## 1.2 Introduction  
Depending on the traffic stream conditions, traffic flows can be categorized as continuous, semi-continuous and periodically interrupted traffic flows. Continuous (uninterrupted) traffic stream conditions are present on open road sections without property access points and intersections. In a continuous traffic flow, vehicle speeds and traffic flow are influenced primarily by traffic density and secondarily by existing speed limits, roadway design elements and roadway environmental features. Under these conditions, vehicles do not stop until traffic density reaches its maximum value (congestion). Semi-continuous traffic stream conditions are typically observed along freeway and motorway segments, particularly near various types of off-grade intersections. These conditions are marked by the additional interactions between vehicles in the main traffic flow and those involved in merging and diverging traffic flows, as well as between vehicles interacting along weaving road segments. In contrast to continuous traffic flow conditions, semi-continuous conditions are characterized by more frequent and substantial fluctuations in the speed, acceleration and deceleration of individual vehicles. Interrupted traffic flow conditions occur on road sections with unsignalized and signalized at-grade intersections, where two or more traffic flows cross-intersect each other. In these traffic stream conditions, the movements of individual vehicles in the conflicting traffic flows on signalizedintersections are separated in time by traffic signals. Due to this fact, vehicles must stop their movement during each red phase to give way to vehicles in conflicting traffic streams. Vehicles also need to stop at minor approaches of unsignalized at-grade intersections in cases where they must yield to vehicles on the major intersection approaches. Due to the presence of periodic vehicle movements, interrupted traffic stream conditions differ significantly from continuous (uninterrupted) and semi-continuous traffic flows. Over the past decades, numerous research works have attempted to describe and visualize the mathematical relationships between fundamental traffic flow parameters in different traffic flow regimes [1; 2], utilizing diverse forms of traffic flow diagrams [3; 4; 5; 6]. However, during this period, few researchers have specifically addressed the contrasts between the various functional forms of fundamental traffic flow diagrams suitable for representing the mathematical relationships between empirical traffic flow parameter values observed in both continuous and periodically interrupted traffic flow conditions. Therefore, in an attempt to reduce this research gap, this paper primarily aims to propose and compare optimal mathematical expressions for deterministic \"speed-density\" and \"flow-density\" fundamental diagrams suitable for describing the relationships between empirical values of relevant macroscopic traffic flow parameters in continuous and periodically interrupted traffic stream conditions on the ZC5210 regional road within the Republic of Croatia. The purpose of this paper is to identify the differences in the expected ranges and in the relevant boundary and critical values of the empirical traffic flow parameter values representative of continuous and periodically interrupted traffic flow conditions on selected road sections of an undivided two-lane regional road and to define appropriate deterministic functions for describing the relationships between mean values of the observed traffic flow parameters.