Speed limitations imposed upon vehicles using the road network are practically brought down to a road administrator defining a certain presupposed discrete value. The latter is typically dependent on the arrangement of linear road infrastructure elements and local development (visibility), the road pavement condition and the roadway geometry. Such a limitation is sometimes consequential to excessive noise or technical specificity of an infrastructure element, which is the case of bridges, tunnels etc. Every officially imposed speed limit set at a too high level against the legitimate requirements (needs) causes considerable losses in terms of traffic flow and affects (also negatively) the behaviour of drivers. From a cognitive perspective, a lower and unjustified speed limit causing irritation among vehicle drivers may not necessarily translate into increased traffic safety. Reduction of traffic flow in road network cross-sections stems from the fact that the permissible speed has been limited to a level below the free flow traffic speed, only based on discrete (step) values, and not always the appropriate ones. This article provides a discussion on a method for determining a legitimate speed limit for a road network based on mobile diagnostics of vehicles.