© 2025 Elsevier LtdState border barriers are proliferating, but their ecological impacts are poorly documented, particularly during construction. More than 50 km of Poland's 186 km long barrier, built in 2022 along the Polish-Belarusian border, bisects Bia?owie?a Forest, a transboundary UNESCO World Heritage site. We surveyed a 5.4 km forest road 50 times (270 km surveyed), during (2022) and after (2023) barrier construction, recording vehicle traffic, roadside tree damages, and wildlife roadkill. We selected this road because it runs along the strict reserve of Bia?owie?a National Park, Poland, and was among roads used to transport building materials to the border. Per kilometer of this road, 28 roadside trees sustained damage and 0.69 vertebrates were killed during the construction period (March–July 2022), which saw up to 133 vehicles/day and an estimated 2870 vehicles (43 % of them construction, 43 % uniformed services). While trees sustained most damage during the construction period, roadkill was higher the following year (2.67 vertebrates/km), when traffic peaked at 40 vehicles/day with 2290 total vehicles (81 % uniformed services; March–October 2023). These findings of variation over time highlight the importance of long-term monitoring of protected areas during and following infrastructure projects given interactions of direct (modification of the landscape) and indirect effects (animal behavior in response to changes, such as traffic).