## 5 Conclusions  
This study presents an evaluation of the 2+1 road design versus the conventional TLTW design, for Middle Eastern conditions. It initiates with the utilization of the SUMO traffic microscopic simulation model to model the movements of passenger cars (PCs) and heavy vehicles (HVs) within the highway network over a designated timeframe. The calibrated SUMO parameters were based on a segment of a TLTW from Egypt. In our analysis, we used a 6.0-km-long road with a lane width of 3.65 m for both designs (i.e., TLTW and 2+1) and an auxiliary passing lane of 1.0 km and a lane width of 3.65 m. In addition, we tested five different speed limits, namely 50, 60, 70, 80, and 90 km/h, and five different traffic volumes (i.e., 250 to 1250 vehicle/hour/direction). Furthermore, the heavy-vehicle effects were explored at four different levels, namely 2.50%, 5.0%, 10.0%, and 15%, of the total traffic volume. Moreover, two speed-limit strategies (i.e., USL and DSL) were investigated, as well. The average network delay and the average travel speed (ATS) were used as the indicators for the mobility performance, while the number of simulated traffic conflicts, based on the time to collision (TTC), was used as the surrogate safety indicator to reflect the crash potential for prohibited overtaking and allowed overtaking in the opposite direction in the 2+1 design. The main conclusions based on the mobility performance assessment are as follows. * For different USLs, the speed improvement was between 1.4% (at low traffic volumes) and 12.5% (at traffic volumes near capacity) and between 3.3% and 13.80% for the 2+1 design that prohibits overtaking and that allows it, respectively. And, the reduction in the average network delay varies from 26% to 50% and from 47% to 74% for the 2+1 design that prohibits overtaking and that allows it, respectively. While the presence of heavy vehicles has a negligible effect on traffic mobility;
* For the DSL strategy, as the HVs percentage increases in the traffic stream, there is a significant change in the ATS and the network delay. The reduction in the average delay was between 26% and 48% and between 26% and 51% for speed limits of 80 km/h and 90 km/h, respectively. In addition, the increase in the ATS was from 1.5% to 12.5% and from 1.4% to 16.9% for speed limits of 80 km/h and 90 km/h, respectively, with approximately the same rate of increase for all % HVs. Furthermore, the main conclusions based on the safety assessment are as follows. * For the 2+1 design that prohibits overtaking in the opposite direction, the simulated conflict ratio (\(\mathbf{\rho}\) value) for almost all scenarios, with both the USL and DSL speed-limit strategies, different traffic volumes, and different TTC thresholds, was less than \"1.0\", which suggests a reduction in the simulated conflicts and, hence, a reduction in the expected crashes compared to the TLTW design. This suggests a safety improvement that would be expected when converting the TLTW roads to 2+1 roads and prohibiting vehicles from overtaking in opposite directions by using an appropriate median barrier; \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**V**} & \multirow{2}{*}{**\%HV**} & \multicolumn{3}{c}{**SL = 80 km/h for PCs and 60 km/h for HVs**} & \multicolumn{3}{c}{**SL = 90 km/h for PCs and 60 km/h for HVs**} \\ \cline{3-8}  & & **TTC \(\leq\) 2.50** & **TTC \(\leq\) 1.50** & **TTC \(\leq\) 0.50** & **TTC \(\leq\) 2.50** & **TTC \(\leq\) 1.50** & **TTC \(\leq\) 0.50** \\ \hline
250 & & 0.20 (0.04) & 0.01 (0.01) & 0.00 (0.00) & 0.09 (0.01) & 0.00 (0.00) & 0.00 (0.00) \\
500 & & 0.26 (0.03) & 0.02 (0.00) & 0.01 (0.00) & 0.16 (0.01) & 0.02 (0.00) & 0.01 (0.00) \\
750 & 15 & 0.39 (0.03) & 0.04 (0.01) & 0.01 (0.00) & 0.33 (0.02) & 0.04 (0.00) & 0.02 (0.00) \\
1000 & & 0.83 (0.04) & 0.10 (0.01) & 0.03 (0.00) & 0.63 (0.03) & 0.08 (0.01) & 0.02 (0.00) \\
1250 & & 2.67 (0.21) & 0.31 (0.03) & 0.06 (0.01) & _1.48 (0.08)_ & 0.19 (0.01) & 0.04 (0.00) \\ \hline \end{tabular}
\end{table}