There are also the vehicles among the other vehicles in road traffic that have been modified without being authorised by their producer. These also include modifications such as structural modifications in the vehicle break system. Besides a brake system of road motor vehicles is one of the main factors influencing the active safety of vehicles. The design of the brake system, its technical condition and additional intervention in its construction may have a positive as well as negative impact on the braking distance length and the value of the mean braking deceleration achieved. The paper focuses on the influence of the brake disc diameter of the front axle on the achieved value of the mean braking deceleration and the braking distance length, while the braking system has been modified for several times without being approved by car manufacturer. The introductory part of the paper describes the braking distance sections and it also explains the term of mean braking deceleration. The following part of the paper deals with the measurement methodology, measuring equipment and the vehicle used during the measurements as well as procedures employed. The results obtained from the measurements are processed and presented in tables and also in graphs for greater clarity. The final part of the paper summarizes and evaluates the measured results. The importance of the paper lies in quantification of the influence of brake discs with different diameters on the vehicle active safety in the case of a particular vehicle.