Roads affects ecological processes at a range of spatial scales, and studies document both negative and positive effects on natural capital and ecological processes. Still there are too large knowledge gaps and geographic and taxonomic biases to draw conclusions and guide road authorities. We used a systematic review approach to synthesise current knowledge and identify knowledge gaps. We used a broad search string and limited our search to the period 2008 -2018. This gave a total of about 2000 unique papers. Title and abstract screening reduced this to 473 papers where we included studies that estimate, model or review effects of roads on population processes, demography, distribution, occurrence, abundance, biodiversity, behaviour and landscape connectivity. Papers that passed the initial screening criteria were screened on full-text, grouped into themes and explored in more detail to detect patterns related to geography, road types, habitat types, management and responses recorded. This grouping was based on whether papers addressed invasive species; population genetics; edge effects, landscape perspectives; population processes; biodiversity; dispersal and connectivity; roadside construction and management; verges as habitats and resource; ecological traps; roadkill; pollution and ecotoxicology; verges as refugees and conservation approaches; noise; urbanisation; or ecosystem services. These groups of papers were then reviewed using a narrative approach. Biodiversity was addressed in 175 papers as the largest category of papers, mainly with a focus on taxonomic diversity and often within a narrow phylogenetic focus. 130 studies addressed different landscape perspectives, some in combination with details on fragmentation, population genetics, dispersal and connectivity. The majority of papers on movement and dispersal, however, addressed processes at smaller spatial scales. A good number of papers (124) explored the role of road verges as habitats or providers of resources. In a subset of the papers, this was linked to the role of roadsides as refuges or their role in conservation. There were over 100 papers addressing demography and population processes involving a broad range of measured or estimated effects and only a few in combination with population genetics. Edge effects were also rather well described for a wide range of organisms (108 papers) sometimes linked to abiotic explanatory variables. We found that, despite strong negative effects of processes and factors such as noise, barrier effects, vehicle collisions and landscape fragmentation, HTI can contain considerable biological diversity and species richness, contribute to structural and resource heterogeneity in the landscape, and function as corridors for a diverse set of organisms. However, evidence of these contributions is fragmented, with high species-specificity in responses (especially in animals), but also a strong impact of the landscape configuration and resources. A shift in focus from species occurrence to processes and functions at population and community level; addressing the importance of connectivity towards and away from the roads, integrating habitats along roads in a larger landscape; and approaches to identify and prioritise critical components and trade-offs during road construction and maintenance are among the major knowledge gaps to be addressed in future research.
