This paper sheds some light on the macroscopic and microscopic characteristic of traffic flow on freeways in the merging areas in presence of Human-Driven Vehicles (HDV) and Connected Autonomous Vehicle (CAV) by thoroughly digging into the literature, exploiting real data and simulating simple scenarios. In particular, in the first step, an extensive literature review on merging behaviour for current and future traffic conditions (i.e. having HDV or HDV and/or CAV on the road) has been carried out. This is followed by an analysis of single vehicle data from a freeway in Lyon to illustrate current speed and headway distributions which are most likely to change with the introduction of CAV's. Finally, some microscopic simulation results are presented which show that traffic flow typically gains from the introduction of CAV's, but may also display negative consequences under certain circumstances. This simulation study focused mainly on the traffic characteristics at the merging area in presence of CAV. It remains for future work to map out precisely where the introduction of CAV's improves the traffic system, and where amendments have to be made to prevent negative side-effects. It should be noted that calibrating and validating the micro-simulation tools which simulate traffic scenarios in presence of CAV's will indeed be challenging in the future. This needs extra attention for research.