Institute of Transportation, MOTC

       In the freeway system, traffic flow from the on-ramp merging to the mainline creates lane-changes and weaving, causing phenomena such as traffic turbulence propagation and capacity drop. Previous empirical studies using VD traffic data or road-side cameras are constrained by the data collection technologies, and the understanding of the traffic operation within the on-ramp merge area is still quite limited. In this study, we use unmanned aerial vehicles (UAV) to observe the traffic flow from the top view. We further use computer vision technology to extract vehicle trajectory data from the video and generate traffic characteristics at the temporal and spatial dimensions. This study presents the traffic data collected from six different sites on Taiwan’s freeway interchange with three mainline lanes. The result shows that the maximum pre-breakdown flow rate over at 5-minute intervals is between 5,700 and 5,950 pcu/hr, and the maximum pre-breakdown flow rate at 15-minute intervals is from 5,500 to 5,810 pcu/hr. These values are significantly lower than the recommended values given by the Taiwanese Highway Capacity Manual (THCM, 2011). Under non-congested traffic flow conditions, the outermost lane has the lowest usage rate, while under congested traffic flow conditions, the usage rates among lanes are closer to each other. If the on-ramp has a higher flow rate, mainline traffic has a higher proportion to use the inner lanes, showing that the lane usage ratios are affected by the ramp traffic flow. To investigate the critical point of the merge area, which is the position with the worst operational performance, we divide the merge area into 50-meter cells and determine the spatial traffic characteristics. It is found that the critical points of the study sites are not at the same position, but in general it is somewhere between the end of the acceleration lane and its 100-meter upstream on the outermost lane, where the speed is the lowest and the lane change frequency is concentrated. The result of this study can be used for the development of capacity analysis methodology for the freeway ramp merge areas.