## 4 Conclusions  
This study describes a simulation study to evaluate the effects of increasing the speed limit of tractor-trailers (TTs) on the safety and mobility of Egyptian three-lane highways. In this study, only highways with speed limits of passenger cars of 90 and 100 km/h were considered. For the 90 km/h speed limit for PCs, the TT speed limits considered were 60, 70, and 80 km/h for the differential speed limit (DSL) strategy, along with the TT speed limit of 90 km/h for the uniform speed limit (USL) strategy. For the 100 km/h speed limit for PCs, the TT speed limits considered were 70, 80, and 90 km/h, for the DSL strategy, and 100 km/h for the USL strategy. The average network delay (second/vehicle) was used as the measure of mobility performance, and the conflict ratio based on both the deceleration rate to avoid crash (DRAC) and the time to collision (TTC) surrogate safety measures were used as indicators of safety performance. The SUMO model was used to model the increase in the TT speed limits and their effects for different traffic volumes along with different percentages of TTs. The SUMO surrogate safety measure (SSM) file, as a direct output of the SUMO model, was used to obtain simulated conflicts with \(\text{TTC}\leq 2.50\) s, representing less severe conflicts; \(\text{TTC}\leq 1.50\) s, representing medium-severity conflicts; and \(\text{TTC}\leq 0.50\) s, representing severe conflicts. Additionally, \(\text{DRAC}\geq 3.35\) m/s\({}^{2}\), representing less severe to medium-severity conflicts, and \(\text{DRAC}\geq 6.0\) m/s\({}^{2}\), representing severe conflicts, were used as well. Based on the simulation results, we can conclude the following results:  
For the speed limit of 90 km/h for PCs, increasing TT speed limit to 70, 80, and 90 km/h would result in statistically significant reduction in the average network delay for all traffic volumes (500 to 2000 vphpl) and for all percentages of TTs (2.5-15% of total traffic volume), compared with the base condition (i.e., TT speed limit of 60 km/h); the relative delay reduction, from 60 to 70 km/h, was between 27.25% and 62.04%. When increasing TT speed limit from 60 to 80 km/h, the relative delay reduction was between 38.07% and 71.75%; and from 60 to 80 km/h, the relative delay reduction was between 38.35% and 76.32%. Additionally, increasing the speed limit for TTs from 70 to 80 km/h would result in delay reduction between 70 to 80 km/h would result in delay reduction between 14.88% and 31.67%, while increasing the speed limit for TTs from 80 to 90 km/h would result in delay reductions between 0.44% and 16.18%. For the speed limit of 100 km/h for PCs, increasing TT speed limit to 80, 90, and 100 km/h would result in a statistically significant reduction in the average network delay for all traffic volumes (500 to 2000 vphpl) and for all percentages of TTs (2.5-15% of total traffic volume) compared with the base condition (i.e., TT speed limit of 70 km/h). The relative reduction, from 70 to 80 km/h, was between 23.10% and 41.94%. When increasing the TT speed limit from 70 to 90 km/h, the relative delay reduction was between 27.15% and 60.33%, and from 70 to 100 km/h, the relative delay reduction was between 29.28% and 58.83%. In addition, increasing the speed limit for TTs from 80 to 90 km/h would \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Volume** & **\%TTs** & **DRAC \(\geq\) 3.35** & **DRAC \(\geq\) 6.0** & **TTC \(\leq\) 0.50** & **TTC \(\leq\) 1.50** & **TTC \(\leq\) 2.50** \\ \hline
500 & & 1.000 (1.414) & 1.000 (0.000) & 1.000 (0.000) & 10.00(10.00) & 0.667 (0.599) \\ \hline
1000 & & 0.333 (0.254) & 1.000 (0.000) & 20.00 (20.00) & 1.000 (0.676) & 0.583 (0.242) \\
1500 & 15 & 2.000 (2.404) & 40.00 (26.67) & 20.00 (20.00) & 0.846 (0.338) & 0.328 (0.064) \\ \hline
2000 & & 2.375 (1.019) & 3.500 (2.672) & 2.125 (1.261) & 0.551 (0.101) & 0.233 (0.025) \\ \hline \hline \end{tabular} The standard deviations are in parentheses.  
\end{table} 
Table 10: Cont.