Flood fatality data show that world‐wide, flood related deaths are increasingly associated with people perishing in vehicles that become unstable driving through floodwaters. Previous research on vehicle stability in flood flows has predominantly focussed on model scale testing. This paper presents novel testing determining stability thresholds of full‐scale, road‐going vehicles in floodwaters. Traction forces for various flow depths and buoyancy conditions were directly measured for three types of prototype vehicles. Friction coefficients for prototype vehicles were determined for different bed surface conditions including concrete, gravel, and sand highlighting that a worst‐case friction coefficient must be considered. Novel direct measurements of drag coefficients were conducted for a range of flow conditions of a model‐scale vehicle complementing the prototype tests. The combination of measured forces provided vehicle stability thresholds applicable to a range of vehicle types. While this study was the most comprehensive stability assessment to date, all tests were conducted under laboratory conditions and more conservative threshold criteria must be applied to account for violent flood waters and uneven surfaces.