Highway merging points are critical elements due to the interactions between merging vehicles and following vehicles on the outermost lane of the highway stream. Such interactions could have significant implications for safety and capacity at ramp locations. The aim of this study was to investigate the spacing adjustment behavior by the interacting drivers at merging locations. In this regard, we relied on the NGSIM trajectory dataset to investigate the impacts of the speed difference between the following and merging vehicles on a space headway, considering different geometric designs and vehicle classes. Nonlinear regression models were estimated to analyze the interactions. The results showed a significant and exponential tendency for headway reduction, particularly when the difference in speed was higher than 30 km/h. In addition, the findings revealed that the highway with an auxiliary lane performed better in terms of headway reduction. Furthermore, the space headway reduction trend was higher when the following vehicle was a truck rather than a car. Policymakers and practitioners aiming to improve road safety at merging locations could use this study’s findings. The resulting parameters can also be utilized in microsimulation models, e.g., for headway adjustment behavior in car-following models.