Accelerating Roundabout Implementation

in the United States – Volume V of VII

Evaluation of Geometric Parameters that

Affect Truck Maneuvering and Stability

PUBLICATION NO. FHWA-SA-15-073

SEPTEMBER 2015

Updated October 2020

i 

FOREWORD

Since the Federal Highway Administration (FHWA) published the first Roundabouts 

Informational Guide in 2000, the estimated number of roundabouts in the United States has 

grown from fewer than one hundred to several thousand. Roundabouts remain a high priority for 

FHWA due to their proven ability to reduce severe crashes by an average of 80 percent. They are 

featured as one of the Office of Safety Proven Safety Countermeasures and were included in the 

Every Day Counts 2 campaign for Intersection & Interchange Geometrics.

As roundabouts became more common across a wide range of traffic conditions, specific 

questions emerged on how to further tailor certain aspects of their design to better meet the needs 

of a growing number and diversity of stakeholders. The substantial work performed for this 

project – Accelerating Roundabout Implementation in the United States – sought to address 

several of the most pressing issues of National significance, including enhancing safety, 

improving operational efficiency, considering environmental effects, accommodating freight 

movement and providing pedestrian accessibility. This work represents yet another notable step 

forward in advancing roundabouts in the United States. 

The electronic versions of each of the seven report volumes that document this project are 

available on the Office of Safety website at http://safety.fhwa.dot.gov/. 

Michael S. Griffith 

Director

Office of Safety Technologies 

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard, 

specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
ii

TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-SA-15-073

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Accelerating Roundabouts in the United States: Volume V of VII

– Evaluation of Geometric Parameters That Affect Truck

Maneuvering and Stability

5. Report Date

September 2015

6. Performing Organization Code:

7. Author(s)

Lee A. Rodegerdts1, Ashleigh Griffin1, Hermanus Steyn1, Mehdi

Ahmadian2, Yunbo Hou2, and Mehdi Taheri2

8. Performing Organization Report No.

11861 Task 7

9. Performing Organization Name and Address

1

Kittelson & Associates, Inc., 610 SW Alder St. Suite 700,

Portland, OR 97205

2

Center for Vehicle Systems and Safety, Virginia Tech, MC-0901,

Blacksburg, VA 24061

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-10-D-00023-T-11002

12. Sponsoring Agency Name and Address

Office of Safety

Federal Highway Administration

United States Department of Transportation

1200 New Jersey Avenue SE

Washington, DC 20590

13. Type of Report and Period Covered

Technical Report

August 2011 through September 2015

14. Sponsoring Agency Code

FHWA HSA

15. Supplementary Notes

The Contracting Officer’s Technical Manager (COTM) was Jeffrey Shaw, HSST Safety Design Team

16. Abstract

This volume is fifth in a series of seven. The other volumes in the series are: Volume I - Evaluation of Rectangular RapidFlashing Beacons (RRFB) at Multilane Roundabouts, Volume II – Assessment of Roundabout Capacity Models for the 

Highway Capacity Manual, Volume III – Assessment of the Environmental Characteristics of Roundabouts, Volume IV – A 

Review of Fatal and Severe Injury Crashes at Roundabouts, Volume VI – Investigation of Crosswalk Design and Driver 

Behaviors, and Volume VII – Human Factor Assessment of Traffic Control Device Effectiveness. These reports document a 

Federal Highway Administration (FHWA) project to investigate and evaluate several important aspects of roundabout 

design and operation for the purpose of providing practitioners with better information, leading to more widespread and 

routine implementation of higher quality roundabouts.

This report presents an evaluation of the combination of horizontal alignment, vertical alignment, cross section, and curb 

treatments that sometimes leads to truckload instability or rollover within a roundabout. The study includes a literature 

review, a review of a sample of incidents involving truck overturns at roundabouts, and a modeling simulation using a 

multibody dynamics simulation model to evaluate the impact of various design and behavior factors that may impact truck 

stability in single-lane and two-lane roundabouts. The most significant factor found to affect truck stability was truck speed, 

followed to a lesser degree by the cross-sectional geometry of two-lane circulatory roadways and the use of the truck apron.

17. Key Words

Roundabouts, , trucks, overturn crashes, instability,

geometric design, apron, curb, simulation

18. Distribution Statement

No restrictions. This document is available to the public at

http://safety.fhwa.dot.gov/

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

93

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
iii

TABLE OF CONTENTS

FOREWORD.................................................................................................................................. 

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NOTICE .. i

QUALITY ASSURANCE STATEMENT ... i

TECHNICAL REPORT DOCUMENTATION PAGE .. Error! Bookmark not defined.

TABLE OF CONTENTS . iii

LIST OF FIGURES .......... vi

LIST OF TABLES ... .x

CHAPTER 1. INTRODUCTION ... .1

CHAPTER 2. LITERATURE REVIEW . .3

ACCOMMODATION OF STANDARD TRUCKS AT MULTILANE ROUNDABOUTS

 3

ACCOMMODATION OF OVERSIZE/OVERWEIGHT VEHICLES 8

EVALUATION OF TRUCK STABILITY ... 8

CHAPTER 3. TRUCK CRASH REVIEW ...... .11

CHAPTER 4. SIMULATION MODEL DESCRIPTION . .13

TRUCK MODELS .. 13

Truck Configurations .. 13

Truck Parameters . 14

Baseline Truck . 15

ROUNDABOUT MODELS ... 15

Modeling Roundabouts . 15

Roundabout Components ... 16

Roundabout Parameters .... 18

Baseline Roundabouts . 19

Baseline Single-lane Roundabout .. . 19

Baseline Two-lane Roundabout . . 21

CHAPTER 5. SIMULATION CASE STUDIES .... 23

TRUCK TRAVEL PATH .. 30

TRUCK SPEED .. 30

CHAPTER 6. SIMULATION RESULTS . 33

ROLLOVER INDEX .... 33
iv

EFFECT OF ROUNDABOUT ICD ON TRUCK STABILITY ......................................... 34

Right Turns .......................................................................................................................... 35

Through Movements ........................................................................................................... 37

Left Turns ............................................................................................................................ 38

EFFECT OF CIRCULATORY ROADWAY CROSS-SECTION ON TRUCK 

STABILITY .............................................................................................................................

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Right Turns . 40

Through Movements ..... 41

Left Turns ..... 42

EFFECT OF TRUCK CONFIGURATION ON TRUCK STABILITY 44

Right Turns .. 45

Through Movements ....... 47

Left Turns ... 48

EFFECT OF LOAD CONFIGURATION ON TRUCK STABILITY .. 49

Right Turns................. 50

Through Movements .. 52

Left Turns................................................................................................ 54

EFFECT OF ROUNDABOUT TILT ON TRUCK STABILITY .. 56

Right Turns .... 57

Through Movements .... 59

Left Turns... 60

EFFECT OF ENTRY SPEED ON TRUCK STABILITY.... 62

Right Turns..... 63

Through Movements .... 65

Left Turns .. 66

SUMMARY OF SIMULATION ANALYSIS .... 68

Effect of Roundabout Inscribed Circle Diameter on Truck Stability 68

Effect of Circulatory Roadway Cross-section on Truck Stability...... 70

Effect of Truck Configuration on Truck Stability .. 71

Effect of Load Configuration on Truck Stability.... 72

Effect of Roundabout Tilt on Truck Stability ......... 75

Effect of Entry Speed on Truck Stability ............. ....................................................... 77

CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS ... .79
v 

DESIGN CONSIDERATIONS .............................................................................................. 

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Circulatory Roadway Cross-Section .. 79

Truck Apron . 79

Speed .. 79

Truck Types and Loads ... 80

EDUCATION PROGRAMS AND DRIVER PRACTICE ..... 80

RECOMMENDATIONS FOR ADDITIONAL STUDY......... 80

REFERENCES .83
vi 

LIST OF FIGURES

Figure 1. Diagram. Accommodation of trucks, case 1: trucks encroach on adjacent 

approaching and circulating lanes. 4 ............................................................................................. 

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Figure 2. Diagram. Accommodation of trucks, case 2: trucks encroach only on adjacent 

circulating lanes. 5

Figure 3. Diagram. Accommodation of trucks, case 3: trucks stay in lane for approach and 

circulating. . 6

Figure 4. Diagram. Basic geometric elements of a roundabout, adapted from Exhibit 6-2 in 

NCHRP Report 672.[11] . 16

Figure 5. Diagram. Roundabout tilt from the perspective of a driver on the approaching 

roadway . 18

Figure 6. Diagram. Horizontal geometry of 42.7-m (140-ft) baseline single-lane roundabout 

model. 20

Figure 7. Diagram. Cross-sectional geometry of 42.7-m (140-ft) baseline single-lane 

roundabout model. . 21

Figure 8. Diagram. Horizontal geometry of 54.9-m (180-ft) baseline two-lane roundabout 

model. 22

Figure 9. Diagram. Cross-sectional geometry of 54.9-m (180-ft) baseline two-lane 

roundabout model. .. 22

Figure 10. Equation. Rollover index. ...... 33

Figure 11. Equation. Left wheel load (rear axles). ..... 33

Figure 12. Equation. Right wheel load (rear axles). .. 33

Figure 13. Graph. Effect of roundabout diameter on rollover index (single-lane 

roundabouts, right turn). . . 36

Figure 14. Graph. Effect of roundabout diameter on rollover index (two-lane roundabouts, 

right turn). .. 36

Figure 15. Graph. Effect of roundabout diameter on rollover index (single-lane 

roundabouts, through movement). . 37

Figure 16. Graph. Effect of roundabout diameter on rollover index (two-lane roundabouts, 

through movement). .................................................................................................................... 37

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Figure 17. Graph. Effect of roundabout diameter on rollover index (single-lane 

roundabout, left turn). ..... . 38

Figure 18. Graph. Effect of roundabout diameter on rollover index (two-lane roundabout, 

left turn). ... . 39

Figure 19. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout (twolane, right turn). . 40
vii 

Figure 20. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout (twolane, through movement). . 41 ..........................................................................................................

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Figure 21. Effect of road crown and truck apron on rollover index for a 54.9-m (180-ft) 

roundabout (two-lane, through movement). . . 42

Figure 22. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout (twolane, left turn). ......................................................................................................................... 43

Figure 23. Effect of road crown and truck apron on rollover index for a 54.9-m (180-ft) 

roundabout (two-lane, left turn). .. 43

Figure 24. Effect of truck apron and road crown on rollover index at different circulating 

speeds (16 km/h [10 mph] and 24 km/h [15 mph])(two-lane, left turn). . 44

Figure 25. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane roundabout, right turn). 46

Figure 26. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane roundabout, right turn). ... 46

Figure 27. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement). ... 47

Figure 28. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement). .................................................................................................. 47

Figure 29. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn).................................................................................................................. 48

Figure 30. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, left turn). .................................................................................................................... 49

Figure 31. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, right turn). .. . 51 ...........................................................................................................

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Figure 32. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout (twolane, right turn). 51

Figure 33. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement). .. 53

Figure 34. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout (twolane, through movement) 53

Figure 35. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn). ... 55

Figure 36. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout (twolane, left turn). ... .. 55

Figure 37. Effect of truck weight on rollover index at different circulating speeds: 16 km/h 

(10 mph) (left plot) and 24 km/h (15 mph) (right plot); (140-ft single-lane roundabout, left 

turn). .. 56

Figure 38. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, right turn). ... 58
viii

Figure 39. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, right turn) . . 58 ................................................................................................................

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Figure 40. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement). . 59

Figure 41. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement). . 59

Figure 42. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn).. 60

Figure 43. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(single-lane, left turn)... 61

Figure 44. Effect of roundabout tilt on rollover index at different circulating speeds: 16 

km/h (10 mph) (left plot) and 24 km/h (15 mph) (right plot); (single-lane roundabout, left 

turn). . 62

Figure 45. Effect of entry speed on rollover index for a 42.7-m (140-ft) roundabout (singlelane, right turn). . 64

Figure 46. Effect of entry speed on rollover index for a 54.9-m (180-ft) roundabout (twolane, right turn). .. . 64

Figure 47. Effect of entry speed on rollover index for a 42.7-m (140-ft) roundabout (singlelane, through movement). 65 ...........................................................................................................

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Figure 48. Effect of entry speed on rollover index for a 54.9-m (180-ft) roundabout (twolane, through movement). 66

Figure 49. Effect of entry speed on rollover index for a 42.7-m (140-ft) single-lane 

roundabout (single-lane, left turn). .. 67

Figure 50. Effect of entry speed on rollover index for a 54.9-m (180-ft) two-lane roundabout 

(two-lane, left turn). .. 67

Figure 51. Effect of roundabout diameters on peak rollover index by movement in singlelane roundabouts. 68

Figure 52. Effect of roundabout diameters on peak rollover index by movement for twolane roundabouts. . 69

Figure 53. Effect of truck apron and road crown on peak rollover index for a 54.9-m (180-

ft) two-lane roundabout by movement. .. 70

Figure 54. Effect of truck configuration on peak rollover index for a 42.7-m (140-ft) 

roundabout by movement. .. 71

Figure 55. Effect of truck configuration on peak Rollover Index for a 54.9-m (180-ft) 

roundabout by movement. . 72 ........................................................................................................ 

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Figure 56. Effect of truck weight on peak Rollover Index for a 42.7-m (140-ft) roundabout 

by movement 73 ................................................................................................................................ 

Figure 57. Effect of truck weight on peak Rollover Index for a 54.9-m (180-ft) roundabout 

by movement. 73
ix

Figure 58. Effect of roundabout tilt on peak rollover index for a 42.7-m (140-ft) roundabout 

by movement. . 76 ..............................................................................................................................

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Figure 59. Effect of roundabout tilt on peak Rollover Index for a 54.9-m (180-ft) 

roundabout by movement. ... 76

Figure 60. Effect of entry speed on peak rollover index for a 42.7-m (140-ft) roundabout by 

movement. .. 77

Figure 61. Effect of entry speed on peak rollover index for a 54.9-m (180-ft) roundabout by 

movement. .... 78
x 

LIST OF TABLES

Table 1. Recommended design parameters for Case 1, Case 2 and Case 3 roundabout 

designs. ........................................................................................................................................... 7

Table 2. Dimensions of WB-67 model. ...................................................................................... 14

Table 3. Weight properties of WB-67 model. ........................................................................... 14

Table 4. Center of gravity estimates of semi-truck (WB-67) model. ...................................... 15

Table 5. Dimensions and weight of single-unit truck (SU-30) model. .................................... 15

Table 6. Dimensions and weight of a tractor with double trailers (B-train) model. ............. 15

Table 7. Roundabout parameters. ............................................................................................. 19

Table 8. Roundabout dynamic modeling cases. ....................................................................... 25

Table 9. Roundabout diameter cases considered for evaluating truck dynamics in 

roundabouts. ................................................................................................................................ 35

Table 10. Roadway cross-section cases considered for evaluating truck dynamics in 

roundabouts. ................................................................................................................................ 40

Table 11. Truck configuration cases considered for evaluating truck dynamics in 

roundabouts. ................................................................................................................................ 45

Table 12. Load configuration cases considered for evaluating truck dynamics in 

roundabouts. ................................................................................................................................ 50

Table 13. Roundabout tilt slope for a 42.7-m (140-ft) and 54.9-m (180-ft) roundabout. ..... 57

Table 14. Entry speed cases considered for evaluating truck dynamics. ............................... 63
1 

CHAPTER 1. INTRODUCTION 

The accommodation of large trucks through a modern roundabout has profound influence on 

many aspects of its design, which in turn affects the overall performance as it concerns all users.

As will be noted in the literature review, guidance is available for the accommodation of trucks 

in the horizontal plane, that is, the swept paths caused by the off-tracking of truck trailers as they 

pass through a roundabout. However, to date, a key aspect of truck performance has not been 

studied in depth in the United States: the combination of horizontal alignment, vertical 

alignment, cross section, and curb treatments that sometimes leads to truckload instability or 

rollover. 

This study attempted to gain insight into the effect that various combinations of these design 

elements have on truck stability. The study includes a literature review, a review of a sample of 

incidents involving truck overturns at roundabouts, and a modeling simulation using a multibody 

dynamics simulation model to evaluate the impact of various design and behavior factors that 

may impact truck stability in roundabouts. The model was used to perform a study of variation of 

key input parameters that: 

• Included three different truck configurations: a WB-67 semi-truck (a tractor with a 16.2-

m (53-ft) box trailer) was the baseline; a single-unit truck (straight truck); and tractor

with double 12.2-m (40-ft) trailers (B-train).

• Included three truckload configurations (full, half full, and empty) that were modeled to

determine their effect on the trucks’ lateral stability.

• Incorporated different speeds at the roundabout’s entry, circulatory roadway, and exit.

Speeds were varied to evaluate the influence of speed on lateral acceleration.

The roadway features considered for the parametric study included: 

• Varying the inscribed circle diameter (ICD).

• Using different circulatory roadway cross-sections. The following scenarios were used:

(1, baseline) 2% cross-slope to the outside; (2) 2% cross-slope to the outside and a truck

apron; and (3) a crowned circulatory roadway with a truck apron.

• Varying the overall tilt of the roundabout (that is, the grade of the roundabout

perpendicular to the approaching direction of the subject vehicle).

This report outlines the study’s parameters and methods and the results of modeling using a 

commercially available truck simulation program.

3 

CHAPTER 2. LITERATURE REVIEW

This section summarizes the literature review of existing studies and reports documenting how 

trucks are affected by the geometric design of roundabouts. The research team reviewed several

reports, presentations, and fact sheets to identify guidance for roundabout design to 

accommodate trucks. The following sections summarize the applicable findings. 

ACCOMMODATION OF STANDARD TRUCKS AT MULTILANE ROUNDABOUTS

The Minnesota and Wisconsin Departments of Transportation (MnDOT and WisDOT) jointly 

funded a study to understand how to better accommodate standard trucks at multilane 

roundabouts (roundabouts with two or more lanes) on state trunk highways by studying current 

design practices, collecting input from the truck industry, and developing guidelines.[1] Standard 

trucks, or trucks that are allowed to use the highway system without needing a permit, were 

defined as a WB-65 in Wisconsin and a WB-62 for Minnesota; oversize/overweight permitted 

loads were not included in the study. The study was conducted in four phases that included a 

synthesis of current design practice as summarized into three design cases, video data collection 

at 18 roundabouts, development of design guidance and recommendations, and a summary of 

findings and recommendations on how to proceed with implementation. The study included an 

evaluation of geometry, operations, and crash data. 

Phase 1 (review of current design practices) identified three ‘case types’ to describe the 

movement of trucks through a multilane roundabout. In Case 1, illustrated in figure 1, the 

roundabout is designed to allow trucks to encroach into adjacent lanes as they approach and 

traverse the roundabout. In Case 2, illustrated in figure 2, the roundabout is designed to 

accommodate trucks in their lane as they approach and enter the roundabout, but may require 

trucks to encroach into adjacent lanes as they circulate and exit the roundabout. In Case 3, 

illustrated in figure 3, the roundabout is designed to accommodate trucks in one lane as they 

approach and traverse the entire roundabout. 
4 

Source: Wisconsin DOT Facilities Development Manual.

[5]

Figure 1. Diagram. Accommodation of trucks, case 1: trucks encroach on adjacent 

approaching and circulating lanes. 
5 

Source: Wisconsin DOT Facilities Development Manual.

[5]

Figure 2. Diagram. Accommodation of trucks, case 2: trucks encroach only on adjacent 

circulating lanes. 
6 

Source: Wisconsin DOT Facilities Development Manual.[5]

Figure 3. Diagram. Accommodation of trucks, case 3: trucks stay in lane for approach and 

circulating. 

The results showed that Case 2 and 3 roundabouts can be designed such that they are consistent

with geometric design principles for roundabouts. Case 1 roundabouts had slightly more truck 

crashes (sideswipe was the most common) and caused delays at entries due to truck 

encroachment. Video review showed that trucks at Case 1 roundabouts encroached on adjacent 

lanes (as expected), and on rare occasions, the trucks drove over outside entry curbs. Trucks at 

Case 2 and Case 3 roundabouts generally stayed in their lane and used the truck apron when 
7 

traffic was present in an adjacent lane. When traffic was not present in an adjacent lane, truck 

drivers typically avoided the use of the truck apron regardless of the design case. One noted 

limitation of the study is that it did not address increases in other crashes at Case 2 and Case 3 

roundabouts due to a larger radius.[3]

Recommended design parameters for each of the cases are summarized in Table 1.

[4]

Table 1. Recommended design parameters for Case 1, Case 2 and Case 3 roundabout 

designs.

Roundabout 

Design Element

Case 1

Parameter

Case 2

Parameter

Case 3

Parameter

Inscribed circle diameter (ICD) 45.7-57.9 m 

(150-190 ft)

48.8-64.0 m 

(160-210 ft)

54.9-67.1 m 

(180-220 ft)

Inner circulatory lane width 3.4-4.0 m 

(11-13 ft)

3.4-4.0 m 

(11-13 ft)

4.0-4.6 m 

(13-15 ft)

Outer circulatory lane width 4.0-4.6 m 

(13-15 ft)

4.0-4.6 m 

(13-15 ft)

4.6-5.5 m 

(15-18 ft)

Entry width 8.5-9.8 m 

(28-32 ft)

9.8-10.4 m 

(32-34 ft)

9.8-10.4 m 

(32-34 ft)

Entry radius 20+ m 

(65+ ft)

20+ m 

(65+ ft)

20+ m 

(65+ ft)

Exit width 8.5-9.8 m 

(28-32 ft)

8.5-9.8 m 

(28-32 ft)

8.5-9.8 

(28-32 ft)*

* where larger radius or tangential exit is used

The Wisconsin DOT Facilities Development Manual (FDM) [5] includes guidance for 

accommodating trucks at roundabouts. The standard design vehicle that WisDOT uses for trucks 

is a WB-65. The document states that some intersections must also check for accommodation of 

WB-92, farm combine, and 24.4-m (80-ft) mobile home transport vehicles.

The FDM provides details on a number of geometric elements.[5]

 Truck aprons should be used, 

with the truck apron sloped outward at 1%. The minimum truck apron width on both single and 

multilane roundabouts is 3.6-m (12-ft). Some cases require additional space for off-tracking. 

Roundabouts generally have a crowned circulatory roadway, 2/3 inward and 1/3 outward. Where 

off-tracking of trucks may occur, a 100-mm (4-in.) sloped curb and gutter on the outside of the 

approach may be used; a traversable concrete pad may also be used, provided it does not conflict 

with the location of pedestrian paths or crossings. The maximum recommended cross-slope in 

the circulatory roadway is 2%. 

The FDM describes the three design cases for accommodating trucks at multilane roundabouts as 

described in the Joint WisDOT-MnDOT study and states that Case 3 is preferred when feasible

and where there are approximately 100 large trucks per day forecasted to use the roundabout. 

Some of the guidance for Case 2 and Case 3 roundabouts includes the use of wider entries than 

Case 1 roundabouts; longer curve lengths on the approach geometry; offset-left alignments (left 

of the center of the roundabout); larger, longer radii with straight tangent sections between 

curves rather than tight entry radii curves; and width transitions. 
8 

ACCOMMODATION OF OVERSIZE/OVERWEIGHT VEHICLES

The WisDOT FDM[5] includes guidance for accommodating oversize/overweight (OSOW) 

trucks at roundabouts. It is recommended that proposed intersections be designed to 

accommodate oversize/overweight (OSOW) multiple-trip-permitted vehicles within the curb 

face, but intersections that did not accommodate these vehicles prior to improvements do not 

have to accommodate the vehicle after improvements. Intersections on the OSOW Freight 

Network should be able to accommodate the anticipated OSOW turning movements. 

A report led by Kansas State University[2] summarized current practice and research from 

different states and countries related to the effects that OSOW vehicles have on roundabout 

location, design, and accommodation to fill in gaps in information about roundabout design and 

operations for these vehicles. 

Based on two surveys of the trucking industry, the report found the following strategies have 

merit in accommodating OSOWs:[2]

• Provide wide truck aprons of at least 3.6-m (12-ft) with a minimal slope and mountable

curb.

• Design center islands to accommodate through and turning movements. This may include

making fixed objects in the central island removable or eliminating them altogether.

• Provide paved area behind curb (right side for off tracking).

• Design signs to be removable and use setbacks for permanent fixtures (light poles).

• Allow trucks to cross over the median (stamped, depressed, or corrugated) before

entering the roundabout, in a counter flow direction, to make a left turn in the opposing

lane and then cross back over after the turn.

• Provide right turn lanes; these are sometimes gated when separated from the roundabout.

The researchers drew a number of general conclusions from the surveys. Vertical clearance in 

general, and curbs in particular, are an issue for large trucks and OSOWs. The design of these 

elements should be considered where OSOWs are accommodated. A maximum height of 900-

mm (3-in.) should be considered for splitter islands, truck aprons, and curbs. The design of 

roundabouts, like bridges and many other roadway network components, should consider the 

needs of OSOW trucks, and the development of statewide freight networks can help ensure that 

key routes are not overlooked. 

EVALUATION OF TRUCK STABILITY

A study of the stability of articulated trucks was conducted in 1978 in the United Kingdom[7] and 

is cited in literature in the United States.

[7] The goal of this study was to determine what vehicle 

characteristics contributed to trucks rolling over, particularly articulated trucks. The study found 

that articulated vehicles were twice as likely to roll over as rigid vehicles in all maneuvers 

leading to injury accidents, and at roundabouts they were four times as likely to roll over.
9 

The research team used three methods to evaluate the roll stability of different trucks. They 

conducted tilt tests in which they tilted the trucks on a platform until the truck reached a point of 

balance. They drove some vehicles into rollover conditions to review the stability of different 

vehicles in these conditions. Finally, they conducted a computer simulation of truck rollovers 

and compared these to the dynamic tests to determine how well the computer simulation 

performed relative to the dynamic tests. 

The results of this study revealed a number of findings. The tilt method was a good indication of 

roll stability. The computer simulation needed improvements on its modeling of the impact of 

springs on stability. Multiple turning maneuvers (an S-curve, or a roundabout) did not appear to 

increase the risk of a rollover above that of maneuvering through a single curve of the same 

radius. The lateral acceleration at rollover ranged from 0.2 g to 0.3 g (0.2 to 0.3 times the 

acceleration of gravity). The articulation angle between the tractor and trailer did not appear to 

have a significant effect on the roll stability of the vehicles. The trailers rolled more than the 

tractors and eventually pulled the tractors over.

The authors found that the height of the center of gravity of the vehicle and the load were 

significant factors in the roll behavior of trucks tested. Loads with high centers of gravity or with 

centers of gravity further forward than the center of the trailer may experience lower rollover 

speeds. When the suspension stiffness was higher, the lateral acceleration associated with 

rollover was higher. Trucks with higher suspension stiffness rolled less relative to the rear axle 

for a given lateral acceleration. Therefore, the authors concluded that some improvement in roll 

stability can be obtained by increasing the spring stiffness and using spring bases and overall 

width and track as wide as permitted. Liquid movement did not appear to have a significant 

effect on vehicle roll. Power steering did not cause more rollover crashes than manual steering.

The authors offered the opinion that the greatest reduction in rollover crashes may be reached by 

educating drivers that high speeds and sharp turns are the primary contributing factors to 

rollovers, and that rollovers can occur even at very low speeds within tight curves.

SUMMARY OF LITERATURE REVIEW

Several common themes from the five studies reviewed were noted and used to develop 

hypotheses for testing in the simulation modeling phase of the study. The impact of the truck’s 

load and center of gravity is thought to have an impact on truck stability. The speed of the truck 

in the roundabout is expected to have a significant impact on the truck’s likelihood of rollover. 

The impact of the circulatory roadway cross-section design is expected to impact the truck’s 

stability, although not all of the studies agreed on what impact it would have. Finally, the design 

and use of the truck apron is expected to impact truck stability in roundabouts.

11 

CHAPTER 3. TRUCK CRASH REVIEW

To help inform the truck stability modeling conducted later in this project, as well as to verify 

that a better understanding of this issue is needed, the project team conducted a review of truck 

crashes. A combination of Internet searches, professional contacts, and personal knowledge from 

project team members, state and local agencies, and FHWA staff helped to identify possible 

truck crashes at roundabouts in the United States. The project team collected and analyzed crash 

reports and news stories for 37 crashes.

The crash review produced the following observations. Of the 37 crashes identified, rollover 

crashes (over 30 crashes) were the most common crash type, followed by trucks getting stuck (3 

crashes), fixed-object (1 crash), and sideswipe (1 crash). Speed was identified as a likely factor 

in five crashes; the presence of a high-speed approach (without confirmation of whether the 

driver was speeding) was identified in an additional crash. No information on actual travel speed 

at the time of the crash was available; therefore, this information is based on whether speed was 

mentioned as a contributing factor. Load shift was identified as a potential contributing factor in 

six crashes. Several truck types were involved in crashes, with truck-and-trailer combinations the 

most common. Other truck types included tankers (4 crashes), logging trucks (3 crashes), dump 

trucks (1 crash), and concrete trucks (1 crash). At least four crashes occurred at roundabouts that 

were reported to be newly constructed and therefore may have been unfamiliar to drivers. It was 

unclear what role the roadway cross-section and truck apron played in the majority of the 

crashes.

Observations from the crash review were consistent with issues identified in prior research. 

Rollover crashes appear to be a commonly reported type of truck crash at roundabouts, and speed

is likely a key contributing factor to rollover crashes.

13 

CHAPTER 4. SIMULATION MODEL DESCRIPTION

TRUCK MODELS

For the simulation modeling portion of this research, trucks were modeled and tested in 

TruckSim®. TruckSim® is a multibody simulation program that is designed for simulating and 

analyzing the dynamic behavior of medium to heavy trucks, buses, and articulated vehicles.[8]

The mathematical models in TruckSim® are built on decades of research in characterizing 

vehicles and reproducing their behavior with mathematical models[10]

, which provides 

reasonably accurate simulation results. In addition, the road input components in TruckSim® can 

create complicated road models such as roundabouts. 

To create truck models in TruckSim®, users first select a configuration and then define the 

vehicle parameters. In general, a vehicle consists of several systems, such as tires, suspension 

system, steering system, powertrain, and so forth. The performance of each of these systems is

defined by several key parameters. In TruckSim®, these key parameters and the dimensions and 

weight properties for the tractor and trailer are used to build a truck model. When the 

configuration and all the parameters are determined by the user, TruckSim® creates a parametric 

truck model and uses the parameters in its mathematical models to derive simulation results.

Truck Configurations

Three truck configurations commonly operated on the roads were selected and modeled for this 

study. The first was a WB-67 interstate semi-trailer, a combination of a (1) three-axle tractor 

with a 5.69 m (18.7-ft) wheelbase and (2) a two-axle, 16.2 m (53-ft) long box trailer. WB-67 

indicates that the combined wheelbase (the distance from tractor’s front axle to the center line of 

trailer’s two rear axles) is 20.4-m (67-ft). In addition, the tractor and trailer are hitched by a fifthwheel coupling, widely used in North America. Hence, in dynamic modeling, the tractor and 

trailer act as two jointed bodies; i.e., when a semi-truck is moving, its tractor and trailer may 

have different dynamic responses to road and steering inputs. 

The second modeled truck configuration is the SU-30 single-unit truck, a two-axle truck with a 

cuboid-shaped cargo area. The wheelbase for this single-unit truck model is 6.1-m (20-ft), and 

the length is 9.1m (30-ft). The single-unit truck’s cargo area is fixed to the vehicle body, and it 

behaves as one body in dynamics, unlike articulated vehicles. According to prior research, the 

single-unit truck may be more stable when traversing roundabouts. 

The last truck configuration is the B-train, a combination of a tractor (identical to that used in the 

WB-67) with double trailers (each 12.2-m (40-ft) long with a wheelbase of 10.9-m (35.8-ft). The 

resulting truck configuration is approximately the same length as the WB-92D design vehicle. A 

fifth-wheel coupling is used between the tractor and the first trailer, and between the first trailer 

and the second trailer. Thus, this combination performs as three jointed bodies in dynamics. This 

configuration can be used to determine whether adding length and pivot points decreases truck 

lateral stability at roundabouts.
14 

Truck Parameters

The parameters for these three truck configurations are listed in table 2 through table 6. The 

dimensions are those for typical design vehicles from “A Policy on Geometric Design of 

Highways and Streets (2011),” published by the American Association of State Highway and 

Transportation Officials.

[9] The weight properties of the three truck configurations were 

determined depending on the truck type. The full weight for the semi-truck was set to meet the 

maximum gross weight limit without special permit that states enforce on the interstate system,

36,300 kg (80,000 lbs). The gross weight for single-unit trucks was determined based on trucks 

commonly operated on roadways. For the tractor with double trailers, the general weight limit for 

all vehicles is 36,300 kg (80,000 lbs); however, overload permits can be issued. This overload 

gross weight limit varies from state to state, and in some states it is not specified. According to a 

report,[10] the gross weight for a tractor with double trailers, representing the average overloaded 

truck weight limits in Minnesota and its neighboring states and provinces, is 63,500 kg (140,000 

lbs). 

The loads fixed to the trailers were assumed to have a uniform density and rectangular shape. For 

simulating full-load conditions, the load was assumed to fill the entire trailer. In the half-full 

cases, the simulated load was assumed to fill the bottom half of the trailer. Shifting loads were

not considered here; however, as a major issue in heavy truck steering dynamics, they should be 

considered in follow-up studies. 

Table 2. Dimensions of WB-67 model.

Item Property Tractor Trailer

1 Wheelbase 5.69 m (224.0 in.) 13.26 m (522.0 in.)

2 Bogie Spread 1.28 m (50.4 in.) 1.22 m (48.0 in.)

3 Track Width 1.84 m (72.5 in.) 1.84 m (72.5 in.)

4 Fifth Wheel Height 1.10 m (43.3 in.) ---

5 Length 7.48 m (294.5 in.) 16.15 m (636.0 in.)

6 Width 1.75 m (96.0 in.) 2.59 m (102.0 in.)

7 Height 2.87 m (113.0 in.) 3.83 m (150.4 in.)

Table 3. Weight properties of WB-67 model. 

Component(s) Weight

Tractor 8435 kg (18,600 lbs)

Trailer (Empty) 6531 kg (14,400 lbs)

Tractor + Trailer (Empty) 14966 kg (33,000 lbs)

Tractor + Trailer (Half full) 25624 kg (56,500 lbs)

Tractor + Trailer (Full) 36281 kg (80,000 lbs)
15 

Table 4. Center of gravity estimates of semi-truck (WB-67) model. 

Component(s) xCG yCG zCG

Tractor -2.35 m (-92.5 in.) 0.0 in. 0.96 m (37.8 in.)

Tractor + Trailer (Empty) -15.86 m (-264.6 in.) 0.0 in. 1.17 m (46.1 in.)

Tractor + Trailer (Half full) -8.94 m (-352.0 in.) 0.0 in. 1.78 m (70.1 in.)

Tractor + Trailer (Full) -12.13 m (-477.6 in.) 0.0 in. 2.33 m (91.7 in.)

Note: The x-axis runs from the back of the trailer to the front of the cab, with zero at the tractor 

front-axle center line; the y-axis runs along the front axle from the left side of the truck to the 

right side of the truck, with zero at the middle of the axle; and the z-axis runs from the ground 

to the top of the truck, with zero at the ground.

Table 5. Dimensions and weight of single-unit truck (SU-30) model. 

Item Property SU-30

1 Wheelbase 6.10 m (240.0 in.)

2 Length 9.14 m (360.0 in.)

3 Height 3.81 m (150.0 in.)

4 Width 2.44 m (96.0 in.)

5 Gross weight 1270 kg (28,000 lbs)

Table 6. Dimensions and weight of a tractor with double trailers (B-train) model. 

Item Property Tractor 1st trailer 2nd trailer

1 Wheelbase 5.69 m (224.0 in.) 10.92 m (430.0 in.) 10.92 m (430.0 in.)

2 Length 7.48 m (294.5 in.) 12.19 m (480.0 in.) 12.19 m (480.0 in.)

3 Height 2.87 m (113.0 in.) 3.81 m (150.0 in.) 3.81 m (150.0 in.)

4 Width 2.44 m (96.0 in.) 2.44 m (96.0 in.) 2.44 m (96.0 in.)

5 Gross weight 63500 kg (140,000 lbs)

(tractor and trailers)

n/a n/a

Baseline Truck

In this study, a baseline truck was chosen, and its characteristics were modified to determine the 

effects of truck characteristics on lateral stability in roundabouts. Of the three commonly 

operated truck configurations chosen and discussed in prior sections, the WB-67 semi-truck is 

used more widely than the tractor with double trailers. Although SU-30 trucks are also common, 

the goal of this study was to evaluate the dynamic behavior of heavy articulated trucks. 

Therefore, the fully loaded WB-67 semi-truck of GVWT 36,300 kg (80,000 lbs) was selected as 

the baseline truck model. Its characteristics are described in table 2 through table 4. 

ROUNDABOUT MODELS

Modeling Roundabouts

The TruckSim® “Road Segment Builder” and “Road Off-Center Elevation” tools were used in 

this study to create roundabout models. The horizontal road geometry for a left-turn movement 

through a roundabout is assembled and simulated using a series of road segments, the first being 
16 

a straight segment (approaching roadway), followed by a segment curving to the right (entry

curve), a segment curving to the left (partial circumference of central island), a segment curving 

to the right (exit curve), and finally a straight segment (exit roadway). After the horizontal 

geometry of a road model is determined, the user inputs its vertical geometry using the “Road 

Off-Center Elevation” tool by specifying the elevation along the road’s lateral coordinate. 

Roundabout Components

Figure 4 shows a roundabout’s basic geometric components and dimensions. 

Figure 4. Diagram. Basic geometric elements of a roundabout, adapted from Exhibit 6-2 in 

NCHRP Report 672.[11]

The roundabout entry plays an important role in roundabout design, for both safety and 

operation. The entry curve is designed to force the vehicle to slow down before entering the 

roundabout. The cross-slope of an approaching roadway is usually 2% outward for drainage, 

meaning the left side of the roadway is higher than the right side. The entry radius is determined 

by the type of roundabout (single-lane or two-lane) and the roundabout’s diameter, among other 

factors. 

The roundabout interior consists of a central island, circulatory roadway, and, for some 

roundabouts, a truck apron. The central island is a non-traversable elevated area in the center of 

the roundabout. Vehicles travel around the central island in a counterclockwise direction along 

the circulatory roadway, which may have one or more lanes. The circulatory roadway’s crosssection can have a constant cross-slope or a crown, where the inner section of the circulatory 
17 

roadway has an inward cross-slope, and the outer section has an outward cross-slope. A truck 

apron is an additional paved area around the central island allowing large vehicles to over-track 

without compromising the deflection for smaller vehicles.[11]

The design of a roundabout, like any intersection, requires designers to consider many factors, 

including operation, safety, and context. This study on truck stability focused on the following 

key design elements for roundabouts, which were used to create the roundabout models for the 

simulations:

Number of lanes: Single-lane or two-lane.

Inscribed circle diameter (ICD): This is the distance across the circle from outer edge to 

outer edge of the circulatory roadway pavement.

Circulatory roadway width: This is the radial distance between the inner edge and the outer 

edge of the circulatory roadway.

Circulatory roadway cross-slope: The two circulatory roadway slope scenarios considered 

for this study consisted of (1) a uniform 2% slope away from inner edge to outer edge, and 

(2) a crowned slope where the inner two thirds of the circulatory roadway sloped to inner

edge at 2% and the outer one third sloped to outer edge at 2%.

Truck apron: The truck apron was designed to be 0.0762-m (3-in.) higher than the 

circulatory roadway, 3.96-m (13-ft) wide, and to have a 2% cross-slope to the outside of the 

roundabout. 

Central island diameter: The central island diameter was calculated as the difference 

between the roundabout ICD and twice the circulatory roadway width.

Roundabout tilt: Some intersections’ topography requires that the roundabout be tilted on a 

constant plane. For a tilted roundabout, the entire roundabout is tilted at a constant slope. 

Three basic tilted slopes were considered, as shown in figure 5: no tilt, positive tilt 

(roundabout is tilted outward), and negative tilt (roundabout is tilted inward). Four tilted 

slopes were considered in this study: 4 %, 2 %, -4 % and -2 %. 
18 

Figure 5. Diagram. Roundabout tilt from the perspective of a driver on the approaching 

roadway. 

Roundabout exit layout is similar to entry layout. The exit curve that connects the circulatory 

roadway and exit roadway also has a cross-slope of 2% outward. In general, the exit radius is

larger than the entry radius. The exit radius is also determined by whether the roundabout is 

single-lane or two-lane, the roundabout’s diameter, and other factors. 

Roundabout Parameters

Roundabout design parameters can change from site to site. The roundabout parameters used in 

this study were determined through consultation with the research team and are intended to be 

representative of more common design elements. 

Single-lane and two-lane roundabout models were created, and their parameters are listed in

table 7. Roundabouts with ICDs of 45.7-m (150-ft) and smaller are single-lane, and those with 

ICDs greater than 45.7-m (150-ft) are two-lane. 
19 

Table 7. Roundabout parameters. 

Case ICD 

[m (ft)] 

Type Entry 

radii

[m (ft)] 

Exit radii

[m (ft)] 

Circulatory 

Roadway 

width

[m (ft)]

Truck 

apron 

Road 

crown

Tilt

1 33.5 (110) Single 22.9 (75) 45.7 (150) 6.1 (20) Applied N/A N/A

2 39.6 (130) Single 22.9 (75) 45.7 (150) 6.1 (20) Applied N/A N/A

3* 42.7 (140) Single 22.9 (75) 45.7 (150) 6.1 (20) Applied N/A N/A

4 42.7 (140) Single 22.9 (75) 45.7 (150) 6.1 (20) Applied N/A Applied

5 45.7 (150) Single 22.9 (75) 45.7 (150) 6.1 (20) Applied N/A N/A

6 51.8 (170) Multi 30.5 (100) 122 (400) 9.1 (30) N/A N/A N/A

7** 54.9 (180) Multi 30.5 (100) 122 (400) 9.1 (30) N/A N/A N/A

8 54.9 (180) Multi 30.5 (100) 122 (400) 9.1 (30) Applied N/A N/A

9 54.9 (180) Multi 30.5 (100) 122 (400) 9.1 (30) Applied Applied N/A

10 54.9 (180) Multi 30.5 (100) 122 (400) 9.1 (30) N/A N/A Applied

11 57.9 (190) Multi 30.5 (100) 122 (400) 9.1 (30) N/A N/A N/A

12 64.0 (210) Multi 30.5 (100) 122 (400) 9.1 (30) N/A N/A N/A

*Baseline single-lane roundabout

**Baseline two-lane roundabout

Baseline Roundabouts 

Baseline Single-lane Roundabout

The baseline single-lane roundabout has an ICD of 42.7 m (140-ft), a typical ICD for single-lane 

roundabouts. The rest of the baseline roundabout’s parameters are listed in row three of table 7. 

The horizontal geometry of the baseline roundabout is shown in figure 6. The hatched area 

encircling the central island represents the truck apron. Vehicles approach from the left-handside to perform the right turn, through-movement, or left turn. 
20 

Figure 6. Diagram. Horizontal geometry of 42.7-m (140-ft) baseline single-lane roundabout 

model. 

The cross-sectional geometry of the baseline roundabout is shown in figure 7. The numbers in 

the rectangles illustrate the vertical height. Detail 1 shows the entry geometry, and detail 2

shows the exit geometry (the exit geometry is the same for all three movements). The 

approaching roadway has a 2% outward cross-slope, thus for a 6.1-m (20-ft) roadway, its left 

edge is 0.12-m (0.4-ft) higher than the right edge. There are two transition areas that transfer the 

road’s vertical geometry from that of the approaching roadway to that of the circulatory 

roadway. Transition area 1 is where the left edge of the approaching roadway is lowered to 

match the outer edge of the circulatory roadway, and is 6.1-m (20-ft) long. Transition area 2 is 

where the lowered left edge of the roadway is raised to meet the circulatory roadway’s inner 

edge, and is 9.6-m (31.5-ft) long. There are also two similar transition areas transferring the 

vertical roadway geometry from that of the circulatory roadway to that of the exit roadway, as 

shown in figure 7. In all transition areas, the elevation increases or decreases linearly with 

respect to the length of the arcs or lines.

Section A-A in figure 6 shows the cross-sectional geometry for the approaching roadway, and 

Section B-B in figure 7 shows the cross-sectional geometry of the circulatory roadway. 
21 

Figure 7. Diagram. Cross-sectional geometry of 42.7-m (140-ft) baseline single-lane 

roundabout model. 

Baseline Two-lane Roundabout

The baseline two-lane roundabout is designed to have two lanes and a 54.9-m (180-ft) ICD. The 

parameters for the baseline roundabout are listed in row seven of table 7. The model of the twolane baseline roundabout is similar to that of the single-lane baseline roundabout, except that the 

geometric parameters, such as entry radii, exit radii, and roadway width, are different.

Figure 8 illustrates the baseline two-lane roundabout’s horizontal geometry and the crosssectional geometry of the entry and circulatory roadways. 

Figure 9 illustrates the baseline two-lane roundabout’s horizontal geometry, transition areas, and 

the cross-sectional geometry of the circulatory roadway. In figure 8, detail 1 shows the entry 

geometry, and detail 2 shows the exit geometry, and there are two transition areas where the 

vertical geometries of the approaching and exit roadways are matched to that of the circulatory

roadway in the same way as for the single-lane roundabout. For the two-lane roundabout, 

transition area 1 is 9.1-m (30-ft) long, and transition area 2 is 12.2-m (40-ft) long. Entry 

transition areas are similar to exit transition areas, and within a transition area the elevation 

increases or decreases linearly with respect to the length of the arcs or lines.
22 

Figure 8. Diagram. Horizontal geometry of 54.9-m (180-ft) baseline two-lane roundabout 

model.

Figure 9. Diagram. Cross-sectional geometry of 54.9-m (180-ft) baseline two-lane

roundabout model.
23 

CHAPTER 5. SIMULATION CASE STUDIES

It is suspected that a combination of vehicle characteristics and roadway design elements, along 

with driver behavior, potentially contribute to the risk of truck overturns at roundabouts. Six 

features were selected to investigate in this study: truck configuration, load configuration, entry 

speed, ICD, circulatory roadway cross-section, and roundabout tilt. To perform a parametric 

study, each case was modeled separately, and only one factor (truck characteristic or roundabout 

parameter, depending on the case) was modified from the baseline, while the other parameters 

were kept constant. Then the results for each case were compared with the baseline to determine 

a parameter’s effect on truck lateral stability. For each test, three truck movements through each 

roundabout were simulated: right turns, through movements, and left turns.

In total, 106 separate cases were created and tested. The details for the cases are summarized in 

table 8. The terms and abbreviations used in table 8 are: 

ICD: Inscribed Circle Diameter.

Lanes: number of lanes (one or two). 

Entry/Exit: entry radius and exit radius. 

Width: roadway width. 

Cross-slope: circulatory roadway cross-slope. 

Crown/Apron/Tilt: whether roundabout has road crown, truck apron or tilted slope. 

Truck Type: WB-67 semi-truck, SU-30 single-unit truck, or a B-train tractor with double 

trailers.

Truckload: truckload conditions – empty, half full or full. 

Truck Speed: truck travel speed. For example, “32/24/32 (20/15/20)” means that the truck 

enters at 32 km/h (20 mph), then slows down to 24 km/h (15 mph) to circulate, and 

accelerates to 32 km/h (20 mph) when exiting the roundabout. 

Maneuver: the movements that truck performs to travel through the roundabout. In two-lane 

cases, ‘through (left)’ means the truck enters in the left lane and then straddles both lanes to 

circulate, whereas ‘through (right)’ means the truck enters in right lane and straddles both 

lanes to circulate. In addition, ‘through (apron)’ or ‘left (apron)’ indicates that the truck 

enters and keeps the in left lane without straddling to drive through the roundabout, as truck 

apron is present to accommodate the off-tracking and therefore no straddling is needed. 

In table 8 below, the base cases are highlighted in yellow (*), and the modified element in each 

case is highlighted in gray (†) or green (^).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 8. Roundabout dynamic modeling cases. 

Case ICD (m [ft]) Lanes 

Entry 

Curve 

Radius 

(m) 

Exit 

Curve 

Radius 

(m) 

Roadway 

Width 

(m) 

Cross -

slope Crown Apron Tilt Truck Type 

Truck 

Load 

Truck Speed 

(entry/circle/ 

exit, km/h) 

Maneuver 

1* 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Right turn 

2* 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Through 

3* 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Left turn 

4 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A B- train^ Full 32/24/32 Right turn 

5 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A B- train^ Full 32/24/32 Through 

6 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A B- train^ Full 32/24/32 Left turn 

7 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A SU -30† Full 32/24/32 Right turn 

8 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A SU -30† Full 32/24/32 Through 

9 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A SU -30† Full 32/24/32 Left turn 

10 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Empty^ 32/24/32 Right turn 

11 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Empty^ 32/24/32 Through 

12 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Empty^ 32/24/32 Left turn 

13 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Half† 32/24/32 Right turn 

14 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Half† 32/24/32 Through 

15 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Half† 32/24/32 Left turn 

16 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 4%^ WB -67 Full 32/24/32 Right turn 

17 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 4%^ WB -67 Full 32/24/32 T hrough 

18 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 4%^ WB -67 Full 32/24/32 Left turn 

19 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 2%† WB -67 Full 32/24/32 Right turn 

20 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 2%† WB -67 Full 32/24/32 Through 

21 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 2%† WB -67 Full 32/24/32 Left turn 

22 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -4%^ WB -67 Full 32/24/32 Right turn 

23 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -4%^ WB -67 Full 32/24/32 Through 

24 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -4%^ WB -67 Full 32/24/32 Left turn 

* = Base Case; ^ = modified element (gray); † = modified element (green)

25 
26 

Entry Exit 

Case ICD (m [ft]) Lanes Curve

Radius 

Curve Roadway 

Radius Width Cross - Truck Truck Speed slope Crown Apron Tilt Truck 

(m) Type Load (entry/circle/ Maneuver 

(m) (m) exit, km/h) 

 

 

25 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -2%† WB -67 Full 32/24/32 Right turn 

26 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -2%† WB -67 Full 32/24/32 Through 

27 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -2%† WB -67 Full 32/24/32 Left turn 

28 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 24/24/24^ Right turn 

29 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 24/24/24^ Through 

30 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 24/24/24^ Left turn 

31 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 40/24/40† Right turn 

32 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 40/24/40† Through 

33 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 40/24/40† Left turn 

34* 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Right turn 

35* 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (left)

36* 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (right)

37* 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Left turn 

38 54.9 (180) 2 30.5 122 9.1 2% N/A Applied^ N/A WB -67 Full 32/24/32 Through (apron) 

39 54.9 (180) 2 30.5 122 9.1 2% N/A Applied^ N/A WB -67 Full 32/24/32 Left turn 

40 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Right turn 

41 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Through (left) 

42 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Through (right) 

43 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Through (apron) 

44 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Left turn 

45 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/24/32 Left turn (apron) 

46 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A B-train^ Full 32/24/32 Right turn 

47 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A B-train^ Full 32/24/32 Through (left) 

48 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A B-train^ Full 32/24/32 Through (right) 

49 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A B-train^ Full 32/24/32 Left turn 

* = Base Case; ^ = modified element (gray); † = modified element (green)
27 

Entry Exit 

Case ICD (m [ft]) Lanes Curve Curve Roadway 

Radius Radius Width Cross - Truck Truck Speed slope Crown Apron Tilt Truck Type Load (entry/circle/ Maneuver 

(m) (m) (m) exit, km/h) 

50 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A SU-30† Full 32/24/32 Right turn 

51 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A SU-30† Full 32/24/32 Through (left) 

52 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A SU-30† Full 32/24/32 Through (right) 

53 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A SU-30† Full 32/24/32 Left turn 

54 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Empty^ 32/24/32 Right turn 

55 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Empty^ 32/24/32 Through (left) 

56 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Empty^ 32/24/32 Through (right) 

57 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Empty^ 32/24/32 Left turn 

58 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Half† 32/24/32 Right turn 

59 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Half† 32/24/32 Through (left) 

60 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Half† 32/24/32 Through (right) 

61 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Half† 32/24/32 Left turn 

62 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 4%^ WB -67 Full 32/24/32 Right turn 

63 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 4%^ WB -67 Full 32/24/32 Through (left) 

64 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 4%^ WB -67 Full 32/24/32 Through (right) 

65 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 4%^ WB -67 Full 32/24/32 Left turn 

66 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 2%† WB -67 Full 32/24/32 Right turn 

67 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 2%† WB -67 Full 32/24/32 Through (left) 

68 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 2%† WB -67 Full 32/24/32 Through (right) 

69 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A 2%† WB -67 Full 32/24/32 Left turn 

70 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -4%^ WB -67 Full 32/24/32 Right turn 

71 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -4%^ WB -67 Full 32/24/32 Through (left) 

72 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -4%^ WB -67 Full 32/24/32 Through (right) 

73 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -4%^ WB -67 Full 32/24/32 Left turn 

74 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -2%† WB -67 Full 32/24/32 Right turn 

75 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -2%† WB -67 Full 32/24/32 Through (left) 

* = Base Case; ^ = modified element (gray); † = modified element (green)
28 

Entry Exit 

Case ICD (m [ft]) Lanes Curve Curve Roadway 

Radius Radius Width Cross - Truck Truck Speed slope Crown Apron Tilt Truck Type Load (entry/circle/ Maneuver 

(m) (m) (m) exit, km/h) 

76 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -2%† WB -67 Full 32/24/32 Through (right) 

77 54.9 (180) 2 30.5 122 9.1 N/A N/A N/A -2%† WB -67 Full 32/24/32 Left turn 

78 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 24/24/24^ Right turn 

79 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 24/24/24^ Through (left) 

80 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 24/24/24^ Through (right) 

81 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 24/24/24^ Left turn 

82 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 40/24/40† Right turn 

83 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 40/24/40† Through (left) 

84 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 40/24/40† Through (right) 

85 54.9 (180) 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 40/24/40† Left turn 

86^ 33.5 (110)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Right turn 

87^ 33.5 (110)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Through 

88^ 33.5 (110)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Left turn 

89† 39.6 (130)† 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Right turn 

90† 39.6 (130)† 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Through 

91† 39.6 (130)† 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/16/32 Left turn 

92^ 45.7 (150)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Right turn 

93^ 45.7 (150)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Through 

94^ 45.7 (150)^ 1 23 150 6.1 2% N/A Applied N/A WB -67 Full 32/24/32 Left turn 

95† 51.8 (170)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Right turn 

96† 51.8 (170)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (left) 

97† 51.8 (170)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (right) 

98† 51.8 (170)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Left turn 

99^ 57.9 (190)^ 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Right turn 

100^ 57.9 (190)^ 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (left) 

101^ 57.9 (190)^ 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (right) 

* = Base Case; ^ = modified element (gray); † = modified element (green)
29 

Entry Exit 

Case ICD (m [ft]) Lanes Curve Curve Roadway 

Radius Radius Width Cross -

(m) (m) (m) slope Crown Apron Tilt Truck Truck Truck Speed 

Type Load (entry/circle/ Maneuver 

exit, km/h) 

102^ 57.9 (190)^ 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Left turn 

103† 64.0 (210)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Right turn 

104† 64.0 (210)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (left) 

105† 64.0 (210)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Through (right) 

106† 64.0 (210)† 2 30.5 122 9.1 2% N/A N/A N/A WB -67 Full 32/24/32 Left turn 

107 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Full 32/16/32† Left turn 

108 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Empty^ 32/16/32^ Left turn 

109 42.7 (140) 1 23 45.7 6.1 2% N/A Applied N/A WB -67 Half† 32/16/32† Left turn 

110 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 4%^ WB -67 Full 32/16/32^ Left turn 

111 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied 2%† WB -67 Full 32/16/32† Lef t turn 

112 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -4%^ WB -67 Full 32/16/32^ Left turn 

113 42.7 (140) 1 23 45.7 6.1 N/A N/A Applied -2%† WB -67 Full 32/16/32† Left turn 

114 54.9 (180) 2 30.5 122 9.1 2% N/A Applied^ N/A WB -67 Full 32/16/32^ Left turn (apron) 

115 54.9 (180) 2 30.5 122 9.1 N/A Applied† Applied† N/A WB -67 Full 32/16/32† Left turn (apron) 

* = Base Case; ^ = modified element (gray); † = modified element (green)
30 

TRUCK TRAVEL PATH

Three movement scenarios were modeled for single-lane roundabouts. The first was a right turn, 

where trucks shifted to the left side of the roadway when approaching the roundabout, kept left 

to accommodate off-tracking, and drove back to the middle of the road to exit the roundabout. 

Next was a through movement, where trucks first made a slight right turn, then a left turn, and 

finally a second right turn. For each turn, drivers would keep the vehicle to the outside edge of 

the lane to accommodate the long wheelbase. Therefore, the vehicle shifted to the left of 

roadway to make the first right turn, then drove to the right edge when circulating the central 

island, and finally swung back to the left edge to complete the second right turn. Last was a leftturn movement, where the path was similar to through movements, but the distance traveled in 

the circulatory roadway was longer. 

A greater number of movement scenarios were modeled for two-lane roundabouts. The right-turn 

movement had vehicles approach from the right lane and then straddle both lanes to complete the 

turn. For through movements, there were three possible paths: in the first, the truck approached 

from the middle of the right lane and then straddled both lanes to make the through movement; 

in the second, the truck entered from the middle of the left lane and then straddled lanes to pass 

through the roundabout; and in the third, which was commonly performed by trucks in

roundabouts with a truck apron, the vehicle kept in the left lane and drove through the 

roundabout without straddling lanes. The left turn movement depended on whether or not a truck 

apron was present; if present, the left turn movement had vehicles approach from the middle of 

the left lane and straddle both circulating lanes when completing the left turn, and when not 

present, the vehicles would approach from the middle of the left lane and use the left lane and 

apron without straddling lanes. 

TRUCK SPEED

Given their long wheelbase, heavy load, and high center of gravity (CG), articulated-truck 

drivers usually drive slowly when negotiating roundabouts. This speed may vary from site to 

site, but is normally 16 km/h (10 mph) for smaller roundabouts and 24 km/h (15 mph) for larger 

roundabouts. 

In practice, semi-truck drivers commonly feel uncomfortable when they experience a lateral 

acceleration of more than 0.2 g. Therefore, for this study, a lateral acceleration of 0.2 g served as 

the limit for deciding a proper circulating speed. When curvature is fixed, lateral acceleration is 

mainly determined by traveling speed, so the proper truck speed in this study was determined 

based on the roundabout diameters. 

In most test cases for this study, the truck entered the roundabout at 32 km/h (20 mph), slowed 

down to a speed determined by the roundabout diameter, and accelerated back to 32 km/h 

(20 mph) when exiting the roundabout. Their acceleration time depended on roundabout 

diameter. For the roundabouts with a 33.5-m (110-ft) ICD and a 39.6-m (130-ft) ICD, the truck 

slowed down to 16 km/h (10 mph) in the 2.5 s after entering the roundabout. For the roundabouts 

with an ICD greater than 39.6 m (130-ft), the truck slowed down to 24 km/h (15 mph) within 2 s. 
31 

Those speeds were used for most of the test cases, except for the test cases that evaluated speed 

and whether the trucks could adequately maneuver the roundabout with an entry speed of 24, 32 

or 40 km/h (15, 20, or 25 mph). 

33 

CHAPTER 6. SIMULATION RESULTS

The roundabout dynamic modeling cases, presented in table 8, consist of 106 separate cases that 

were tested to obtain the truck lateral dynamics for each scenario. In each case, three parameters 

were used to evaluate the truck dynamics: roll angle, lateral acceleration, and rollover index. The 

roll angle and lateral acceleration directly represent the vehicle dynamics; for example, lateral 

acceleration is strongly influenced by the speed, curvature of the turn, and road vertical 

geometry. Large lateral acceleration leads to large roll angle, which results in large side-to-side 

wheel unloading and high possibility of rollover. The dynamic relationship between lateral 

acceleration, roll angle, and the likelihood of rollover is influenced by both the vehicle and 

roadway design. If the lateral acceleration and vehicle body roll do not cause significant wheel 

unloading, then rollover will not occur. 

ROLLOVER INDEX

Rollover index indicates the likelihood of rollover, and is calculated using the equation in figure 

10: 

|Fz

Rollover Index = 

left − Fzright|

Fzleft + Fzright

Figure 10. Equation. Rollover index.

Where: 

Fz_left = Left wheel load (rear axles)

Fz_right = Right wheel load (rear axles)

For a WB-67 semi-truck, the left and right wheel loads (above) are the sum of the individual 

wheel loads for the dual wheels on the tandem axles (four on each side) of the tractor and trailer. 

For example, for the trailer we get the equations in figure 11 and figure 12: 

Fz_left = Fz_4li + Fz_4lo + Fz_5li + Fz5lo

Figure 11. Equation. Left wheel load (rear axles). 

Fz_right = Fz_4ri + Fz_4ro + Fz_5ri + Fz5ro

Figure 12. Equation. Right wheel load (rear axles). 

Where: 

Fz_4li = Wheel load, left side, inner wheel, trailer axle 1

Fz_4lo = Wheel load, left side, outer wheel, trailer axle 1

Fz_5li = Wheel load, left side, inner wheel, trailer axle 2
34 

Fz_5lo = Wheel load, left side, outer wheel, trailer axle 2

Fz_4ri = Wheel load, right side, inner wheel, trailer axle 1

Fz_4ro = Wheel load, right side, outer wheel, trailer axle 1 

Fz_5ri = Wheel load, right side, inner wheel, trailer axle 2

Fz_5ro = Wheel load, right side, outer wheel, trailer axle 2

Rollover index varies between zero and one. If there is no load transfer, the vertical loads on 

both left and right side are equivalent, and rollover index = 0. If all loads are transferred to one 

side, the terms of the equation in figure 10 become equal, resulting in rollover index = 1. 

Therefore, a rollover index equal to 0 describes a perfectly weight-balanced vehicle. A vehicle 

with a rollover index of 0.8 or less is capable of traveling through a roundabout while 

maintaining stability. Between 0.8 and 0.9 the vehicle becomes unstable, and if peak rollover 

index exceeds 0.9, the vehicle has a high likelihood of rollover. A peak rollover index equal to 1 

describes a vehicle on the threshold of rolling over 

EFFECT OF ROUNDABOUT ICD ON TRUCK STABILITY

The purpose of this test was to determine the effect of different ICDs on truck lateral dynamics.

The truck model in this test was the baseline full-load semi-truck with an entry speed of 32 km/h 

(20 mph) for all cases. For roundabouts with a 33.5 m (110-ft) ICD and a 39.6-m (130-ft) ICD, 

the truck slowed down to 16.1 km/h (10 mph) after entering the roundabout. For the roundabouts 

with an ICD greater than 39.6 m (130-ft), the truck slowed down to 24 km/h (15 mph). For 

through movements at the two-lane roundabouts, two paths were followed. For one, the truck 

approached from the left lane, and then straddled both lanes to pass through the roundabout. For 

the other, the truck approached from the right lane, and then straddled both lanes to pass through 

the roundabout. All of the single-lane roundabout models had truck aprons, but the two-lane 

roundabout models did not. 

The 21 cases testing roundabout ICD’s effect on truck stability are summarized in table 9; all 

dimensions not shown are unchanged within the set of single-lane and two-lane cases (see table 8 

for details). 
35 

Table 9. Roundabout diameter cases considered for evaluating truck dynamics in 

roundabouts.

Case 

No.

Roundabout 

Diameter - ICD

[m (ft)]

Type Movement

Speed at

entry/roundabout/exit

[km/h (mph)]

86 33.5 (110) Single-lane Right turn 32/16/32 (20/10/20) 

87 33.5 (110) Single-lane Through 32/16/32 (20/10/20) 

88 33.5 (110) Single-lane Left turn 32/16/32 (20/10/20) 

89 39.6 (130) Single-lane Right turn 32/16/32 (20/10/20) 

90 39.6 (130) Single-lane Through 32/16/32 (20/10/20) 

91 39.6 (130) Single-lane Left turn 32/16/32 (20/10/20) 

92 45.7 (150) Single-lane Right turn 32/24/32 (20/15/20) 

93 45.7 (150) Single-lane Through 32/24/32 (20/15/20) 

94 45.7 (150) Single-lane Left turn 32/24/32 (20/15/20) 

95 51.8 (170) Two-lane Right turn 32/24/32 (20/15/20) 

96 51.8 (170) Two-lane Through (left lane) 32/24/32 (20/15/20) 

97 51.8 (170) Two-lane Through (right lane) 32/24/32 (20/15/20) 

98 51.8 (170) Two-lane Left turn 32/24/32 (20/15/20) 

99 57.9 (190) Two-lane Right turn 32/24/32 (20/15/20) 

100 57.9 (190) Two-lane Through (left lane) 32/24/32 (20/15/20) 

101 57.9 (190) Two-lane Through (right lane) 32/24/32 (20/15/20) 

102 57.9 (190) Two-lane Left turn 32/24/32 (20/15/20) 

103 64.0 (210) Two-lane Right turn 32/24/32 (20/15/20) 

104 64.0 (210) Two-lane Through (left lane) 32/24/32 (20/15/20) 

105 64.0 (210) Two-lane Through (right lane) 32/24/32 (20/15/20) 

106 64.0 (210) Two-lane Left turn 32/24/32 (20/15/20) 

Right Turns

The results for rollover index for trucks performing right turns are shown in figure 13 and figure 

14.
36 

Figure 13. Graph. Effect of roundabout diameter on rollover index 

(single-lane roundabouts, right turn).

Figure 14. Graph. Effect of roundabout diameter on rollover index 

(two-lane roundabouts, right turn).

Figure 13 shows that, for single lane roundabouts, the rollover indices for right turns with a 33.5-

m (110-ft) ICD and 39.6-m (130-ft) ICD were similar, but those for right turns with a 45.7-m 

(150-ft) ICD were much larger. This was because trucks negotiated a 45.7-m (150-ft) ICD 

roundabout at 24 km/h (15 mph), but negotiated the other two roundabouts at a slower 16 km/h 

(10 mph). 

Figure 14 shows that, for two-lane roundabouts, the rollover indices for right turns were nearly 

identical for the three ICDs. 
37 

Through Movements 

The results for rollover index for trucks performing through movements are shown in figure 15

and figure 16

Figure 15. Graph. Effect of roundabout diameter on rollover index 

(single-lane roundabouts, through movement).

Figure 16. Graph. Effect of roundabout diameter on rollover index 

(two-lane roundabouts, through movement).

Figure 17 shows the trucks making through movements in single-lane roundabouts. The three 

ICDs have the same lateral dynamics for the first 13 s. After 13 s, the rollover indices diverge. 

The 39.6-m (130-ft) ICD again had the lowest rollover index, and the 45.7-m (150-ft) ICD had 
38 

the largest rollover index, again due to the higher speed. At around 19 s, the trucks moved up 

onto the truck apron, rapidly changing the vertical load on both sides of the trucks and rocking 

them side-to-side, as shown by the narrow peaks and troughs between 19 s and 26 s in figure 18. 

Figure 18 indicates that roundabouts with larger ICDs had smaller rollover indices and a lower 

likelihood of rollover. When the truck approached from the right lane and then straddled lanes, 

the rollover likelihood was smaller than when the truck approached from the left lane. 

Left Turns

The results for rollover index for trucks performing left turns are shown in figure 17 and figure 

18. 

Figure 17. Graph. Effect of roundabout diameter on rollover index 

(single-lane roundabout, left turn).
39 

Figure 18. Graph. Effect of roundabout diameter on rollover index 

(two-lane roundabout, left turn).

Figure 17 and Figure 18 show that the left-turn cases followed the same pattern as the through 

movement cases: for single-lane roundabouts, the 45.7-m (150-ft) roundabout had the highest 

rollover index because of the higher circulating speed of 24 km/h (15 mph) rather than 16 km/h 

(10 mph); and for the two-lane roundabouts, the 64.0-m (210-ft) roundabout had the lowest 

rollover index because it had the largest ICD.

EFFECT OF CIRCULATORY ROADWAY CROSS-SECTION ON TRUCK STABILITY

The purpose of this test was to determine the effect of roundabout cross-section on truck 

stability. The truck model for this test was the baseline full-load semi-truck with an entry speed 

of 32 km/h (20 mph). Twelve cases using two-lane roundabouts were created and tested as 

summarized in table 10; all dimensions not shown are unchanged (see table 8 for details). For the 

baseline cross-section, the entire circulatory roadway had a constant 2% cross-slope to the 

outside of the roundabout. For the crowned cross-section, the inner two-thirds of the circulatory 

roadway had a 2% inward cross-slope, and the outer third of the circulatory roadway had a 2% 

outward cross-slope. A truck apron was either present or absent. For the cases with a truck apron, 

an additional path, called ‘apron,’ was added for through movement. For the apron movement, 

the vehicle remained in the left lane to ensure it passed over the truck apron. 
40 

Table 10. Roadway cross-section cases considered for evaluating truck dynamics in 

roundabouts.

Case # Roundabout Diameter -

ICD [m (ft)]

Truck Apron CrossSectionMovement

34 54.9 (180) No Baseline Right turn

35 54.9 (180) No Baseline Through (left lane) 

36 54.9 (180) No Baseline Through (right lane) 

37 54.9 (180) No Baseline Left turn

38 54.9 (180) Yes Baseline Through (apron) 

39 54.9 (180) Yes Baseline Left turn (apron)

40 54.9 (180) Yes Crown Right turn

41 54.9 (180) Yes Crown Through (left lane) 

42 54.9 (180) Yes Crown Through (right lane) 

43 54.9 (180) Yes Crown Through (apron) 

44 54.9 (180) Yes Crown Left turn

45 54.9 (180) Yes Crown Left turn (apron)

Right Turns

The results of rollover index for right turns are shown in figure 19. 

Figure 19. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, right turn).

As shown in figure 19, the crowned roundabout had a higher peak rollover index and lower truck 

lateral stability than the baseline roundabout. This was mainly because, for the baseline case, the 

left side of the roadway was higher than the right side of the roadway. Thus, when the truck 
41 

made a right turn, the roadway cross-section countered the centrifugal accelerations at the truck’s 

CG, reducing body roll and improving truck stability. For the crowned roundabout, the left edge 

of the inner two-thirds of the roadway was lower than the right edge. When the truck straddled 

the lanes to make a right turn, its left wheels ran on the inner, lower part of the roadway, 

increasing the truck’s tendency to lean to the left side and reducing its lateral stability. 

Through Movements 

The results of rollover index for through movements are shown in figure 20 and figure 21. 

Figure 20. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout (twolane, through movement).
42 

Figure 21. Effect of road crown and truck apron on rollover index for a 54.9-m (180-ft) 

roundabout (two-lane, through movement).

Figure 20 shows that when the truck passed through the roundabout starting in the left lane, it 

experienced a higher peak rollover index with the baseline cross-section than with the crowned

cross-section (although both are less than 0.8). This is because when the truck was in the left lane 

on a crowned road, the inward cross-slope on the inner two-thirds of the roadway reduced the 

truck’s lateral acceleration. When the truck started in the right lane, however, the truck 

experienced a higher rollover index with the crowned cross-section than with the baseline 

condition (although both are less than 0.8). Figure 21 shows that when the truck passed over the 

truck apron (light gray traces) to move through the roundabout, it caused rapidly oscillating and 

high rollover indices, because moving onto the truck apron caused rocking. For the truck apron 

movement, the baseline cross-section had a higher rollover index than the crowned cross-section, 

and both were high, above 0.8. 

Left Turns

The results of rollover index for left turns are shown in figure 22 and figure 23. 
43 

Figure 22. Effect of road crown on rollover index for a 54.9-m (180-ft) roundabout (twolane, left turn). 

Figure 23. Effect of road crown and truck apron on rollover index for a 54.9-m (180-ft) 

roundabout (two-lane, left turn).

Figure 22 shows that without a truck apron, a left-turning truck in a 54.9-m (180-ft) two-lane 

roundabout experienced a lower rollover index under the crowned section than under the 

baseline section, with the rollover index for the baseline section slightly exceeding 0.8 at 24 

km/h (15 mph). Figure 22 shows that the semi-truck experienced a stronger likelihood of rollover 
44 

when making left turns with a truck apron at 24 km/h (15 mph). Therefore, two additional cases 

at a lower speed were created and tested, with the other parameters kept constant. For the lowspeed cases, the circulating speed was reduced from 24 km/h (15 mph) to 16 km/h (10 mph). The 

rollover indices are shown in figure 24. 

Figure 24. Effect of truck apron and road crown on rollover index at different circulating 

speeds (16 km/h [10 mph] and 24 km/h [15 mph])(two-lane, left turn). 

Figure 24 shows that a fully loaded semi-truck passing over the truck apron to perform a left turn 

at 24 km/h (15 mph) in the 54.9-m (180-ft) two-lane roundabout resulted in a peak rollover index 

of almost 1, and a significant risk of rollover. The inward cross slope of the crown reduced the 

peak rollover index, but it still exceeded 0.9. However, when the circulating speed was lowered

to 16.1 km/h (10 mph), the peak rollover indices for the two cross-sections were lower than 0.8, 

indicating that the semi-truck could safely negotiate the roundabout at the lower speed. 

EFFECT OF TRUCK CONFIGURATION ON TRUCK STABILITY

In this test, three different truck configurations, a WB-67 semi-truck, a SU-30 single-unit truck, 

and a B-train tractor with double trailers, were simulated to determine their lateral dynamics 

when driving through the baseline single-lane 42.7-m (140-ft) ICD roundabout and the baseline 

two-lane 54.9-m (180-ft) roundabout. Each truck entered the roundabout at 32 km/h (20 mph), 

then slowed down to 24 km/h (15 mph) and followed the same path. The cases considered are 

summarized in table 11; all dimensions not shown are unchanged within the set of single-lane 

and two-lane cases (see table 8 for details). 
45 

Table 11. Truck configuration cases considered for evaluating truck dynamics in 

roundabouts.

Case # Roundabout Diameter 

- ICD [m (ft)]

Movement Truck configuration

1 42.7 (140) Right turn WB-67

2 42.7 (140) Through WB-67

3 42.7 (140) Left turn WB-67

4 42.7 (140) Right turn B-train

5 42.7 (140) Through B-train

6 42.7 (140) Left turn B-train

7 42.7 (140) Right turn SU-30

8 42.7 (140) Through SU-30

9 42.7 (140) Left turn SU-30

34 54.9 (180) Right turn WB-67

35 54.9 (180) Through (left lane) WB-67

36 54.9 (180) Through (right lane) WB-67

37 54.9 (180) Left turn WB-67

46 54.9 (180) Right turn B-train

47 54.9 (180) Through (left lane) B-train

48 54.9 (180) Through (right lane) B-train

49 54.9 (180) Left turn B-train

50 54.9 (180) Right turn SU-30

51 54.9 (180) Through (left lane) SU-30

52 54.9 (180) Through (right lane) SU-30

53 54.9 (180) Left turn SU-30

Right Turns

The results of the rollover index for right turns in a 42.7-m (140-ft) and a 54.9-m (180-ft) 

roundabout are shown in Figure 25 and Figure 26, respectively. 
46 

Figure 25. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane roundabout, right turn). 

Figure 26. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane roundabout, right turn). 

As shown in figure 25 and figure 26, the SU-30 single-unit truck had the highest roll stability, 

followed by the WB-67 semi-truck. The B-train double trailer truck had the lowest roll stability. 

All three truck configurations, however, could safely make a right turn at a roundabout at the 

speeds considered here. 
47 

Through Movements 

The results of the rollover index for through movements in a 42.7-m (140-ft) and a 54.9-m (180-

ft) roundabout are shown in Figure 27and Figure 28, respectively. 

Figure 27. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement). 

Figure 28. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement). 

According to figure 27 and figure 28, the SU-30 single-unit truck had the smallest rollover index, 

indicating that it was more stable than the other configurations when moving through 

roundabouts. Even though the B-train tractor with double trailers had a greater overall length 

than the WB-67 semi-truck, it had a slightly lower rollover index, because the B-train’s 
48 

constituent trailers were both shorter than the WB-67 semi-truck’s trailer, allowing it to better 

conform to the circulatory roadway. 

Left Turns

The results of the rollover index for left turns in 42.7-m (140-ft) and 54.9-m (180-ft) roundabouts 

are shown in figure 29 and figure 30, respectively. Note that a value of 1 or greater indicates a 

rollover condition, and as a result, the graph and all other similar graphs show a maximum value 

of 1. 

Figure 29. Effect of truck configuration on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn).
49 

Figure 30. Effect of truck configuration on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, left turn).

Figure 29 and figure 30 illustrate that the SU-30 single-unit truck was more stable than the other 

two configurations. 

The WB-67 semi-truck and B-train tractor with double trailers had similar rollover indices in 

both roundabouts. In the 42.7-m (40-ft) ICD roundabout, the baseline WB-67 semi-truck with a 

full load (GVWT = 36300 kg [80,000 lbs]), making a left turn and circulating at 24 km/h (15 

mph), had a high likelihood of rollover. 

EFFECT OF LOAD CONFIGURATION ON TRUCK STABILITY

In this test, three load configurations were studied for the WB-67 semi-truck (the baseline 

vehicle) to determine whether the load would impact truck stability. The baseline truck was

evaluated in empty, half-full, and full load conditions, as summarized in table 12; all dimensions 

not shown are unchanged within the set of single-lane and two-lane cases (see table 8 for 

details). The loads were assumed to be fixed, and no shifting loads were considered. 
50 

Table 12. Load configuration cases considered for evaluating truck dynamics in 

roundabouts.

Case # Roundabout Diameter -

ICD [m (ft)]

Movement Load

1 42.7 (140) Right turn Full

2 42.7 (140) Through Full

3 42.7 (140) Left turn Full

10 42.7 (140) Right turn Empty

11 42.7 (140) Through Empty

12 42.7 (140) Left turn Empty

13 42.7 (140) Right turn Half full

14 42.7 (140) Through Half full

15 42.7 (140) Left turn Half full

34 54.9 (180) Right turn Full

35 54.9 (180) Through (left lane) Full

36 54.9 (180) Through (right lane) Full

37 54.9 (180) Left turn Full

54 54.9 (180) Right turn Empty

55 54.9 (180) Through (left lane) Empty

56 54.9 (180) Through (right lane) Empty

57 54.9 (180) Left turn Empty

58 54.9 (180) Right turn Half full

59 54.9 (180) Through (left lane) Half full

60 54.9 (180) Through (right lane) Half full

61 54.9 (180) Left turn Half full

Right Turns

The results of rollover index for right turns in 42.7-m (140-ft) single-lane roundabout and the 

54.9-m (180-ft) two-lane roundabout are shown in figure 31 and figure 32, respectively, for the 

three load conditions. 
51 

Figure 31. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, right turn).

Figure 32. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, right turn). 

Figure 31 and figure 32 show that the half-full truck had the lowest rollover index for both the 

single-lane and two-lane roundabouts, and surprisingly, the empty truck had the highest rollover 

index. The lighter axle load for the empty truck resulted in a smaller denominator in figure 10. 

Therefore, a larger rollover index occurred for the side-to-side weight transfer that occurred due 

to lateral accelerations at the truck’s CG. Another way to think of this phenomenon is by 

considering that a lighter axle has less vertical load holding it down against the road, and 

therefore it could be more easily unloaded. A heavier axle load, such as the fully loaded truck, is
52 

more heavily loaded against the road and requires more unloading before it reaches zero wheel 

loads. 

The peaks in figure 31 and figure 32 happened during the transition from one steering 

configuration (roadway curvature) to another, when the truck changed from turning left to 

turning right and vice versa. The rapid change in roadway elevation at the left edge of the 

roadway at curvature transitions contributed significantly to the lateral acceleration and rollover 

index peaks. For instance, in roundabouts with 54.9-m (180-ft) ICDs, the left edge of the 

roadway changed from 0 to 0.15-m (6-in.) in elevation within 9.1-m (30-ft) of travel. Such a 

rapid change in roadway geometry acted like a lateral impulse force, which in turn acted as a 

large lateral force at the CG, resulting in proportionally large lateral accelerations, wheel 

unloading, and rollover indices. The higher CG for the fully loaded truck resulted in a somewhat 

larger rollover index, as compared with the half-full truck. 

The half-full truck’s configuration optimized the load on the wheelbase and the mass and 

position of the truck’s CG to create a condition with the lowest peak rollover index. 

Through Movements 

The results of rollover indices for through movements in 42.7-m (140-ft) and 54.9-m (180-ft) 

roundabouts are shown in figure 33 and figure 34, respectively, for the three load conditions. 
53 

Figure 33. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement).

Figure 34. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement).

Figure 33 shows that, for through movements in the single-lane roundabout, the empty truck had 

the highest rollover index similar to the results for right turns. However, for the empty truck, the 

smaller turning radius of the 42.7-m (140-ft) roundabout and passing over the truck apron 

resulted in rollover indices exceeding the rollover limit of 1 at the roadway curvature transitions. 

The peaks in rollover index for the loaded truck reached as high as 0.9. Thus, the empty and full 

trucks both have a high risk of rollover in this condition. The half-full truck had a peak rollover 

index of about 0.55, indicating it could safely move through the single-lane roundabout. 
54 

For the two-lane roundabout, figure 34 shows that none of the rollover indices exceeded 0.80. 

The rollover index followed the same trend for trucks starting in the right and left lanes. For full 

and half-full trucks, those starting in the left lane had higher peak indices than those starting in 

the right lane due to the tighter curvature of the truck’s path and increased lateral acceleration. 

The half-load truck had the lowest peak rollover index. The empty truck’s rollover index 

exhibited different trends in peaks, and had a peak rollover index between that of the full load 

and half load trucks. 

Left Turns

The results of rollover index for left turns in 42.7-m (140-ft) single-lane and 54.9-m (180-ft) 

two-lane roundabouts are shown in figure 35 and figure 36, respectively, for the three load 

conditions. 
55 

Figure 35. Effect of truck weight on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn).

Figure 36. Effect of truck weight on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, left turn).

The results in figure 35 and figure 36 are similar in trend to those for through movements. 

However, for left turns in the single-lane roundabout (figure 35), both full and empty trucks had 

peak rollover indices of over 1, indicating a high likelihood of rollover, and the half-full truck 

also exhibited a high peak rollover index of 0.75. 

The results in figure 36 show that, for trucks turning left in the two-lane roundabout, all three 

load conditions resulted in rollover indices that were below 1. Similar to the other turning 
56 

maneuvers, in the 54.9-m (180-ft) roundabout, the full truck had the highest rollover index, the 

empty truck had the lowest, and the half-full truck fell in between. 

For the single-lane, 42.7-m (140-ft) roundabout, an additional set of cases was tested with the 

circulating speed reduced to 16 km/h (10 mph) from 24 km/h (15 mph). The rollover indices of 

the three additional cases are shown in figure 37. 

Figure 37. Effect of truck weight on rollover index at different circulating speeds: 16 km/h 

(10 mph) (left plot) and 24 km/h (15 mph) (right plot); 

(140-ft single-lane roundabout, left turn). 

Figure 37 shows that for both the half-full and full trucks, rollover was significantly less likely 

when travelling at 16 km/h (10 mph) than at 24 km/h (15 mph), as their peak rollover indices 

decreased to less than 0.7. As for the empty truck, even though it travelled at a lower speed, its 

light axle load caused it to have a peak rollover index of 1. The high rollover index is believed to 

occur when the truck transitions onto the truck apron, which is modeled as a three-inch vertical 

face. An alternate truck apron design was not modeled in this study. 

EFFECT OF ROUNDABOUT TILT ON TRUCK STABILITY

In this test, the entire roundabout was tilted at four constant angles to determine the effect of 

tilted slope on truck stability. Tilt was with respect to the approaching roadway. Positive tilt 

indicated that, from the perspective of a driver on the approaching roadway, the entire 

roundabout was tilted down to the right, i.e., ‘outward’ toward the roadway’s right shoulder. 

Negative slope indicates that the roadway was tilted, from the perspective of a driver on the 

approaching roadway, down to the left, i.e., ‘inward’ toward the road’s centerline, as shown in 

figure 5. 
57 

The baseline semi-truck model was used for all of the tilt cases, as summarized in table 13; all 

dimensions not shown are unchanged within the set of single-lane and two-lane cases (see table 8 

for details). 

Table 13. Roundabout tilt slope for a 42.7-m (140-ft) and 54.9-m (180-ft) roundabout.

Case # Roundabout Diameter 

- ICD [m (ft)]

Movement Percent Tilt 

(slope)*

16 42.7 (140) Right turn +4

17 42.7 (140) Through +4

18 42.7 (140) Left turn +4

19 42.7 (140) Right turn +2

20 42.7 (140) Through +2

21 42.7 (140) Left turn +2

22 42.7 (140) Right turn -4

23 42.7 (140) Through -4

24 42.7 (140) Left turn -4

25 42.7 (140) Right turn -2

26 42.7 (140) Through -2

27 42.7 (140) Left turn -2

62 54.9 (180) Right turn +4

63 54.9 (180) Through (left lane) +4

64 54.9 (180) Through (right lane) +4

65 54.9 (180) Left turn +4

66 54.9 (180) Right turn +2

67 54.9 (180) Through (left lane) +2

68 54.9 (180) Through (right lane) +2

69 54.9 (180) Left turn +2

70 54.9 (180) Right turn -4

71 54.9 (180) Through (left lane) -4

72 54.9 (180) Through (right lane) -4

73 54.9 (180) Left turn -4

74 54.9 (180) Right turn -2

75 54.9 (180) Through (left lane) -2

76 54.9 (180) Through (right lane) -2

77 54.9 (180) Left turn -2

* “+” indicates outward tilt and “-” indicates inward tilt

Right Turns

The rollover indices for the baseline truck turning right at roundabouts with the various tilted

slopes are shown in figure 38 and figure 39, for a single-lane and a two-lane roundabout, 

respectively. 
58 

Figure 38. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, right turn). 

Figure 39. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, right turn)

Figure 38 and figure 39 show that the rollover index—and therefore, the likelihood of truck 

rollover—increased as the roundabout tilt decreased, indicating that tilting the roundabout 

outward had a stabilizing effect for right-turn movements. The peak rollover index for the semitruck in the single-lane, 42.7-m (140-ft) roundabout with no tilt (0.50, figure 25), fell between 

the positive and negative peak rollover indices in figure 38. Similarly, for the semi-truck in the 

two-lane 42.7-m (140-ft) roundabout with no tilt (0.43, figure 26), the peak rollover index fell 

between the positive and negative peak rollover indices in figure 39. 
59 

Through Movements 

The rollover indices for the baseline truck going straight through roundabouts with the various 

tilted slopes are shown in figure 40 and figure 41, for a single-lane and a two-lane roundabout, 

respectively.

Figure 40. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement). 

Figure 41. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement). 

Figure 40 and figure 41 indicate that at the roundabout entry and exit, a more negative/inward tilt 

increased rollover index. The opposite was true when the truck was in the roundabout, where a 
60 

more negative/inward tilt decreased rollover index. For the single-lane roundabout (figure 40), 

all peak rollover indices occurred while the truck was on the circulatory roadway inside the 

roundabout, and not at the entrance or exit. Thus, for single-lane roundabout, a negative/inward

tilt was less likely to cause rollover. For the two-lane roundabout (figure 41), peak rollover 

indices also occurred while the truck was on the circulatory roadway inside the roundabout, 

again suggesting that a negative/inward tile is preferable. 

Left Turns

The rollover indices for the baseline truck turning left at roundabouts with the various tilted 

slopes are shown in figure 42 and figure 43, for a single-lane and a two-lane roundabout, 

respectively.

Figure 42. Effect of roundabout tilt on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, left turn).
61 

Figure 43. Effect of roundabout tilt on rollover index for a 54.9-m (180-ft) roundabout 

(single-lane, left turn). 

The results for semi-trucks making left turns in the roundabouts, shown in figure 42 and figure 

43, followed the same trends as those for trucks moving through the roundabouts, (figure 40 and 

figure 41). A more positive/outward tilt led to higher rollover indices in the circulatory roadway, 

but lower rollover indices at the roundabout entrance and exit. For the single-lane roundabout 

(figure 42), negative/inward tilt resulted in lower peak rollover indices, but for all tilted slopes, 

the peak rollover index reached 1, indicating a high risk of rollover. For the two-lane roundabout 

(figure 43), a negative/inward tilt resulted in lower peak rollover indices in the circulatory 

roadway, but did not raise the peak rollover indices at the entry above those in the circulatory 

roadway. Thus, for this roundabout, a negative/inward tilt is preferable. 

The baseline WB-67 truck had a high risk of rollover when turning left in the 42.7-m (140-ft) 

single-lane roundabout at 24 km/h (15 mph), where the peak rollover indices for all tilted slopes 

reached 1 (figure 42). Hence, four additional cases with a lower circulating speed were created 

and tested. In these cases, the circulating speed was decreased from 24 km/h (15 mph) to 16 

km/h (10 mph). The rollover indices for the reduced speed cases are shown in figure 44. 
62 

Figure 44. Effect of roundabout tilt on rollover index at different circulating speeds: 16 

km/h (10 mph) (left plot) and 24 km/h (15 mph) (right plot); 

(single-lane roundabout, left turn). 

As figure 44 shows, it is evident that the peak rollover indices for all cases with a (16-km/h (10-

mph) circulating speed were less than 0.80, indicating that the truck could navigate through the 

tilted roundabouts at 16 km/h (10 mph). Therefore, a lower circulating speed could result in 

better truck lateral stability in tilted roundabouts. 

EFFECT OF ENTRY SPEED ON TRUCK STABILITY

This test intended to evaluate the effect of entry speeds on truck stability. Three entry speeds 

were considered for the baseline truck, as summarized in table 14; all dimensions not shown are 

unchanged within the set of single-lane and two-lane cases (see table 8 for details). For cases 

with 32 km/h (20-mph) and 40 km/h (25-mph) entry speeds, the truck slowed down to 24 km/h 

(15 mph) to negotiate the roundabout. For cases with an entry speed of 24 km/h (15 mph), the 

truck maintained that speed for the entire run. 
63 

Table 14. Entry speed cases considered for evaluating truck dynamics. 

Case #

Roundabout Diameter 

- ICD [m (ft)]

Movement Speed at 

entry/roundabout/exit

[km/h (mph)]

1 42.7 (140) Right turn 32/24/32 (20/15/20)

2 42.7 (140) Through 32/24/32 (20/15/20)

3 42.7 (140) Left turn 32/24/32 (20/15/20)

28 42.7 (140) Right turn 24/24/24 (15/15/15)

29 42.7 (140) Through 24/24/24 (15/15/15)

30 42.7 (140) Left turn 24/24/24 (15/15/15)

31 42.7 (140) Right turn 40/24/40 (25/15/25)

32 42.7 (140) Through 40/24/40 (25/15/25)

33 42.7 (140) Left turn 40/24/40 (25/15/25)

34 54.9 (180) Right turn 32/24/32 (20/15/20)

35 54.9 (180) Through (left lane) 32/24/32 (20/15/20)

36 54.9 (180) Through (right lane) 32/24/32 (20/15/20)

37 54.9 (180) Left turn 32/24/32 (20/15/20)

78 54.9 (180) Right turn 24/24/24 (15/15/15)

79 54.9 (180) Through (left lane) 24/24/24 (15/15/15)

80 54.9 (180) Through (right lane) 24/24/24 (15/15/15)

81 54.9 (180) Left turn 24/24/24 (15/15/15)

82 54.9 (180) Right turn 40/24/40 (25/15/25)

83 54.9 (180) Through (left lane) 40/24/40 (25/15/25)

84 54.9 (180) Through (right lane) 40/24/40 (25/15/25)

85 54.9 (180) Left turn 40/24/40 (25/15/25)

Right Turns

The rollover indices for the right turns and the various entry speeds in the two roundabouts are 

shown in figure 45 and figure 46. 
64 

Figure 45. Effect of entry speed on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, right turn). 

Figure 46. Effect of entry speed on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, right turn). 

The different speeds of the trucks caused them to encounter roadway features, such as curves and 

slope, at different times, so the peaks in rollover index corresponding to those roadway features 

are not aligned in the figures comparing truck speed. 

As shown in figure 45 and figure 46, the peak in rollover index when the truck entered the 

roundabout was higher for the truck traveling at 40 km/h (25-mph) than it was for the other 

speeds. The peak when the truck exited the roundabout was largely unaffected by truck speed. 

The reason that trucks traveling at 24 km/h (15 mph) and 32 km/h (20 mph) had generally the 
65 

same magnitude of peaks in rollover index was because the truck with an entry speed of 32 km/h 

(20 mph) slowed down to 24 km/h (15 mph) within a very short period of time. 

The differences in peaks in rollover index can be explained by the trucks’ speeds when they 

navigated the curvature at the roundabout entry and exit. The trucks traveling at 40 km/h 

(25 mph) and 32 km/h (20 mph) both slowed to 24 km/h (15 mph), but the truck traveling at 40 

km/h (25 mph) took longer to do so, and was thus was traveling faster when it navigated the 

curvature at the roundabout entry, resulting in a higher rollover index. The truck traveling at 32 

km/h (20 mph) was able to slow to 24 km/h (15 mph) by the time it reached the curvature at the 

roundabout entry, so it exhibited the same rollover-index behavior as the truck traveling at a 

constant 24 km/h (15 mph). All trucks were traveling at 24 km/h (15 mph) at the roundabout 

exit, and all had similar peaks in rollover index at that point. 

A direct correlation between entry speed and rollover index in at the roundabouts’ entry was as 

expected. For both roundabouts, the rollover index peak remained below the critical limit of 1.0 

for the speeds considered in the simulation. 

Through Movements 

The rollover indices for the through movements and the various entry speeds in the two 

roundabouts are shown in figure 47 and figure 48. 

Figure 47. Effect of entry speed on rollover index for a 42.7-m (140-ft) roundabout 

(single-lane, through movement).
66 

Figure 48. Effect of entry speed on rollover index for a 54.9-m (180-ft) roundabout 

(two-lane, through movement). 

The results shown in figure 47 and figure 48, above, are similar to those for right turns in the 

previous section. The peak in rollover index was higher for the truck traveling at 40 km/h (25 

mph) than for those traveling at the other speeds, because it encountered the curvature at the 

roundabout’s entry while traveling faster. The peak at the roundabout exit was almost the same 

height for the three speeds, because the three trucks were all traveling 24 km/h (15 mph) before 

exiting the roundabout. 

Left Turns

The rollover indices for the left turns at the various entry speeds in the two roundabouts are 

shown in figure 49 and figure 50. 
67 

Figure 49. Effect of entry speed on rollover index for a 42.7-m (140-ft) single-lane 

roundabout 

(single-lane, left turn). 

Figure 50. Effect of entry speed on rollover index for a 54.9-m (180-ft) two-lane roundabout 

(two-lane, left turn). 

The results in figure 49 and figure 50 show the same trends as the other movements, but the 

magnitude of the rollover indices was different. For the single-lane roundabout, trucks traveling 

at all three speeds experienced rollover indices of 1 at the roundabout exit, indicating a high 

likelihood of rollover. For the two-lane roundabout, the peak rollover indices at the roundabout 

exit were 0.80. 
68 

SUMMARY OF SIMULATION ANALYSIS

The results of all six tests and some additional cases are summarized in this section. For these 

analyses, peak rollover index indicates the moment of largest rollover likelihood during the 

entire movement. A zero rollover index represents a perfectly weight-balanced vehicle. A vehicle 

with a rollover index lower than 0.8 is capable of traveling through a roundabout and 

maintaining stability. Between 0.8 and 0.9, the vehicle becomes less stable, and if peak rollover 

index exceeds 0.9, the vehicle has a high likelihood of rollover. 

Effect of Roundabout Inscribed Circle Diameter on Truck Stability

Figure 51. Effect of roundabout diameters on peak rollover index by movement in singlelane roundabouts. 

Figure 51 shows that the peak rollover indices were lower for the 39.6-m (130-ft) ICD 

roundabout than the 33.5-m (110-ft) roundabout. However, the peak rollover indices for the 

45.7-m (150-ft) roundabout were much higher than the other ICDs. This was because trucks 

negotiating the 45.7 (150-ft) roundabout traveled at 24 km/h (15 mph), compared to 16 km/h (10 

mph) for the other two roundabout ICDs. 
69 

Figure 52. Effect of roundabout diameters on peak rollover index by movement for twolane roundabouts. 

Figure 52 shows that for two-lane roundabouts, all three roundabout ICDs had nearly the same 

rollover indices for right turns. For through movements and left turns, rollover indices decreased 

as diameters increased. When the truck performed a through movement from the right lane and 

straddled lanes, the peak rollover index was lower than when the truck approached from the left 

lane. Left turns had the highest peak rollover indices of the movements. 

In summary, when performing right-turn movements, a roundabout’s ICD does not greatly affect 

the likelihood of rollover, because the vehicle need not negotiate the central island. Therefore, 

regardless of the cross-sectional design of the roadway, the elements that mainly determine the 

vehicle lateral dynamics for making right turns are entry radii, exit radii, and especially traveling 

speed, as indicated in figure 51. 

For through movements and left-turn movements, a larger ICD decreased the possibility of 

vehicle rollover. For the same vehicle speed in the roundabout, lateral accelerations are smaller 

in a roundabout with a larger diameter. The results of this test show that speed plays a highly 

significant role in truck lateral dynamics in roundabouts. 
70 

Effect of Circulatory Roadway Cross-section on Truck Stability

Three scenarios with different roundabout cross-sections were tested to determine their effect on 

truck stability. The peak rollover indices of each scenario for a 54.9-m (180-ft) roundabout are 

shown in figure 53. 

Figure 53. Effect of truck apron and road crown on peak rollover index for a 54.9-m (180-

ft) two-lane roundabout by movement. 

According to figure 53, for right-turn movements, the crowned cross-section caused the truck to 

have relatively less lateral stability than the baseline cross-section. This was because for the 

crowned roundabout, the left edge of the inner two-thirds of the roadway was lower than the 

right edge. When the truck straddled the lanes to make a right turn, its left wheels ran on the 

inner, lower part of the roadway, increasing the truck’s tendency to lean to the left side and 

reducing its lateral stability. However, the rollover indices for both the baseline and crowned 

roundabout were low, indicating a low likelihood of rollover. 

For the through movements and left-turn movements, the crowned cross-section had a smaller 

peak rollover index and lower likelihood of rollover than the baseline cross-section. When the 

trailer rear tires moved onto the truck apron (denoted by ‘(apron)’ in figure 53’s legend), the 

rollover index was significantly higher than when the truck did not move onto the truck apron. 

This was mainly because the apron’s outward slope caused the truck to lean to the outside in the 

direction of the centrifugal forces against the trailer, and was also due to transient dynamics 

caused by the truck moving onto and off of the apron. If the truck did not straddle lanes to make 

a through movement or left turn, the truck apron and road crown combination resulted in a lower 

likelihood of rollover than the truck apron and baseline combination. 
71 

Because trucks turning left in these test cases had high rollover indices, additional cases at 

reduced speed were performed to determine whether the extent reduced speed would reduce the 

rollover indices. When the semi-truck traveled at a slower 16 km/h (10 mph), it had a much 

lower likelihood of rollover. It should be noted that load configuration, truck suspension 

properties, and the truck’s path all affect the truck’s lateral dynamics. For instance, a truck with a 

shifting load may have less lateral stability than a truck with a fixed load, and more research is 

needed to cover a larger number of possible cases. 

Effect of Truck Configuration on Truck Stability

Three different truck configurations were simulated to determine their lateral dynamics. The 

peak rollover indices for the truck configurations for a 42.7-m (140-ft) single-lane roundabout 

and a 54.9-m (180-ft) two-lane roundabout are shown in figure 54 and figure 55, respectively.

Figure 54. Effect of truck configuration on peak rollover index for a 42.7-m (140-ft) 

roundabout by movement.
72 

Figure 55. Effect of truck configuration on peak Rollover Index for a 54.9-m (180-ft) 

roundabout by movement. 

Based on figure 54 and figure 55, a SU-30 single-unit truck was more stable than the other two 

configurations: a WB-67 semi-truck and a B-train tractor with double trailers. Even though a Btrain tractor with double trailers was longer than a WB-67 semi-truck, each of the trailers was 

shorter than the semi-truck’s single trailer, and the hitch connection between them allowed the 

tractor with double trailers to better conform to the roundabout. Additionally, the longer 

wheelbase meant the tractor with double trailers’ second trailer circulated at a lower speed than 

the semi-truck’s trailer, resulting in lower lateral accelerations. Thus, in general, a tractor with 

double trailers had similar lateral stability to the semi-truck that was modeled. 

Effect of Load Configuration on Truck Stability

Three load configurations were studied for the baseline semi-truck to determine whether the load 

configuration impacted truck lateral stability. The peak rollover indices for each load 

configuration are shown in figure 56 and figure 57 for the single-lane and two-lane roundabout, 

respectively. 
73 

Figure 56. Effect of truck weight on peak Rollover Index for a 42.7-m (140-ft) roundabout

by movement. 

Figure 57. Effect of truck weight on peak Rollover Index for a 54.9-m (180-ft) roundabout

by movement.

75 

Figure 56 indicates that in the 42.7-m (140-ft) single-lane roundabout, the half-full truck had the 

lowest rollover indices and the lowest likelihood of rollover. That is because a lighter axle load 

has less load weight holding the truck down against the road, and can be more easily unloaded. A 

heavier axle load, such as in the fully loaded truck, is more heavily loaded against the road and 

requires more unloading to reach a high likelihood of rollover. 

The results in figure 57 show that all of the three load conditions resulted in rollover indices 

reasonably below 1. For the 54.9-m (180-ft) roundabout, the full truck yielded the largest 

rollover index, and the half-full truck the smallest. 

In summary, trucks with different load configurations had different dynamic responses to the 

road input, as one might intuit. Somewhat less intuitively, results indicated that an empty truck 

reached its rollover limit sooner than a loaded or partially loaded truck, particularly in 

roundabouts with smaller diameters. In practice, maintaining safety would necessitate designing 

the roadway for the empty truck condition or advising lower speeds. 

In addition, changes in roadway curvature and side-to-side elevation resulted in transient lateral 

dynamics, causing large lateral accelerations, roll angles, and rollover index peaks. This 

phenomenon was most noticeable when an empty truck moved straight through or turned left at a 

single-lane roundabout with a truck apron. For larger roundabouts, the balance between the 

truck’s CG height (due to the load condition) and the wheel loads was the primary factor 

determining rollover index. For the 54.9-m (180-ft) roundabout, the full truck exhibited the 

largest peaks in rollover index, although the rollover index remained well below the 1 rollover 

limit. The half-load truck exhibited the lowest rollover index. 

Reduced speed cases were added to test truck lateral dynamics with different load configurations 

at a lower speed. When the circulating speed was decreased to 16 km/h (10 mph), the fully and 

half loaded truck both exhibited reasonably better lateral stability. However, the lower 

circulating speed did not greatly reduce the rollover index for the empty truck, due to the lighter 

load on its axles. Hence full- and half-loaded semi-trucks were more sensitive to the speed and 

could obtain better stability when traveling at a lower speed. For the empty semi-trucks, a lower 

speed had relatively little effect on lateral dynamics.

Effect of Roundabout Tilt on Truck Stability

The entire roundabout was tilted at four separate constant slopes to determine the effect of tilted 

slopes on truck stability. The peak rollover indices for the tilted-slope cases are shown in figure 

58 and figure 59 for the single-lane and two-lane roundabouts, respectively. 
76 

Figure 58. Effect of roundabout tilt on peak rollover index for a 42.7-m (140-ft) roundabout

by movement. 

Figure 59. Effect of roundabout tilt on peak Rollover Index for a 54.9-m (180-ft) 

roundabout by movement.
77 

Figure 58 and figure 59 indicate that for right turns, the positive (outward) tilted slopes resulted 

in better truck lateral stability than negative (inward) titled slopes. The opposite effect of tilt 

occurred for through movements and left turns; at the roundabout entry and exit, negative/inward 

tilt increased rollover index, but in the circulatory roadway, negative (inward) tilt reduced 

rollover index. This outcome presents a challenge in terms of applying the results. A positive

(outward) tilt would reduce rollover index at the entry and exit, but a negative (inward) tilt would 

reduce rollover index in the circulatory roadway. For through movements and left turns, the 

rollover index peaks in the circulating roadway were larger than those at the entry and exit, so 

moderately tilting the roundabout negatively (inward) would increase truck lateral stability in the 

areas where it experienced the highest rollover index. Several elements interact with roundabout 

tilt to affect truck lateral dynamics, but, in general, negative (inward) tilting resulted in better 

truck lateral stability. 

Effect of Entry Speed on Truck Stability

The effect of entry speeds on truck stability was evaluated. The peak rollover indices for 

different entry speeds are shown in figure 60 and figure 61 for the single-lane and two-lane 

roundabouts, respectively. 

Figure 60. Effect of entry speed on peak rollover index for a 42.7-m (140-ft) roundabout by 

movement.
78 

Figure 61. Effect of entry speed on peak rollover index for a 54.9-m (180-ft) roundabout by 

movement. 

For right turning trucks, peak rollover indices occurred at the roundabout entry. According to 

figure 60 and figure 61, when trucks entered the roundabout at 40 km/h (25 mph) to make a right 

turn, they experienced much larger rollover indices than at the other two speeds. That was 

because the truck entering at 32 km/h (20 mph) slowed to 24 km/h (15 mph) within a short time

and therefore had behavior at the roundabout entry very similar to that of the truck traveling at a 

constant 24 km/h (15 mph). However, the truck traveling at 40 km/h (25 mph) did not decelerate 

to 24 km/h (15 mph) before it reached the roundabout entry, and its peak rollover index at that 

point was higher because of the higher speed. 

The peak rollover indices for through movements and left turns were nearly identical at the three 

entry speeds. That is because trucks experienced peak rollover index while navigating the central 

island, and at that point, all trucks had slowed to 24 km/h (15 mph), regardless of entry speed. 

Therefore, even though the truck entry speed significantly affected the truck’s lateral dynamics at 

the roundabout entry, it did not influence the truck’s lateral dynamics while in the circulatory 

roadway or the roundabout exit. 
79 

CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS

Based on the literature review, truck crash review, and simulation results, the following 

conclusions and recommendations were identified. 

DESIGN CONSIDERATIONS 

The following sections summarize design features that were identified as having the potential to 

impact truck stability based on this study. These should be considered in roundabout design, 

along with other site-specific elements, to determine the most appropriate design for each 

location. 

Circulatory Roadway Cross-Section

Two different circulatory roadway cross-sections were tested during the simulation modeling to 

determine their effect on truck stability. In two-lane roundabouts, when using both travel lanes to 

complete through movements or left turns, trucks had better lateral stability with crowned 

circulatory roadways than they did in roundabouts with a 2% cross slope to the outside. Although 

the simulation modeling showed that the crowned circulatory roadway increased stability for left 

turns and through movements at roundabouts, the crowned circulatory roadway resulted in a 

slightly higher rollover index for trucks making right turns when compared to roundabouts with a 

2% cross-slope to the outside. Therefore, it is important to consider truck movements at the 

roundabout to determine an appropriate design for each site. 

Truck Apron

The use of a truck apron is a well-established design practice for accommodating large vehicles

at roundabouts, while also providing adequate speed control for passenger cars. During the 

simulation modeling, trucks performing through or left-turn movements experienced a higher 

rollover index and rollover risk on the truck trailer at the moment when mounting and 

dismounting a truck apron modeled with a 3-in. vertical face. The design of the truck apron 

interface with the circulatory roadway could not be evaluated as part of this study, but the shape 

of the truck apron curb (e.g., height, slope, etc.) is believed to be a contributing factor. 

Speed

The literature review and truck crash review both indicated that speed was a contributing factor 

in some of the reported truck crashes at roundabouts, with trucks traveling too fast to safely 

move through the roundabout. Simulation results and the truck crash review indicated that speed 

within the circulatory roadway was an influential factor in truck stability at roundabouts. The 

simulation results indicated that speeds of 24 km/h (15 mph) were sufficient to experience a high 

risk of rollover in some of the single-lane roundabout scenarios tested. In these scenarios, truck 

stability was improved by reducing the vehicle speed within the circulatory roadway to 16 km/h 
80 

(10 mph). The speed within the circulatory roadway was more influential on truck stability than 

speeds at the entry and exit. Because truck speeds of 24 km/h (15 mph) may be too high for some 

roundabout configurations, drivers and police officers may not always recognize that speed was a 

contributing factor in rollover crashes. Education programs for truck drivers to inform them of 

the effect of truck speed on truck stability would encourage slower truck speeds in roundabouts. 

Truck Types and Loads 

The literature review indicated that center of gravity and truckload has an impact on truck 

stability. The crash history review found several crashes that noted load shift as a contributing 

factor to a rollover incident. The simulation results indicated that truck configurations and load 

have some influence on truck stability at roundabouts. WB-67 trucks were found to be less stable 

than SU-30 trucks and B-trains at smaller, single-lane roundabouts, while B-train and WB-67 

trucks were found to have similar stability in larger roundabouts. Empty trucks were found to 

have a higher risk of rollover, particularly for WB-67 and B-train configurations, in single-lane 

roundabouts, while fully loaded trucks, with higher center of gravity, were found to have the 

highest rollover indexes in two-lane roundabouts. The balance between the truck’s center of 

gravity height and the wheel loads appeared to be a factor influencing the rollover index for 

larger two-lane roundabouts. Non-static loads were not tested as part of this effort. 

Educating drivers about these influences on stability may impact driver behavior at roundabouts. 

For instance, the lower stability of empty trucks may be counterintuitive, and better education 

may inform drivers and impact driver behavior, leading to fewer rollovers. 

EDUCATION PROGRAMS AND DRIVER PRACTICE

Several of the conclusions from the simulation testing and information obtained from the crash 

history review and literature review indicated that factors in truck crashes at roundabouts 

involved vehicle types, vehicle loads, and driver behavior. Programs focused on educating truck 

drivers and other vehicle drivers on these findings would help promote appropriate driver 

behavior related to trucks at roundabouts. These efforts can build upon the collaboration taking 

place within the trucking industry in a number of states. 

RECOMMENDATIONS FOR ADDITIONAL STUDY 

This study provided a review of factors associated with truck rollover crashes at roundabouts by 

reviewing existing literature, by studying crash history involving trucks at roundabouts, and by 

completing a modeling analysis to evaluate trucks’ rollover index under various roadway, 

vehicle, and driver characteristics. There are several areas of future study that should be 

considered to continue this work. 

An evaluation of the impact of dynamic loads on truck stability was not performed, but would be 

informative, because unlike in the simulations performed here, loads may shift during travel if 
81 

not completely secured. Further research should consider the impact of this load shift and 

different load materials (liquid, solid) on truck stability. 

Further work on the effect of the truck apron curb design on stability is recommended. The 

simulation modeling analysis used for this study could not consider the difference between 

different curb designs, including more gradually sloped curbs, which may result in a less abrupt 

point of impact for the truck. Future studies should evaluate the impact of different cross-section 

slopes and curb heights on lowboys, which may be more likely to get stuck in roundabouts. 

Finally, this study could only evaluate one variable at a time, so further studies should consider 

various combinations of factors studied here, as well as additional factors, to better understand 

their combined impact on truck stability in roundabouts. These combination studies may be best 

conducted in collaboration with the trucking industry in lab and/or field settings.

83 

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PUBLICATION NO. FHWA-SA-15-073