Method for estimating the damping characteristics of the shock absorbers of a vehicle in an active or semi-active suspension system comprising: - a plurality of adjustable damping shock absorbers, each including a pressure chamber containing a damping fluid, inside of which a piston is slidable, the position of which defines a lower pressure chamber and an upper pressure chamber, a by-pass chamber in communication with the upper pressure chamber, and at least one control valve, arranged to control the passage of the damping fluid between the pressure chamber and the by-pass chamber; - sensor means for detecting the relative acceleration or relative movement between the body of the vehicle and each wheel with which a shock absorber is associated, as well as the accelerations and movements of the body; and - electronic processing and control means, adapted to receive signals indicative of said relative acceleration or said relative movement, and arranged to emit driving signals of the control valves of the shock absorbers to achieve predetermined damping characteristics of the shock absorbers, characterized in that the method comprises: - providing a reference model indicative of a nominal relation between a road severity index, RSI, related to the vertical acceleration values experienced by the wheels of the vehicle while driving on a road section, and a quantity representative of the magnitude of the driving force of the control valves of the shock absorbers; - acquiring respective relative acceleration or speed data of at least the front wheels of the vehicle with respect to the body of the vehicle; - determining a value of the road severity index starting from said relative acceleration or speed data of the front wheels of the vehicle with respect to the body of the vehicle; - acquiring values of a quantity representative of the magnitude of the driving force of the control valve of each shock absorber; - for at least one shock absorber, comparing the acquired value of said quantity representative of the magnitude of the driving force with an expected value of said quantity representative of the magnitude of the nominal driving force determined as a function of said road severity index according to said reference model; and - determining a degradation condition of a shock absorber if the acquired value of the respective magnitude of the driving force does not correspond to said expected value of the magnitude of the nominal drive force. - a plurality of adjustable damping shock absorbers, each including a pressure chamber containing a damping fluid, inside of which a piston is slidable, the position of which defines a lower pressure chamber and an upper pressure chamber, a by-pass chamber in communication with the upper pressure chamber, and at least one control valve, arranged to control the passage of the damping fluid between the pressure chamber and the by-pass chamber; - sensor means for detecting the relative acceleration or relative movement between the body of the vehicle and each wheel with which a shock absorber is associated, as well as the accelerations and movements of the body; and - electronic processing and control means, adapted to receive signals indicative of said relative acceleration or said relative movement, and arranged to emit driving signals of the control valves of the shock absorbers to achieve predetermined damping characteristics of the shock absorbers, - providing a reference model indicative of a nominal relation between a road severity index, RSI, related to the vertical acceleration values experienced by the wheels of the vehicle while driving on a road section, and a quantity representative of the magnitude of the driving force of the control valves of the shock absorbers; - acquiring respective relative acceleration or speed data of at least the front wheels of the vehicle with respect to the body of the vehicle; - determining a value of the road severity index starting from said relative acceleration or speed data of the front wheels of the vehicle with respect to the body of the vehicle; - acquiring values of a quantity representative of the magnitude of the driving force of the control valve of each shock absorber; - for at least one shock absorber, comparing the acquired value of said quantity representative of the magnitude of the driving force with an expected value of said quantity representative of the magnitude of the nominal driving force determined as a function of said road severity index according to said reference model; and - determining a degradation condition of a shock absorber if the acquired value of the respective magnitude of the driving force does not correspond to said expected value of the magnitude of the nominal drive force.