The Millau Viaduct (French: le Viaduc de Millau - pronounced "mee-yoh") is a large cable-stayed road-bridge that spans the valley of the river Tarn near Millau in southern France. Designed by the structural engineer Michel Virlogeux and British architect Norman Foster, it is the tallest vehicular bridge in the world, with one mast's summit at 343 metres (1,125 ft) - slightly taller than the Eiffel Tower and only 38 m (125 ft) shorter than the Empire State Building. The viaduct is part of the A75-A71 autoroute axis from Paris to Béziers. It was formally dedicated on 14 December 2004, inaugurated the day after and opened to traffic two days later. The bridge won the 2006 IABSE Outstanding Structure Award.

The bridge's construction broke three world records:

The highest pylons in the world: pylons P2 and P3, 244.96 metres (803 ft 8 in) and 221.05 metres (725 ft 3 in) in height respectively, broke the French record previously held by the Tulle and Verrères Viaducts (141 m/460 ft), and the world record previously held by the Kochertal Viaduct (Germany), which is 181 metres (590 ft) at its highest;

The highest mast in the world: the mast atop pylon P2 peaks at 343 metres (1,130 ft).

The highest road bridge deck in the world, 270 m (890 ft) above the Tarn River at its highest point. It is nearly twice as tall as the previous tallest vehicular bridge in Europe, the Europabrücke in Austria. It is slightly higher than the New River Gorge Bridge in West Virginia in the United States, which is 267 m (880 ft) above the New River. Only the bridge deck of the Royal Gorge Bridge in Colorado, United States (mainly a pedestrian bridge over the Arkansas River, occasionally also used by motor vehicles) is higher with 321 m (1,050 ft), and is considered the highest bridge in the world.

The Millau Viaduct is located on the territory of the communes of Millau and Creissels, France, in the département of Aveyron. Before the bridge was constructed, traffic had to descend into the Tarn River valley and pass along the route nationale N9 near the town of Millau, causing heavy congestion at the beginning and end of the July and August vacation season. The bridge now traverses the Tarn valley above its lowest point, linking two limestone plateaux, the Causse du Larzac and the Causse Rouge, and is inside the perimeter of the Grands Causses regional natural park.

The bridge forms the last link of the A75 autoroute, (la Méridienne) from Clermont-Ferrand to Béziers. The A75, with the A10 and A71, provides a continuous high-speed route south from Paris through Clermont-Ferrand to the Languedoc region and through to Spain, considerably reducing the cost of vehicle traffic travelling along this route. Many tourists heading to southern France and Spain follow this route because it is direct and without tolls for the 340 kilometres (210 mi) between Clermont-Ferrand and Pézenas, except for the bridge itself.

The Eiffage group, which constructed the viaduct, also operates it, under a government contract which allows the company to collect tolls for up to 75 years.

For nearly thirty years prior to the construction of the Millau Viaduct, the A75 autoroute had remained unfinished. Before the bridge, a crossing of the River Tarn was provided by a bridge situated in the valley bottom, in the town of Millau. Millau was then known and dreaded as a ‘great black spot’ of motoring. Kilometres of congestion and hours of waiting to transit the town recurred each year with the great surge in traffic in summer months. These slowdowns meant that the advantages of the A75 were lost. The A75 was meant to be a positive example of spatial planning, a modern, direct highway entirely free along its 340 km (210 mi) length. As it was, the traffic from the autoroute brought pollution and danger to the town of Millau.

Design and construction of the bridge took a long time. In this region, climatic conditions are tough, with violent winds. Geological characteristics of the high plateaus of Larzac are peculiar, and, because the Tarn Valley is so deep, crossing is difficult. Different approaches were investigated, and all of them were found to be very technically demanding. Ten years of research and four years of implementation were required for completion of the Millau Viaduct.

The Millau Viaduct consists of an eight-span steel roadway supported by seven concrete pylons. The roadway weighs 36,000 tons and is 2,460 m (8,100 ft) long, measuring 32 m (100 ft) wide by 4.2 m (14 ft) deep, making it the world's longest cable-stayed deck. The six central spans each measure 342 m (1,120 ft) with the two outer spans measuring 204 m (670 ft). The roadway has a slope of 3% descending from south to north, and curves in a plane section with a 20 km (12 mi) radius to give drivers better visibility. The pylons range in height from 77 m (250 ft) to 246 m (810 ft), and taper in their longitudinal section from 24.5 m (80 ft) at the base to 11 m (36 ft) at the deck. Each pylon is composed of 16 framework sections, each weighing 2,230 tons. These sections were assembled on site from pieces of 60 tons, 4 m (13 ft) wide and 17 m (56 ft) long, made in factories in Lauterbourg and Fos-sur-Mer by Eiffage. The pylons each support 97 m (320 ft) tall masts.

The pylons were assembled first, together with some intermediate temporary pylons, before the decks were slid out across the piers by satellite-guided hydraulic rams that moved the deck 600 mm every 4 minutes. Then the masts were driven over the new deck, erected on top of the pylons, connected to the deck and the temporary pylons removed.

Construction began on 10 October 2001 and was intended to take three years, but weather conditions put work on the bridge behind schedule. A revised schedule aimed for the bridge to be opened in January 2005. The viaduct was inaugurated by President Chirac on 14 December 2004 to open for traffic on 16 December, several weeks ahead of the revised schedule.

The metallic deck, which appears very light despite its total mass of around 36,000 metric tons (40,000 short tons), is 2,460 m (8,100 ft) long and 32 m (100 ft) wide. It comprises eight spans. The six central spans measure 342 m (1,120 ft), and the two outer spans are 204 metres (670 ft). These are composed of 173 central box beams, the spinal column of the construction, onto which the lateral floors and the lateral box beams were welded. The central box beams have a 4 m (13 ft) cross-section and a length of 15-22 m (49-72 ft) for a total weight of 90 metric tons (99 short tons). The deck has an inverse airfoil shape, providing negative lift in strong wind conditions.

The seven masts, each 87 m (290 ft) high and weighing around 700 metric tons (770 short tons), are set on top of the pylons. Between each of them, eleven stays (metal cables) are anchored, providing support for the road deck.

Each mast of the viaduct is equipped with a monoaxial layer of eleven pairs of stays laid face to face. Depending on their length, the stays were made of 55 to 91 high tensile steel cables, or strands, themselves formed of seven strands of steel (a central strand with six intertwined strands). Each strand has triple protection against corrosion (galvanisation, a coating of petroleum wax and an extruded polyethylene sheath). The exterior envelope of the stays is itself coated along its entire length with a double helical weatherstrip. The idea is to avoid running water which, in high winds, could cause vibration in the stays and compromise the stability of the viaduct.

To allow for deformations of the metal deck under traffic, a special surface of modified bitumen was installed by research teams from Appia. The surface is somewhat flexible to adapt to deformations in the steel deck without cracking, but it must nevertheless have sufficient strength to withstand motorway conditions (fatigue, density, texture, adherence, anti-rutting, etc.). The "ideal formula" was found only after ten years of research.

The electrical installations of the viaduct are impressive, in proportion to the immensity of the bridge. There are 30 km (19 mi) of high-current cables, 20 km (12 mi) of fibre optics, 10 km (6.2 mi) of low-current cables and 357 telephone sockets allowing maintenance teams to communicate with each other and with the command post. These are situated on the deck, on the pylons and on the masts.

As far as instrumentation is concerned, the viaduct is state of the art. The pylons, deck, masts and stays are equipped with a multitude of sensors. These are designed to detect the slightest movement in the viaduct and measure its resistance to wear-and-tear over time. Anemometers, accelerometers, inclinometers, temperature sensors are all used for the instrumentation network.

Twelve fibre optic extensometers were installed in the base of pylon P2. Being the tallest of all, it is therefore under the most intense stress. These sensors detect movements on the order of a micrometre. Other extensometers, electrical this time, are distributed on top of P2 and P7. This apparatus is capable of taking up to 100 readings per second. In high winds, they continuously monitor the reactions of the viaduct to extreme conditions. Accelerometers placed strategically on the deck monitor the oscillations that can affect the metal structure. Displacements of the deck on the abutment level are measured to the nearest millimetre. The stays are also instrumented, and their ageing meticulously analysed. Additionally, two piezoelectric sensors gather traffic data: weight of vehicles, average speed, density of the flow of traffic, etc. This system can distinguish between fourteen different types of vehicle.