Netherlands' National Wildlife Crossing Network — Meerjarenprogramma Ontsnippering (MJPO)

Starting with badger tunnels built in 1974 and the first large green bridges over the A50 motorway in 1988, the Netherlands built one of the world's most extensive wildlife-crossing networks under Rijkswaterstaat and ProRail, the national roads and rail agencies.
The Multi-Year Programme for Defragmentation (Meerjarenprogramma Ontsnippering, MJPO), run 2005–2018, targeted 176 official ecological barriers where roads, railways and waterways cut through the National Ecological Network. By the end of 2018, 126 barriers were fully removed and 40 more partially addressed, through a mix of ecoducts, eco-culverts, wildlife underpasses, overpasses at tree-crown level and adapted bridges. The country's motorway and provincial-road network now carries more than 2,100 dedicated wildlife crossings.
A peer-reviewed, multi-method evaluation (van der Grift et al., Landscape Ecology, 2020) found that ecoducts are ecologically effective — about 62% of the species they were designed for actually use them — and that fenced crossing structures cut large-mammal roadkill by roughly 83%. The programme raised an estimated 1,734 T-EQA of threat-weighted ecological quality at a total cost of about €283 million.
The same study found ecoducts to be the most expensive mitigation type per unit of ecological gain: large fauna tunnels and shared-use viaducts delivered better cost-effectiveness, and, taken purely on cost-effectiveness grounds, buying and restoring agricultural land outperformed building new crossings. The Dutch government and independent researchers treat this as an argument for mixing cheaper measures with ecoducts rather than a case against the network, which remains the reference model cited across Europe.

Read the full analysis: https://www.clo.nl/en/indicators/en205113-habitat-defragmentation-measures-for-infrastructure-2018