The influence of combined alignments on lateral acceleration on mountainous freeways: a driving simulator study

doi:10.1016/j.aap.2015.01.003

Accident Analysis & Prevention

Volume 76, March 2015, Pages 110-117

https://doi.org/10.1016/j.aap.2015.01.003 Get rights and content

Highlights

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Design guidelines that consider the safety impact of combined alignments are not currently available.
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A controlled parametric study is needed that examines lateral acceleration as drivers adjust their speeds across a range of combined horizontal and vertical alignments.
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A study was conducted in Tongji University’s eight- degree-of-freedom driving simulator to duplicate the wide range of combined alignments used on a mountainous freeway in China.
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Multiple linear regression models were developed to estimate the effects of the combined alignments on lateral acceleration.
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Domains were calculated to illustrate the results and to assist engineers in selecting better design alternatives for safer mountainous freeways.

Abstract

Combined horizontal and vertical alignments are frequently used in mountainous freeways in China; however, design guidelines that consider the safety impact of combined alignments are not currently available. Past field studies have provided some data on the relationship between road alignment and safety, but the effects of differing combined alignments on either lateral acceleration or safety have not systematically examined. The primary reason for this void in past research is that most of the prior studies used observational methods that did not permit control of the key variables. A controlled parametric study is needed that examines lateral acceleration as drivers adjust their speeds across a range of combined horizontal and vertical alignments. Such a study was conducted in Tongji University’s eight-degree-of-freedom driving simulator by replicating the full range of combined alignments used on a mountainous freeway in China. Multiple linear regression models were developed to estimate the effects of the combined alignments on lateral acceleration. Based on these models, domains were calculated to illustrate the results and to assist engineers to design safer mountainous freeways.

Introduction

China’s highway system has grown by about 5700 km annually since 1997 and by 2013, it reached 98,000 km in length (China Freeway News, 2014). Much of the new construction has been occurring in the mountainous areas of western China. In Hunan, by 2011 there were 4046 km of freeways and plans call for 8720 km of freeways by 2030 (Hunan Province Department of Transportation, 2010). These mountainous freeways require numerous combined horizontal and vertical alignments. Designing such alignments at acceptable costs is difficult because of the challenging terrain, and therefore engineers frequently lower the design standard of horizontal curves and combine them with short vertical curves and steep grades. However, such solutions may reduce safety because there are no quantitative guidelines for combined alignments to guide designers.

The design of horizontal curve alignments plays an important role in freeway safety. As Lamm et al. (1991a) pointed out, more than half of the fatalities on rural two-lane highways in the U.S. occur on curved roadway sections. This safety problem particularly arises on sharp horizontal curves where considerable lateral acceleration increases the difficulty of controlling the vehicle (Peter and Iagnemma, 2009). The lateral acceleration experienced by the driver when traversing a curve is a primary design parameter of horizontal curves. A too large lateral acceleration causes discomfort for drivers as they brake on curves and increases the risk of running off the road or colliding with other vehicles. Once lateral acceleration reaches a critical level, the vehicle becomes at risk of a skid or rollover (Furtado et al., 2002). Lateral acceleration is determined by both the horizontal curve radius and the vehicle speed. Speed, while controlled by the driver, is significantly influenced by vertical alignment. Therefore, vertical alignment needs to be considered when determining an acceptable horizontal radius for combined horizontal and vertical alignments.

Prior field studies (Wilson, 1968, Pei and Ma, 2003, Abdel-Aty et al., 2006, Park et al., 2010, Hanno, 2004) have examined the relationships between the alignment elements and crash data, but they have not considered the specific impacts of differing combined alignments on freeway safety. Other field studies (Said et al., 2007, Cafiso and Cava, 2009) used driving performance measures, such as operating speed, lateral offset, and lateral acceleration as safety surrogates to study the relationships between combined alignments and safety. Such studies, however, have not generated sufficient data to quantify the role of these variables. Also, because these studies were observational, they did not systematically manipulate the key variables as would be possible in an experimental setting. In contrast, driving simulator-based experiments provide the opportunity to fully control the variables expected to affect lateral acceleration. Previous driving simulator-based studies (Yang et al., 2011, Bella, 2005, Easa and Ganguly, 2005) have focused on relationships between road alignment and operating speed and between road alignment and lateral offset. However, these studies did not consider the effects of different combined horizontal and vertical alignment types on lateral acceleration.

The current study measured the effects of combined alignments on speed selection and ultimately on lateral acceleration. A mountainous freeway in Hunan Province with 71 combined horizontal and vertical alignments was modeled using Tongji University’s Driving Simulator. Four combined alignment types were separately examined: upslope-curve, downslope-curve, crest vertical curve-curve, and sag vertical curve-curve. Vehicle motion data including speed, lateral offset to the central axis of the road, vertical loads on tires, and steering angle were collected. Lateral acceleration was calculated for each combined alignment type for further analysis. Multiple linear regression analyses were used to estimate the effects of alignment on lateral acceleration, and domain analyses were performed to quantify acceptable ranges of lateral acceleration for each alignment type studied.

Section snippets

Freeway road alignment and its effects on safety

Roadway alignment has been shown to influence freeway safety (Lamm et al., 1991b). Wilson (1968) found that the crash rates on curves with radii less than 200 m were about four to five times greater than on curves with radii greater than 900 m. In China, a similar study was conducted by Pei and Ma (2003), who concluded the existence of a power relationship between the reciprocal of the curve radius and the crash rate.

Statistical models have also been developed to quantify the relationship between

Methodology

The Tongji University high-fidelity driving simulator was used to model a four-lane (two-way) mountainous freeway that incorporated 71 vertical and horizontal combined alignments. Participating drivers experienced each of these alignments as they drove the simulated mountainous freeway while their lateral acceleration was continuously acquired. Multiple linear regression was used to examine the effects of the combined alignments on lateral acceleration.

Results–regression models and domain analyses

This section presents the obtained regression models and the results of the domain analyses for the four types of combined alignments. We performed these analyses at the segment level. As stated above, each combined segment was defined as the overlapped section of the horizontal curve and the vertical alignments.

The 85th percentile of the maximum lateral acceleration (LA85) for the 21 participants was computed to represent the vehicle stability for each segment. A linear regression analysis was

Domain analysis

In accordance with the regression model, the ranges for LA85 were calculated under different combinations of R and G, and the results are shown in Fig. 5 below.

It can be observed that grade G had little influence on lateral acceleration on the upslope-curve sections while the radius of the horizontal curve substantially influenced lateral acceleration. The estimated thresholds show that for lateral acceleration to remain in the safe range, the radius of the horizontal curve should exceed 1000 m;

Domain analysis

Fig. 7 shows the ranges of LA85 as calculated under different combinations of R and G using the regression model.

The data show that adequate lateral acceleration for the safety radius depends on the grade. For example, the horizontal curve radius should exceed 1800 m on the 6% downslope to keep lateral acceleration within the safe limit. Lateral acceleration tends to be unsafely high on the same grade if the radius of the horizontal curve is shorter than 700 m.

Domain analysis

Fig. 9 shows the safety domains between the LA85 thresholds estimated with the regression model under various combinations of R and L.

The results helped to determine the safe radii of horizontal curves given the segment length. Larger curve radii would be required for longer crest combined alignments. For example, a 500 m crest combined alignment requires the curve radius to be larger than 860 m to keep lateral acceleration at a safe level, while a 1000 m crest combined alignment would require the

Domain analysis

Fig. 11 shows the safety domains between the thresholds values of LA85 calculated with the regression model for various values of R.

Unlike the other alignment types, 1/R was the only significant variable on the sag vertical curve-curve segment. Thus, the shapes of the corresponding safety domains differ from the domains obtained for other types. The results show that the lateral acceleration of the sag vertical curve-curve section was in the safe range as long as the horizontal curve radius

Summary

Designing safe mountainous freeways presents a special challenge to the engineer because the terrain often does not allow the preferred solutions. The objective of this study was to determine the domains for combined horizontal and vertical alignments on mountainous freeways by examining the effects of these alignments on the lateral acceleration–a surrogate measure of safety. This study employed a driving simulator to recreate a mountainous freeway in Hunan Province using four types of

Discussion

The advantages of using a surrogate measure of safety as a means to improve design guidelines was demonstrated in this study. The surrogate measure used, lateral acceleration, has been shown in previous research to be predictive of skid and rollover crashes (NHTSA, 2006, AASHTO, 2011).

The research findings show that each of the four types of combined curves examined influence lateral acceleration differently. These differences have usually been ignored in current design practice, and instead a

Acknowledgement

This research was sponsored by the Foundation of Road and Traffic Key Laboratory, Ministry of Education, Tongji University.

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View full text

The influence of combined alignments on lateral acceleration on mountainous freeways: a driving simulator study

Highlights

Abstract

Introduction

Section snippets

Freeway road alignment and its effects on safety

Methodology

Results–regression models and domain analyses

Domain analysis

Domain analysis

Domain analysis

Domain analysis

Summary

Discussion

Acknowledgement

Accid. Anal. Prevent.

Accid. Anal. Prevent.

Assessing crash occurrence on urban freeway by applying a system of interrelated equations

Trans. Res. Rec.

A Policy on Geometric Design of Highways and Streets

Verification of the coordination of horizontal alignment and profile at the driving simulator

Driving performance, alignment consistency, and road safety: real-world experiment

Trans. Res. Rec.

Actual driving data analysis for design consistency evaluation

Trans. Res. Rec.

Radius requirements for reverse horizontal curves on three-dimensional alignments

Am. Soc. Civil Eng.

Design radius requirements for simple horizontal curves on three-dimensional alignments

NRC Res. Press

Modeling driver visual demand on complex horizontal alignments

J. Transport. Eng.

Vehicle stability on combined horizontal and vertical alignment

Bayesian Data Analysis

Effect of the Combination of Horizontal and Vertical Alignments on Road Safety