Viaduc de Sylans, Truss bridge in Le Poizat-Lalleyriat, France.

Viaduc de Sylans is a 1,266-metre concrete viaduct on the A40 motorway between Charix and Le Poizat-Lalleyriat, above Lac de Sylans. It consists of two parallel roadways on tall vertical pillars, sweeping in a broad S-curve up a steep slope. The deck has a W-shaped cross-section with X-shaped ribbing, and the northern carriageway stands several metres higher than the southern one.

The bridge was built in the late 1980s using advanced prestressing methods to reinforce the concrete structure. Engineers designed it with external tensioning cables to handle the weight of traffic crossing the valley.

The viaduct's construction, with the nearby Nantua viaduct and Chamoise tunnel, features in the documentary "Les Routes de tous les records" by Alex Gary, broadcast in May 2025 on Czech Television as part of its megaprojects series. Below the motorway, the older D 1084 road runs along the shore of Lac de Sylans, past the remains of 19th-century icehouses.

The bridge is accessible only to motorway traffic and best viewed from surrounding mountain paths or the valley below. Photo opportunities are available from nearby viewpoints along regional roads in the area.

The structure uses a three-dimensional external prestressing cable system that was innovative when it was completed. This approach allows the bridge to carry heavy traffic loads while remaining relatively lightweight.