Highway traffic bridges have been subjected to increasing loads over the past decades. Some of the movable bridge leafs in these highway bridges were constructed in the 1960s and they are now close to the end of their intended life span. These bridge leafs were typically designed and executed with hardwood deck boards. In the initial design calculations, traffic loads were smaller, complex calculation by computer models did not yet exist. Therefore, some phenomena like for example fatigue and global hybrid interaction were never considered. Since some movable highway bridges are now due to be re-assessed, there is a growing need to gain knowledge on the way timber decks are functioning within the movable bridge leaf. These bridge decks have sustained all loads throughout the years without significant damage, but we don’t know whether these timber deck boards help the main load bearing steel structure as well. Or in other words; do the timber deck boards interact with the steel girders in the current situation? In this research, the degree of hybrid interaction was measured in terms of the magnitude of the main girder deflection for the reference case of the Bridge across the Beneden Merwede. The main girder deflection for bolted connections, having a certain free slip and embedding stiffness, was determined by running a linear sequential secant stiffness analysis. This analysis approximates a physically non-linear analysis. The maximum deflection of the main girder was compared to the maximum main girder deflections for no hybrid interaction versus full hybrid interaction. The resulting global deflections from the Secant Stiffness Analysis have shown that practically no hybrid interaction can be found between the current deck and the steel structure. For retrofitting options, the level of hybrid interaction can be significant. An increasing or decreasing degree of hybrid interaction did not only change the forces and moments in the steel girders, but also had a considerable effect on the deck and its connections. There is a gain and a loss for every retrofitting option considered. Decision-making for retrofitting timber bridge decks should take multiple contradicting criteria into account. An optimisation could be necessary in order to take a well-advised decision.