Modular expansion joints are structures that are submitted to severe fatigue load conditions. This may lead to unexpected premature damage of the structure which, besides the economic cost of repair, may limit the regular service of the bridge. To better understand the dynamic behaviour of modular expansion joints, different over rolling tests have been performed at the modular expansion joint of the Martinus Nijhoff Bridge in the Netherlands. The tests were part of a research program for developing an early warning monitoring system for expansion joints. Strain measurements were performed during the passage of a vehicle at different speeds, for two different scenarios regarding the fixations (sliding springs and sliding bearings) of the cross beams. At the same time, a numerical model was developed and validated by means of the experimental data. This article presents and discusses the measurements and the numerical analysis. The results highlight on the effect of the cross beam fixation, and the effect of the vehicle speed on the strain distribution along the centre beam, together with the changes in modal properties of the structure.