Method for determining a speed limit in force on a road taken by a motor vehicle, said motor vehicle being equipped: - with a navigation system comprising a geolocation module and a road map in which speed limits associated with at least some of the roads are stored, and - with an image processing system comprising a means for acquiring images of the road taken and a processing module able to identify, in said images, speed limit signs located close to the road taken, characterized in that it comprises steps, implemented by a computer, consisting in: a) acquiring, at each speed limit sign identified, a first value of a parameter relating to the nature of the road taken, and prior to step a), if no speed limit sign has been identified, the speed limit associated with the road taken is read in the navigation system, said parameter comprising at least one of the following data: - the number of traffic lanes on the road, - the type of environment - urban, peri-urban or rural, - the type of road, - the shape of the markings on the ground. b) acquiring, when the distance travelled by the motor vehicle since the last speed limit sign was identified exceeds a predetermined threshold (Ds), a second value of the parameter relating to the nature of the road taken, c) if the first and second values (Z1, Z2) of said parameter are different, reading, in the navigation system, the speed limit associated with the road taken, in which, between steps a) and b), the speed limit considered is that read on the last speed limit sign identified, in which, in steps a) and b), either the first and second values of said parameter are determined using the image processing system, or the first and second values of said parameter are read in the navigation system, the navigation system memorizing values of the parameter relating to the nature of at least some of the mapped roads. - with a navigation system comprising a geolocation module and a road map in which speed limits associated with at least some of the roads are stored, and - with an image processing system comprising a means for acquiring images of the road taken and a processing module able to identify, in said images, speed limit signs located close to the road taken, characterized in that it comprises steps, implemented by a computer, consisting in: a) acquiring, at each speed limit sign identified, a first value of a parameter relating to the nature of the road taken, and prior to step a), if no speed limit sign has been identified, the speed limit associated with the road taken is read in the navigation system, said parameter comprising at least one of the following data: - the number of traffic lanes on the road, - the type of environment - urban, peri-urban or rural, - the type of road, - the shape of the markings on the ground. b) acquiring, when the distance travelled by the motor vehicle since the last speed limit sign was identified exceeds a predetermined threshold (Ds), a second value of the parameter relating to the nature of the road taken, c) if the first and second values (Z1, Z2) of said parameter are different, reading, in the navigation system, the speed limit associated with the road taken, in which, between steps a) and b), the speed limit considered is that read on the last speed limit sign identified, in which, in steps a) and b), either the first and second values of said parameter are determined using the image processing system, or the first and second values of said parameter are read in the navigation system, the navigation system memorizing values of the parameter relating to the nature of at least some of the mapped roads. a) acquiring, at each speed limit sign identified, a first value of a parameter relating to the nature of the road taken, and prior to step a), if no speed limit sign has been identified, the speed limit associated with the road taken is read in the navigation system, said parameter comprising at least one of the following data: - the number of traffic lanes on the road, - the type of environment - urban, peri-urban or rural, - the type of road, - the shape of the markings on the ground. - the number of traffic lanes on the road, - the type of environment - urban, peri-urban or rural, - the type of road, - the shape of the markings on the ground. b) acquiring, when the distance travelled by the motor vehicle since the last speed limit sign was identified exceeds a predetermined threshold (Ds), a second value of the parameter relating to the nature of the road taken, c) if the first and second values (Z1, Z2) of said parameter are different, reading, in the navigation system, the speed limit associated with the road taken,