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Providing an appropriate length of highway entrance ramps is an important feature that should be considered in freeway design. The recommended minimum acceleration lane length in the Green Book of the American Association of State Highway and Transportation Officials (AASHTO) is currently used for designing entrance ramp terminals. However, in some cases, traffic congestion and crashes are observed when heavy trucks enter freeways, which indicates they could not accelerate enough and did not reach the merge speed at the end of the acceleration lane. Therefore, several scenarios were simulated using a dynamic simulation model (TruckSim) for heavy vehicles with weight to power ratios of 61, 67, 86, and 108 (kg/kW) and grades of zero, ±3, and ±5%. Subsequently, a linear regression model was presented to calculate acceleration lane lengths. The results showed that the acceleration rate of heavy vehicles is not adequate, especially in upgrades. For example, at +5% upgrades, heavy trucks cannot accelerate to the merge speed mentioned in the AASHTO. Conversely, the acceleration rate of heavy vehicles increased in downgrades; therefore, some heavy vehicles were able to achieve merge speed along the proposed length of AASHTO.

Horizontal curves, as one of the most accident-prone parts of the road, have always attracted special attention. When a vehicle is moving on a road, the driver has to be constantly on the lookout for small deviations from the main route, possibly due to disturbances such as wind, road roughness, and unwanted entrances by the driver. These types of deviations have a corrosive effect on the driver and will eventually lead to the overturning and slipping of vehicles, so they should be minimized. AASHTO standard procedures, as the most valid road design bylaw, do not consider the effect of lateral wind, in which the effect of lateral wind in arcs has been investigated in this research. The work conducted in this research is to increase knowledge about the directional stability of vehicles that are affected by strong winds and to investigate the parameters of the study using accurate models of dynamic simulation of vehicles that are subject to winds. This study aimed to increase the understanding of vehicle sensitivity to wind due to strong lateral winds. For this purpose, the process of passing different types of vehicles through horizontal arcs in different longitudinal slopes with different velocities of the vehicle under lateral wind is simulated and the mechanical properties of vehicles under such conditions are calculated to compare with the results obtained from the passage of vehicles through a smooth horizontal arc (without longitudinal slope) and without lateral wind. Overturning angle, slip angle, lateral acceleration, and lateral coefficient of friction of vehicles in this study were calculated through software outputs, and the effect of lateral wind on the safety of arcs has been investigated. [ABSTRACT FROM AUTHOR]

Steep downgrades are known as a part of mountain roads with a significant number of severe crashes. Generally, using speed limiters such as speed bumps and speed humps are considered the most effective (considering both the time and cost) ways to reduce the number of victims. The efficiency of speed limiters depends on the driver's consciousness and the car's braking system; the latter could be problematic in wet and dark conditions of the road. The main goal of the current study is to propose an approach to correct the longitudinal profile of roads by replacing the consecutive downgrades and upgrades with a continuous downgrade. The vehicle's speed is reduced by using this method before entering an accident-prone site without the driver's involvement and breaking. To this end, a well-known software program is used under two speeds (70 and 100 km/h) in 68 different routes to analyze speed along the proposed routes. Also, another computer model is used for dynamic simulation. Dynamic analyses and regression models show that the vehicle's speed in the new proposed route is significantly reduced compared with the continuous downgrade. Moreover, results indicate that the upgrade slope of the proposed route has the most impact on reducing the final speed of the vehicles. [ABSTRACT FROM AUTHOR]