RSUs are scheduled as a whole and buses will be introduced as moving infrastructure points (or relay stations) to take some of the burden of the RSUs in real-time traffic conditions. We import the Variable-Length Contention Free Period (VL-CFP) for MAC access and also define Time-to-Live (TTL) value to guarantee the data credibility on bus. Using both Math analysis and NS-2 simulation, we can prove our DRAS can achieve a better QoS performance and adequate utilisation of RSU resources than legacy 802.11p and WAVE-based HCCA. These advantages become more obvious as the vehicles number increases.