Despite the increasing number of heavy vehicles (HVs) on the road, little attention is paid to the impact of HVs on traffic flow. Due to their different physical characteristics (length and size) and operational characteristics (acceleration and deceleration) compared to other vehicles, HVs have a physical and psychological impact on the traffic around them. On the other hand, many studies done on changes in road capacity yield a single value of road capacity. In this study, however, the road capacity is analyzed with probability distributions. This study found that the road capacity increases since HV reduction has a constant value with 85% and 50% cumulative probabilities. On major roads, reducing HV by 10%, 20%, 30% increases road capacity by 3-4%, 8% and 11-12% respectively. Lower HVs ratios are less likely to reduce road capacity. However, the speed of HV and other modes does not directly affect the cumulative probabilities. In addition, as the ratio of HV increases, the speed difference between vehicle types decreases. This study is limited to rush hour traffic flows so that a further study can consider the total duration of mixed traffic conditions.