Southeastern Finland's road network has experienced problems due to an increasing number of heavy vehicles. A main road simulation model was built so that the effects of heavy traffic on traffic performance could be studied. Traffic performance was studied with current traffic volumes and in two future scenarios using the simulation model. There is a decrease in average speed with current traffic volumes, especially on road sections around major cities. Queues at border stations cause the majority of cross border traffic delays. Traffic conditions in the growth scenarios weaken most close to the Russian border and around cities. There is a three to four percent increase in study area travel times to border stations, with increased border waiting time not included. The southeastern main road network simulation model generally was able to fairly realistically express traffic conditions. The accuracy of estimated traffic volumes and distributions around cities, however, was not high enough to use in microsimulation, and simulation reliability is decreased due to lack of a validated overtaking model on two lane roads. Despite this, with minor changes future projects can utilize the network model built in this project.