As a result of forest fragmentation, ecotones have become an important contribution to the landscape. The areas that include ecotones are constantly growing, thus creating new habitats for plants and animals. The factor that has the greatest impact on the configuration and extent of an ecotone is called the “edge effect”. The aim of this study was to determine the width of the ecotones in forests that are adjacent to an expressway and to characterize the edge effect they produced. The research was performed along ten transects situated at the edge of a forest and in the interior part of a forest (plots from 0 to 200 m). The structures of the forest stand, trees, shrubs, dwarf shrubs and herbs were determined and data on the species composition and species cover were also collected. Results of the statistical analyses (linear mixed models as well as detrended and canonical correspondence analyses) indicated significant changes in the structure, species composition, and species turnover that were dependent on the distance from the edge of the forest. It was shown that shrubs (cover, density, species richness) were the most affected while vascular plants were the least affected. However, the changes were ambiguous. There was either an increase or decrease in the individual parameters of the forest structure and the coverage of forest species. The extent of the forest edge began at a minimum distance of 0–20 m.