## 3 Spatial and Temporal Scope of Research  
The study area includes one macro- and two micro-observation zones (Figure 1). The macro-observation zone includes the urban road network of the city of Novalja in the Republic of Croatia, its bypass and the main radial roads in the vicinity of the city, including state road D106 and regional roads ZC5210 and ZC5151. The micro-observation zones include two road sections of regional road ZC5210 with the following specific road design elements and traffic flow characteristics:  
1. Road section A: The road section of regional road ZC5210, spanning from a three-leg unsignalized intersection (connecting regional road ZC5210 to Ciponjac South Street) to a four-leg roundabout (linking regional road ZC5210 with state road D106), covering 310 m. The traffic flows were recorded on a straight section of this road, sufficiently distanced from adjacent intersections to ensure that all empirical traffic flow parameter values, representative of continuous traffic stream conditions, could be accurately extracted from the captured video files.
2. Road section B: The road section of regional road ZC5210, situated near a signalized intersection (connecting regional road ZC5210 to Caskin Way Street), 205 m in length. The traffic flows were recorded on a straight segment of the road, directly adjacent to the four-leg signalized intersection to ensure that the empirical traffic flow parameter values, indicative of interrupted traffic stream conditions, could be accurately extracted from the captured video files.  
The reason behind selecting this particular geographic region and road category for this study stems from the following facts: (1) To date, there have been few studies in the world and in Croatia, which have developed the specific forms of \"speed-density\" and \"flow-density\" fundamental diagrams representative of modeling traffic flow characteristics on lower tier (regional and local) bi-directional undivided roads; (2) Due to high seasonal traffic flow fluctuations in the selected geographic region, i.e., higher traffic flow volumes on lower tier roads during the tourist summer season, it was possible to capture the different patterns of traffic flow dynamics present in various traffic flow regimes (free flow, normal and saturated traffic flow) and to obtain a sufficiently large empirical sample necessary for development and calibration of \"speed-density\" and \"flow-density\" fundamental diagrams, which would not be possible in other geographical regions of Croatia, where traffic flow on lower tier roads is typically very low during the whole year.  
The empirical values of traffic flow parameters--essential for conducting statistical analysis to determine the optimal functional forms of \"speed-density\" and \"flow-density\" fundamental diagrams--were calculated on the basis of data obtained from the aerial photographs taken by unmanned aerial vehicles on two selected road sections of regional road ZC5210 at representative 15 min intervals during peak hours (in the morning period from 7:00 to 9:00 a.m. and in the afternoon period from 15:00 to 17:00 p.m.) and off-peak hours (the period from 11:00 a.m. to 13:00 p.m.). Figure 1 shows the macro- and micro-observation zones situated within the study area, together with the indicated time periods of the aerial video recordings conducted and the size of the empirical samples collected on road sections A and B.