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The friction coefficient resulting from the road surface condition directly affects the vehicle braking condition and distance. In the American Association of State Highway and Transportation Officials (AASHTO), the braking distance for different speeds, upgrades, and downgrades is recommended. However, the effects of the variations in the friction coefficient, type, and weight of vehicles are not directly taken into account. Therefore, the present study firstly examines the effects of the variations in vehicle speed, friction coefficient, longitudinal grade (upgrade and downgrade) in AASHTO 2018, and the vehicle type on braking distance. Secondly, a nonlinear analytical approach is considered to estimate braking distance and determine a critical friction coefficient that causes decreases in the braking distance. The findings indicate that under dry road surface conditions (with a friction coefficient of 0.6), the weight of the vehicle predominantly influences braking distance, resulting in heavier vehicles having shorter braking distances than lighter vehicles across all selected grades, except at a − 6%. However, when the road surface friction coefficient decreases from 0.6 to 0.18, the impact of vehicle weight surpasses the effect of the longitudinal grade, leading to an increase in braking distance. Consequently, when designing a road, it's essential to consider not only the technical road parameters and vehicle dimensions but also the vehicle weight and the road surface conditions in areas prone to snowfall, among other factors. If we wish to account for snowy and icy conditions, in addition to AASHTO's classification of wet conditions, in order to refine predictions of braking distance, the results of this study can provide valuable insights for road designers. [ABSTRACT FROM AUTHOR]

Two-way two-lane roads have a significant impact on road transportation infrastructure. Platoon formation on two-lane roads is one of the factors that affect the quality of traffic flow on two-lane roads. More specifically, the creation of a platoon increases the density of vehicles and the number of overtaking maneuvers and decreases the traffic performance of two-lane roads. The present study made an effort to investigate the effect of the platoon characteristics on the traffic flow of vehicles on two-lane roads. Moreover, it strived to develop the nonlinear regression model with a new approach to capacity calculation. Finally, a method for estimating the level of service (LOS) based on the number of followers per capacity (NFPC) and LOS classification using the KNN method was presented. Considering these aims, first, the relevant variables (which were related to platoon) including time headway (Ht), average travel speed (ATS), platoon size (PS), average platoon speed (APS), percentage of heavy vehicles (HV), percent time spent following (PTSF), number of overtaking (NO), density (ρ), and traffic flow were investigated on the examined roads. The results showed that the speed and the Ht were the most effective and the least effective platoon characteristics, respectively. Moreover, it was accompanied by the increase in PTSF which resulted in the increase in overtaking maneuver. Finally, the results regarding the developed model showed that the NFPC measure was able to predict the traffic flow of two-lane roads in a more satisfactory way compared to the two criteria, namely, PTSF and ATS in the Highway Capacity Manual (2016) since it increased ATS by 65% and decreased the capacity of two-lane roads by 21%. [ABSTRACT FROM AUTHOR]