Truck platooning is a means to significantly reduce the fuel consumption for the follower vehicle as the air drag is reduced when the inter-vehicle gap between the trucks is reduced. As each truck is assigned with different start and end locations, platoons will be frequently formed and split, while driving to their respective destinations. Additionally, the trucks are not the only ones driving on the road as there are other road users, which may influence how well a platoon can be formed. In this paper, an experimental study is conducted to investigate how traffic may affect a merging maneuver of two trucks trying to form a platoon on a public highway during rush hours. We obtained traffic data from Stockholm's motorway control system to determine the traffic condition for each testrun. Furthermore, we tried different truck speeds to study if it had any impacts on merge delay. Even in light traffic condition, a platoon merge could be delayed with over 10 % compared to the ideal case with the absence of traffic. This is partially caused by persistent drivers in which we encountered them in a fourth of the runs.