Bridge weigh-in-motion uses an instrumented bridge as a scale to weigh vehicles. Several types of bridges may be used, if being sensitive to wheel or axle loads. Here some strains of steel orthotropic deck bridges are measured by extensometers and analyzed by software, to calculate axle loads and gross vehicle weights. Between 2009 and 2011, several large scale tests were done on the Millau viaduct (France), the tallest cable stayed bridge in the world. These tests showed that the system meets the accuracy class C(15)/D+(20) of the European WIM Specifications. The data obtained can be used for fatigue checks, because it assesses truck sizes and weights of the traffic and the behaviour of the bridge. We show here that bridge lifetimes under current and possible future traffic loads, after an increase of the GVW limit from 40 tons to 44 tons in France, could be reduced by 20%.