1. Introduction
Several studies indicate that most road accidents occur at horizontal curves [
1,
2,
3,
4,
5,
6]. At horizontal curves, the risk of accidents is significantly higher than that in other sections of the road [
7]. Several critical factors are often involved in road accidents, i.e., road characteristics, vehicle performance, driver behavior, and environmental factors [
8]. Among these factors, the geometric characteristic is investigated in this study. Improvement of the road’s geometric design and proper evaluation of its dependent variables would significantly contribute to designing safer roads for reducing the number of accidents [
9]. The curve radius is the most critical parameter in the geometric design of horizontal curves [
10]. Recent studies indicate that the number of accidents has a direct connection with the curve radius [
8,
10,
11]. Based on “American Association of State Highway Transportation Officials (AASHTO) Green Book 2018” [
12], the minimum radius of a horizontal curve can be calculated by as follows.
where R is curve radius (m), V is speed (km/h), e is superelevation, and f is lateral friction. This equation is known as the point-mass model. Although Equation (1) is the basis of the geometric design in horizontal curves, the authors believe that this equation does not consider several important aspects. For example, this model does not regard the changes in the weight and dimensions of vehicles. The authors believe that the values of friction in tires can be different for various vehicles [
13,
14]. On the other hand, AASHTO only considered cross-sectional slopes (superelevation) in this model, whilst, longitudinal grades sometimes coincide with horizontal curves. In such states, the model can not apply the impact of longitudinal grades on the radius. Moreover, the point-mass model determines the radius for a specific speed (V). However, the behavior of the driver does not follow a constant pattern, and sometimes the speed of the vehicle is more or less than the designed value. Unfortunately, this difference in the driver’s behavior was not considered in the model. Regarding the above, it is evident that the main concern of authors is safety because a perfect, flawless geometric design can increase safety. Therefore, this paper has focused on assessing the above vague points to improve safety in the horizontal curves.
Recently, many studies have been carried out to measure the safety of horizontal and vertical curves. Instead of applying the lateral friction as the only parameter related to the pavement in designing curves, a more practical parameter called the safety margin has been used [
15]. The safety margin is defined as the difference between friction supply and friction demand. Safety margin analysis is a suitable method to evaluate the safety of a curve in terms of geometric performance and pavement friction [
16]. Due to changes in vehicle speed and superelevation along the curve, the safety margin should be evaluated throughout the curve [
17]. The safety margin obtained from the tire–pavement lateral friction has been introduced as an efficient method for the proper assessment of the design theory and geometric characteristics of the horizontal curve. Some studies have estimated the safety of horizontal curves based on tire–pavement friction with the help of the ADAMS/Car (Automated Dynamic Analysis of Mechanical System) simulator and the safety margin theory [
18]. Recent studies about the estimation of the safety margin have mainly focused on friction supply and friction demand on horizontal curves [
6,
10,
15,
16,
19,
20,
21]. On the other hand, the use of the clothoid transition curve between the tangent and circular curve has been suggested to increase the safety margin against skidding, especially when superelevation exceeds 12% [
16]. Transition curves improve the safety of horizontal curves because they help to gradually implement superelevation and centrifugal force. Therefore, the application of clothoid transition curves is another way to improve the safety of horizontal curves.
In this paper, the authors simulated two commonplace types of horizontal curves by using ADAMS/Car: simple circular and clothoid–circle–clothoid. They aimed to examine the safety of these types of curves based on the safety margin parameter and lateral friction.
The first innovation of this paper is the accurate assessment of the proposed lateral friction by AASHTO. For all types of vehicles, longitudinal grades, and driver behaviors (various speeds), AASHTO recommends constant friction. However, this paper examines the changes in weight and dimensions of vehicles and the impacts of these variations on the friction value. Furthermore, this study assesses the effect of longitudinal grade and changes in speed. The other novelty of this study is to present a novel methodology for evaluating safety in horizontal curves. This methodology is named the multi-body dynamic simulation process. In this methodology, the safety of the transition curve can be assessed by using the dynamic response of vehicles. The methodology uses the safety margin parameter, based on the side friction, to examine safety. By using this methodology, engineers can evaluate every type of horizontal curve. Therefore, the methodology makes easy the evaluation of safety in horizontal curves for engineers.
4. Validation
This section of the paper examines the validation of the simulation’s results. Validation was conducted in two steps: (1) validation based on longitudinal forces and (2) validation based on lateral forces. Firstly, the authors selected references for validating their simulation. These references were the study conducted by Li et al. [
31] for longitudinal forces, and also the study conducted by Li and He [
18] for lateral forces.
The results of the Pacejka Model, simulated by the ADAMS/Car, were compared by experimental data of the study by Li et al. For experimental data, Li et al., plotted “Longitudinal Force/Normal Force versus Longitudinal Slip” diagrams for a sideslip of 20 degrees [
31]. To validate the values of longitudinal forces, the authors cited
Figure 2 of the study by Li et al. Similar to thementioned figure in the study of Li et al., the authors drew a diagram for their study. Both diagrams can be seen in
Figure 17. This figure verified that the results obtained from simulation with ADAMS/Car in this study coincide with the results of Li et al. In the second step, the lateral force was used as the criteria of validation. For a constant radius and with different speeds, simulation of the tire was done. The lateral forces attained from the simulation were compared with the results of the study by Li and He in
Figure 18. Similar to
Figure 17, this figure also verified that the simulation conducted in this study is reliable.
5. Conclusions
Safety always is the most important aim that engineers and designers of roads follow. Most of the accidents on roads occur in horizontal curves. Therefore, the guarantee of safety in these parts of roads is essential. The principal model in the designing of horizontal curves (Equation (1)) has had some questionable points. This equation does not consider three important issues: the changes in weight and dimensions of vehicles, the coincidence of longitudinal grades with the horizontal curve, and differences in driver behavior. This study aimed to assess these issues. In fact, it can be said that the main concern of the authors was the improvement of safety in horizontal curves. On the other hand, one of the ways of increasing safety is the use of transition curves in geometric designs.
Consequently, for evaluating the proposed method of AASHTO standard, the lateral friction supply and lateral friction demand at various speeds of 50, 80, 110, and 130 km/h were calculated. Based on the obtained results, the margin of safety was determined for both simple circular and clothoid–circle–clothoid curves. Horizontal curves with different characteristics were simulated by ADAMS/Car. Moreover, various longitudinal grades (−6%, −3%, 0%, 3%, and 6%) were designed in the location of the horizontal curves. The basis of this simulation was the dynamic responses of vehicles. The results can be summarized as follows:
The comparison of the values of safety margin in both curves indicates that the existence of the clothoid in curves with a low radius does not have a considerable impact on increasing safety margin. In contrast, in curves with a high radius, clothoid can improve the safety margin.
For all vehicles and on the small radii, the safety margin values of both horizontal curves showed a small difference. However, by increasing the radius, the clothoid–circle–clothoid curve had better safety margin values than the simple circular curve, especially for heavy vehicles. As a result, it is a necessity to use a clothoid transition curve in the curves with a high radius. Albeit, in the case of high radii, the safety margin in both curves was nearly equal for the sedan.
On simple circular curves, instantaneous changes in the safety margin are much greater than those in the clothoid–circle–clothoid curves. The reduction of the safety margin on the clothoid–circle–clothoid curves is much less than that on the simple circular curves.
Most studies related to lateral friction factors have focused on simple circular curves, and there is no study related to the combination of horizontal and transition curves. In this study, the friction factors were applied to the clothoid–circle–clothoid system. These friction factors can be used as a control regulation of safety in geometric design for all combinations of curves. In fact, the proposed methodology can be applied for evaluating safety in any new form of curves that will be presented by engineers.
The maximum lateral friction demand for simple circular curves indicate that the obtained friction demand for a sedan on low-radius curves does not significantly differ from those proposed by AASHTO. However, for heavy vehicles (bus and 3-axle truck), the maximum lateral friction factor showed more critical values for vehicle safety. Therefore, the designers must differentiate between light and heavy vehicles.
According to the results, the maximum lateral friction factor decreases with the increasing radius in both horizontal curves.
According to the maximum lateral friction factor for simulated longitudinal grades, a maximum reduction of 13.9% was observed in the maximum lateral friction for a sedan on a 951 m clothoid curve. The corresponding reductions to 560, 252, and 79 m curves were 7.49%, 0.78%, and 0.8%, respectively. The maximum reduction of the lateral friction factor on a 79, 252, and 560 m clothoid curve for the bus was respectively 5.5%, 23.8%, and 14% relative to a simple circular curve. The corresponding values for a 3-axle truck on the designed curves were 4.1%, 17%, and 19.7%, respectively. The comparison of these values indicates that the impact of the transition curve on heavy vehicles is more significant than that on light vehicles. As the final part of the paper, the authors have two attractive proposals for future studies. By more examination of the point-mass model, it can be found that the model has two other major challenging issues:
In the point-mass model, vehicles are considered as a point mass. However, a vehicle has four tires (or more) that can have various behaviors. Therefore, a new proposal that authors have selected for their future studies is to analyze the behavior of various tires of design vehicles.
The AASHTO green book assumes that superelevation is constant in simple circular curves. On the other hand, this standard proposes that a part of superelevation implementation distance occurs inside the curve. Therefore, the initial and final parts of the curves have different superelevation than the middle part. This issue can create safety challenges for vehicles, especially vehicles with long length. Therefore, another suggested proposal of the authors is related to superelevation in simple circular horizontal curves.