## 2 Materials and Methods  
### Test Route  
The basis of the study is the measurement and analysis of the fuel consumption and the tailpipe emissions of a PHEV representing real-world driving conditions in the Stuttgart metropolitan area. Stuttgart is known for its highly congested roads and, in this context, for air quality issues by frequently exceeding air pollutant limit values in previous years [14]. Thus, a test route is selected that includes several traffic hotspots such as the _Neckarpark Dreieck_, the _Hauptsattter Strasse_ and the federal motorway A8 [15]. The selected test route contains vehicle speed-dependent urban, rural and motorway parts and has a distance of approx. 55 km. Figure 1 gives an overview of the test route, starting in the southwestern area of Stuttgart (Vaihingen) with the major urban parts of the test route leading across the city, following by the main rural sections and ending with the motorway parts. Colors indicate the vehicle speed ranges of the test route for one exemplary test run.  
Another point to highlight is that the boundary conditions of the RDE test procedure are considered as a baseline for the selection of the test route with the prioritization to include the local circumstances of the Stuttgart metropolitan area. Therefore, the selected test route is not fully RDE-compliant. Table 1 presents the characteristics of the selected Stuttgart route in comparison to the respective RDE trip requirements.  
The classification of an RDE trip into the urban, rural and motorway distance sections is conducted according to the vehicle speed. The urban part includes distances driven with less or equal to 60 km/h, the rural part covers distances driven with a vehicle speed of 60-90 km/h and the motorway part comprises distances driven above 90 km/h of vehicle speed. A total number of 11 real-world driving emissions tests have been performed during a period of approx. 20 weeks. As the boundary conditions such as weather, congestion, roadworks, etc. vary within this period, the shares of the vehicle speed groups also differ. Hence, Table 1 presents the ranges of the Stuttgart test route specifications for the entire test series. The comparison shows that the Stuttgart route has a relatively high share of urban  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **RDE Trip Requirements** & **Stuttgart Test Route** \\ \hline Urban driving distance & \(\geq\)16 km & 20.2–27.3 km \\ Rural driving distance & \(\geq\)16 km & 19.1–29.9 km \\ Motorway driving distance & \(\geq\)16 km & 2.7–17.5 km \\ Urban distance share & 29–44\% & 35–46\% \\ Rural distance share & 23–44\% & 34–53\% \\ Motorway distance share & 23–44\% & 5–30\% \\ Total trip duration & 90–120 min & 69–87 min \\ Average vehicle speed & - & 30.3–50.7 km/h \\ Cumulative positive elevation & \(<\)1200 m/100 km & 1100 m/100 km \\ \multicolumn{2}{c}{gain, total} \\ Cumulative positive elevation & \(<\)1200 m/100 km & 1600 m/100 km \\ gain, urban & \multicolumn{1}{c}{} & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the Stuttgart test route characteristics and the real driving emissions (RDE) trip requirements.  
Figure 1: Overview of the selected real-world driving test route in the Stuttgart metropolitan area (and an exemplary vehicle speed profile).  
and rural parts, while the motorway distance share is relatively low. Additionally, the total trip duration is below the RDE range of 90-120 min. The Stuttgart route is furthermore characterised by an area featuring a rather variable topography with height differences of approx. 300 m. The cumulative positive elevation gain of the entire test route is approx. 100 m below the limit of the RDE test conditions while the cumulative positive elevation gain for the urban part of the test route is about 1600 m/100 km and therefore exceeds the limit of 1200 m/100 km.