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Publication Number: FHWA-HRT-13-047
Date: August 2013

 

Simulator Study of Signs for A Complex Interchange and Complex Interchange Spreadsheet Tool

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FOREWORD

The overall goals of this research on complex interchanges were to increase understanding of motorists’ expectations when navigating complex interchanges, determine how those expectations affect their behavior, and discover how the safety of the interchanges can be effectively increased through the use of better signing and marking practices.

Based on the initial literature review task and other ongoing work, the project was divided into two studies: (1) conduct a driving simulator study and (2) develop a metric that can score, rate, or otherwise categorize interchange complexity. This report documents a Federal Highway Administration project that identified potential improvements to current signing practices for complex interchanges and developed a spreadsheet decision tool for defining and quantifying interchange complexity.

This report is of interest to engineers, planners, and other practitioners who are concerned about implementing signing treatments for freeways as well as city, State, and local authorities who have a shared responsibility for ensuring public safety.

Monique R. Evans
Director, Office of Safety
Research and Development

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 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.

 

Technical Report Documentation Page

1. Report No.

FHWA-HRT-13-047

2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Simulator Study of Signs for a Complex Interchange and Complex Interchange Spreadsheet Tool

5. Report Date

August 2013

6. Performing Organization Code
7. Author(s) FHWA-HRT-13-047

Kay Fitzpatrick, Susan T. Chrysler, Marcus A. Brewer, Alicia Nelson, and Vichika Iragavarapu

8. Performing Organization Report No.

 

9. Performing Organization Name and Address

Texas Transportation Institute
The Texas A&M University System
College Station, TX 77843-3135

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-08-D-00032, Task Order #6

12. Sponsoring Agency Name and Address

Office of Safety Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

September 2010–September 2012

14. Sponsoring Agency Code

 

15. Supplementary Notes

The Contracting Officer's Technical Representative (COTR) was Jim Shurbutt, HRDS-30.

16. Abstract

This report documents a Federal Highway Administration (FHWA) project to identify potential improvements to current signing practices for complex interchanges. Based on the initial literature review task along with discussions on other ongoing work, FHWA and the research team divided the project into the following two studies: (1) conduct a driving simulator study and (2) develop a metric or tool that can score, rate, or otherwise categorize interchange complexity.

 

In the first study, test signs were introduced as six topics in a simulation along freeway roadways to evaluate drivers’ real-time response to the signs. Topic 1 tested the understanding and use of different methods to sign for an option lane. Almost all participants made the correct decision to exit or stay on the freeway; however, many unnecessary lane changes were made with each of the three sign sets (SSs). Topic 2 studied sign methods when two interstate exits were within close proximity and a need existed to sign for three destinations (two interchanges/exits and the through lanes). For the SS that had an arrow-per-lane design, all participants made correct lane change decisions. Topic 3 evaluated signing for an upcoming exit that had a Y-split into two directions. While several incorrect lane changes were made for each SS, the SS that used split exit signs at all three sign bridge (SB) locations had the fewest incorrect lane changes and was judged superior in comparison to the other two arrangements. Topic 4 evaluated whether it was better to fill an advance single sign with supplemental way-finding information or to spread the information among multiple signs. An observation from this topic was that spreading information about the next exit across multiple signs on a single bridge may have unintended consequences if the SB also includes a sign for another exit that is located to the left of the preferred lane. Topic 5 evaluated the effectiveness of sign spreading when there were many pieces of information on one SB. Similar to topic 4, it was determined that the lateral position of a sign on the SB is important. Topic 6 evaluated driver understanding of left exit signs. The difference between the two SSs that were tested was minimal.

 

In study 2, the complexity rating tool focused on geometric design factors and related effects on driver expectancy and driver workload. After several revisions, researchers settled on a spreadsheet tool that considered the effects of 32 weighted factors that were based on site characteristics. To determine how well the spreadsheet tool would evaluate interchanges, the research team used the spreadsheet to review 28 existing sites in 11 States. The sites were submitted by State transportation departments based on their perceived complexity. For the characteristics included in the spreadsheet, the results provided a general sense of the relative complexity of the interchanges studied.

17. Key Words

Complex Interchanges, Signs, Simulator Study, Spreadsheet Tool

18. Distribution Statement

No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161

19. Security Classification
(of this report)

Unclassified

20. Security Classification
(of this page)

 

21. No. of Pages

225

22. Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS

LIST OF FIGURES

Figure 1. Equation. Relationship between sign reading time and number of words
Figure 2. Equation. Revised formula for relationship between sign reading time and number of words
Figure 3. Illustration. Arrow-per-lane guide sign for a multilane exit
Figure 4. Illustration. Arrow-per-lane sequence for a split
Figure 5. Illustration. Diagrammatic guide sign for a multilane exit
Figure 6. Photo. Roadway scene shown to subjects without guide sign information.
Figure 7. Photo. Roadway scene shown to subjects with guide sign information
Figure 8. Illustration. Lane designation sign
Figure 9. Illustration. Advance guide sign located approximately 0.5 mi in advance of exit and centered over the four approach lanes
Figure 10. Illustration. Advance guide sign located 1 mi in advance of exit and centered over four approach lanes
Figure 11. Illustration. Geometry for topic 1
Figure 12. Illustration. Geometry for topic 2
Figure 13. Illustration. Geometry for topic 3
Figure 14. Illustration. Geometry for topic 4
Figure 15. Illustration. Geometry for topic 5
Figure 16. Illustration. Geometry for topic 6
Figure 17. Illustration. Example of labels used to code lane changes
Figure 18. Illustration. SS 1-A: arrow-per-lane sign
Figure 19. Illustration. SS 1-B: down arrow-per-lane through sign
Figure 20. Illustration. SS 1-C: no pull through sign
Figure 21. Illustration. SS 2-A: multiple signs with exit only panels
Figure 22. Illustration. SS 2-B: arrow-per-lane sign
Figure 23. Illustration. SS 2-C: diagrammatic sign
Figure 24. Illustration. SS 3-A: shared exit signs
Figure 25. Illustration. SS 3-B: split exit signs
Figure 26. Illustration. SS 3-C: shared exit advance signs with split exit gore sign
Figure 27. Graph. Topic 3 lane change location 3X_Left_2
Figure 28. Graph. Topic 3 lane change location 3X_Left_3
Figure 29. Graph. Topic 3 lane change location 3X_Right_2
Figure 30. Graph. Topic 3 lane change location 3X_Right_3
Figure 31. Illustration. SS 4-A: single sign with multiple destinations
Figure 32. Illustration. SS 4-B: sign spreading across multiple signs on a single bridge
Figure 33. Illustration. SS 4-C: sign spreading across multiple SBs
Figure 34. Illustration. SS 5-A: no sign spreading
Figure 35. Illustration. SS 5-B: sign spreadin
Figure 36. Illustration. SS 6-A: yellow plaque at top left
Figure 37. Illustration. SS 6-B: yellow panel at bottom of sign
Figure 38. Photo. Example of missing movements
Figure 39. Photo. Configuration of site OH-2
Figure 40. Photo. Configuration of site SC-3
Figure 41. Photo. Example view of simulator
Figure 42. Illustration. Facilitator instructions
Figure 43. Illustration. Topic 1 lane change coding
Figure 44. Graph. Topic 1 lane change location 1X_E_1
Figure 45. Graph. Topic 1 lane change location 1X_E_2
Figure 46. Graph. Topic 1 lane change location 1X_T_2
Figure 47. Graph. Topic 1 lane change location 1X_T_3
Figure 48. Illustration. Topic 2 lane change coding
Figure 49. Graph. Topic 2 lane change location 2X_T_3
Figure 50. Graph. Topic 2 lane change location 2X_T_4
Figure 51. Graph. Topic 2 lane change location 2X_1st_2
Figure 52. Graph. Topic 2 lane change location 2X_2nd_2
Figure 53. Graph. Topic 2 lane change location 2X_2nd_4
Figure 54. Graph. Topic 2 lane change location 2X_2nd_5
Figure 55. Illustration. MUTCD Figure 2E-34: Example of signing for a two-lane exit
ramp with two dropped lanes and a bifurcation beyond the mainline gore

Figure 56. Illustration. Topic 3 lane change coding
Figure 57. Illustration. Topic 4 lane change coding
Figure 58. Graph. Topic 4 lane change location 4X_CNV_1
Figure 59. Graph. Topic 4 lane change location 4X_CNV_2
Figure 60. Graph. Topic 4 lane change location 4X_2nd_2
Figure 61. Graph. Topic 4 lane change location 4X_1st_2
Figure 62. Illustration. Topic 5 lane change coding
Figure 63. Graph. Topic 5 lane change location 5X_T_4
Figure 64. Graph. Topic 5 lane change location 5X_Oak_1
Figure 65. Graph. Topic 5 lane change location 5X_Leon_3
Figure 66. Screenshot. SS 5-A: no sign spreading
Figure 67. Screenshot. SS 5-B: initial signs viewed when sign spreading is used
Figure 68. Screenshot. SS 5-B: sign spreading across SBs
Figure 69. Illustration. Topic 6 lane change coding
Figure 70. Graph. Topic 6 lane change location 6X_T_1
Figure 71. Graph. Topic 6 lane change location 6X_E_1
Figure 72. Graph. Topic 6 lane change location 6X_E_3
Figure 73. Screenshot. Portion of spreadsheet containing interchange information
Figure 74. Screenshot. Portion of spreadsheet containing ramp description
Figure 75. Screenshot. Portion of spreadsheet containing interim calculations
Figure 76. Screenshot. Example of factor scores
Figure 77. Screenshot. Example of weighted factor scores
Figure 78. Screenshot. Example of approach and overall complexity scores
Figure 79. Photo. Configuration of site SC-2
Figure 80. Photo. Aerial view of site AZ-1
Figure 81. Photo. Aerial view of site AZ-2
Figure 82. Photo. Aerial view of site AZ-3
Figure 83. Photo. Aerial view of site DE-1
Figure 84. Photo. Aerial view of site DE-2
Figure 85. Photo. Aerial view of site DE-3
Figure 86. Photo. Aerial view of site GA-2
Figure 87. Photo. Aerial view of site GA-3
Figure 88. Photo. Aerial view of site GA-4
Figure 89. Photo. Aerial view of site IN-1
Figure 90. Photo. Aerial view of site IN-3
Figure 91. Photo. Aerial view of site IA-1
Figure 92. Photo. Aerial view of site IA-2
Figure 93. Photo. Aerial view of site IA-3
Figure 94. Photo. Aerial view of site MD-1
Figure 95. Photo. Aerial view of site NY-1
Figure 96. Photo. Aerial view of site NY-3
Figure 97. Photo. Aerial view of site OH-1
Figure 98. Photo. Aerial view of site OH-2
Figure 99. Photo. Aerial view of site OH-3
Figure 100. Photo. Aerial view of site OR-1
Figure 101. Photo. Aerial view of site OR-2
Figure 102. Photo. Aerial view of site SC-1
Figure 103. Photo. Aerial view of site SC-2
Figure 104. Photo. Aerial view of site SC-3
Figure 105. Photo. Aerial view of site VA-1
Figure 106. Photo. Aerial view of site VA-2
Figure 107. Photo. Aerial view of site VA-3

LIST OF TABLES

Table 1. Number of testing scenarios
Table 2. Characteristics of the control subtask
Table 3. Characteristics of the guidance subtask
Table 4. Characteristics of the navigation subtask
Table 5. Driver response accuracy to varying information loads and exposure times
Table 6. Desirable reading times for overhead guide signs
Table 7. Maximum amount of information per sign structure
Table 8. Labels used to code lane changes shown in plots
Table 9. Topic 1: number of participants with lane change type by test variation
Table 10. Topic 2: number of participants with lane change type by test variation
Table 11. Topic 3: number of participants with lane change type by test variation
Table 12. Topic 4: number of participants with lane change type by test variation
Table 13. Topic 5: number of participants with lane change type by test variation
Table 14. Topic 6: number of participants with lane-change type by test variation
Table 15. Summary of study sites
Table 16. Ramp-specific characteristics for spreadsheet tool
Table 17. Factors (questions) and threshold values used in the spreadsheet
Table 18. Factors (numeric) and threshold values used in the spreadsheet
Table 19. Factors and weights used in spreadsheet sorted by weight
Table 20. Complexity scores for study sites
Table 21. Test scenarios by topic divided into participant groups
Table 22. Experimental order by participant group
Table 23. Texas licensed drivers by age and gender (2009)
Table 24. Texas educational background based on total population 18+ years old
Table 25. Variables collected in the simulator study
Table 26. Demographic information collected
Table 27. Demographic information for 42 participants
Table 28. Topic 1 testing variations
Table 29. Topic 1 scores for follow-up questions
Table 30. Topic 2 testing variations.
Table 31. Summary of responses to topic 2 question, “What about the signs influenced your decision to change lanes or not change lanes?”
Table 32. Summary of response to topic 2 question, “What about the signs influenced your decision to change lanes or not to change lanes?”
Table 33. Summary of responses to topic 2 question, “What about the signs influenced your decision to change lanes or not to change lanes?”
Table 34. Summary of response to topic 2 question, “Why did you change lanes (if they moved out of lane 3)?”
Table 35. Topic 3 testing variations.
Table 36. Topic 3 question, “Which lane do you think you need to be in to get to your destination: right, left, or either?”
Table 37. Topic 3 question, “Which exit do you think is coming first: Winner/Edison/Mission or Groton/Victor/Walker?”
Table 38. Topic 3 question, “Do you think Winner/Edison/Mission is on the left or right ahead (if the participant took the exit)?”
Table 39. Topic 3 question, “Do you think Groton/Victor/Walker is on the left or right ahead (if the participant took the exit)?”
Table 40. Topic 4 testing variations
Table 41. Topic 4 question, “What lane would you have gotten in to go to Kenston/Wright/Aspen?”
Table 42. Topic 4 question, “Should you have exited if you wanted to go to the Convention Center?”
Table 43. Summary of responses to topic 4 question, “How did you know to exit (if they took the exit)?”
Table 44. Summary of responses to topic 4 question, “How much longer do you drive until you will exit (if they did not take the exit)?”
Table 45. Topic 5 testing variations
Table 46. Topic 5 responses to questions
Table 47 Topic 6 testing variations
Table 48. Topic 6 scores for follow-up questions
Table 49. Characteristics of AZ-1
Table 50. Characteristics of AZ-2
Table 51. Characteristics of AZ-3
Table 52. Characteristics of DE-1
Table 53. Characteristics of DE-2
Table 54. Characteristics of DE-3
Table 55. Characteristics of GA-2
Table 56. Characteristics of GA-3
Table 57. Characteristics of GA-4
Table 58. Characteristics of IN-1
Table 59. Characteristics of IN-3
Table 60. Characteristics of IA-1
Table 61. Characteristics of IA-2
Table 62. Characteristics of IA-3
Table 63. Characteristics of MD-1
Table 64. Characteristics of NY-1
Table 65. Characteristics of NY-3
Table 66. Characteristics of OH-1
Table 67. Characteristics of OH-2
Table 68. Characteristics of OH-3
Table 69. Characteristics of OR-1
Table 70. Characteristics of OR-2
Table 71. Characteristics of SC-1
Table 72. Characteristics of SC-2
Table 73. Characteristics of SC-3
Table 74. Characteristics of VA-1
Table 75. Characteristics of VA-2
Table 76. Characteristics of VA-3

     

LIST OF ABBREVIATIONS

AASHTO  American Association of State and Highway Transportation Officials
C Correct lane change
C-D Collector-distributor
FHWA Federal Highway Administration
G Pregore undetermined
H Lane change to be in the lane as instructed as the starting lane
IC Lane change to correct an incorrect lane change
IL Incorrect lane change to the left
IR Incorrect lane change to the right
IS Incorrect lane change to go through
L Last recording for the participant (to estimate/double check PE)
MUTCD Manual on Uniform Traffic Control Devices
N Number of participants driving a particular testing variation
NCHRP National Cooperative Highway Research Program
NHTSA National Highway Traffic Safety Administration
PC Lane change leading to the correct lane change
PE Lane change to move into the end lane/pulling over to end simulation
PI Lane change leading to the incorrect lane change
PS Lane change to move into the start lane
PU Lane change leading to the unnecessary lane change
S Swerve (swerve from left lane to right and back to left is counted as two)
SB Sign bridge
SCL Speed change lane
SL Start lane
SS Sign set
sw Stroke width
TRB Transportation Research Board
TTI Texas Transportation Institute
TxDOT Texas Department of Transportation
U Unnecessary lane change
UC Lane change to correct an unnecessary lane change (other than swerve)
W Representing indecision (i.e., S, IC, or UC)

     

EXECUTIVE SUMMARY

BACKGROUND

As transportation agencies struggle with adding freeway lane capacity in times of limited resources and shrinking right-of-way, new interchange designs are being built beyond the traditional diamond and cloverleaf configurations. Freeway interchanges with lane drops, double lane exits with optional lanes, and other unusual geometries confuse drivers and may result in late lane changes and erratic movements near the gore.

OBJECTIVES

This project was initiated to identify potential improvements to current signing and marking practices for complex interchanges. Two approaches were used to investigate complex interchanges—a driving simulator study and a decision tool. The driving simulator task identified driver lane changing behavior for six research questions related to freeway guide signing. The decision tool was developed to measure complexity. If traffic control device practice is to vary by complexity, a way to define complexity is needed.

PROJECT OVERVIEW AND FINDINGS

Driving Simulator Study of Signing for Complex Interchanges

In the driving simulator task, 42 drivers from rural and urban areas in Texas used a desktop driving simulator to navigate to fictional destinations by following test guide signs. Driver peformance measures included lane change proximity to (theoretical) gore as well as the number of unnecessary lane changes. In addition, subjective measures of comfort and confidence were obtained.

The research team created a list of potential topics or research questions for sign sequences used at complex interchanges. A driving simulator was considered for those topics where it was important to know how quickly a driver would make a lane choice. Other topics were investigated using focus groups conducted in a separate collaborative project. The simulator was also considered when it was important to view signs in a sequence and for drivers to see their spatial placement on the roadway. A priority order was determined for the list of topics, and the top six topics were selected. Table 1 lists the six topics investigated in this study along with the number of sign sets (SSs) considered within each topic. The table also provides the number of testing variations (e.g., start lane (SL) and destination combinations).

Table 1. Number of testing scenarios.
Topic Number General Description Number of
SSs
Number of
Testing Variations
Total
Test Scenarios
1 Use of option lane 3 4 12
2 Close proximity of two interstate exits 3 6 18
3 Y-split 3 4 12
4 Information spreading (more signs per bridge) 3 4 12
5 Information spreading (multiple sign bridges) 2 3 6
6 Left exit 2 3 6
Total Not applicable 16 24 66

Topic 1

Topic 1 tested the understanding and use of different sign methods for an option lane. The topic evaluated driver understanding of three different SSs: arrow per lane, a down arrow per lane, and a sign only for the exit and not the through movement. Almost all participants made the correct decision to exit or stay on the freeway; however, many unnecessary lane changes were made with each of the three SSs by people whose SL was either the on the far left or the far right. Interestingly, drivers who started in the center lanes and who were told to exit moved to the far right lane, which included an unnecessary lane change. However, drivers who started in the center lane and given the through destination did not move to the far left lane. This may be due to some reluctance on their part to move into the left lane, which is typically used for high-speed passing.

Topic 2

Topic 2 studied methods for creating signs when two interstate exits are within close proximity and there is a need for signs for three destinations (two interchanges/exits and the through lanes). For the SS that had an arrow per lane design, all participants (42) made correct lane change decisions. A sign adapted from the Manual on Uniform Traffic Control Devices (MUTCD)-style diagrammatic sign also had many correct lane change decisions with five or more of the seven participants in a group (with same SL and destination) making the correct decision.(1) Of the 42 participants who viewed this SS, only three made incorrect lane change decisions. The SS with multiple signs with exit only panels did not have as favorable results (e.g., 6 of the 42 participants made incorrect lane change decisions). This sign array also had more of the participants wanting additional information to make a lane change decision.

Topic 3

Topic 3 evaluated signing for an upcoming exit that then has a Y-split into two directions. Signing options included a split sign to explore if it helps to guide drivers into the appropriate lane for the Y-split in advance of the initial exit. The split sign shows the two destinations side-by-side with a vertical white line separation. One SS (3-B) had the split sign used for the two advance signs and at the gore. Another SS (3-C) only used the split sign at the gore with the two advance signs showing the destinations vertically stacked. The third SS tested (3-A) used the vertical stacked format for both the two advance signs and the gore sign. The lateral location of the destination on the sign was used by the participants in making a lane change decision. Several lane changes were made at the first appearance of the split exit sign. While several incorrect lane changes were made for each SS, SS 3-B, which used split exit signs at all three sign bridge (SB) locations, had the fewest and was deemed superior in comparison to the other two arrangements.

Topic 4

Topic 4 evaluated whether it is better to fill an advance single sign with supplemental way-finding information, such as exit information for a convention center, or to spread the information among multiple signs, including ground-mounted signs. Gore signs with advance signs at 1 mi were used to explore if sign spreading on a single bridge or on multiple bridges improved where the lane change was occurring. For most of the variations studied, a SS with the supplemental information on a separate sign located between the 1-mi advance and the exit gore had the most participants make the correct lane change decision. However, another SS presenting information for the next exit stacked on one sign also had many of the participants correctly making lane positioning decisions. When the destination information was spread across multiple signs on a single bridge, the supplemental sign ends up being located in the center of the SB. When this variation was tested, several participants made incorrect lane changes to the left when the instructions were to go to the second destination. These drivers may have been positioning their vehicles in the lane under the sign with their intended destination. This finding indicates that spreading information about the next exit across multiple signs on a single bridge may have unintended consequences if the SB also includes a sign for another exit that is located to the left of the preferred lane.

Topic 5

Topic 5 evaluated the effectiveness of sign spreading when there are many pieces of information on one SB. One SS did not have sign spreading (SS 5-A), and the other SS had sign spreading longitudinally across multiple SBs (SS 5-B). The lateral position of a pull-through sign on the SB is important. SS 5-A had more unnecessary lane changes as compared to SS 5-B. Half of the participants with SS 5-A had unnecessary lane changes, while SS 5-B had no unnecessary lane changes. Because SS 5-A had more signs on a single-SB, the sign for the through destination was farther to the left, which may have resulted in participants trying to position themselves below the destination name, resulting in an unnecessary (but not incorrect) lane change.

Topic 6

Topic 6 evaluated driver understanding of the 2009 MUTCD left exit standards.(1) Only 1-mi and 0.5-mi advance signs were used to test how quickly a driver identifies the left exit and changes lanes and whether there is confusion if it is an exit only or optional exit. SS 6-A had a yellow plaque at the top left, which is the new MUTCD standard, while SS 6-B had a yellow panel at the bottom of the sign, which is the old standard. Generally, for the two SSs tested under this topic, participants understood which side of the road the exit was located. It is unclear if this was because the participants were cued by the placement of the sign over the left lane, read the word “left” on the signs, or a combination of the two. The placement of the sign over the left lane resulted in the participants correctly avoiding moving across multiple lanes to make a right exit. However, when the participants did not need to make a left exit, they frequently moved out of the leftmost lane due to personal preference even though the lane was not an exit-only lane. A few more of the non-exiting participants who saw SS 6-B with the yellow panel at bottom of sign moved out of the leftmost lane (8 of 14) as compared to the participants who saw SS 6-A with a yellow plaque at the top left (5 of 14). For this study, the difference between these two SSs was minimal.

Decision Tool to Define and Quantify Interchange Complexity

Because complexity is typically a qualitative characteristic, the ability to objectively evaluate the complexity of an interchange is somewhat difficult. That difficulty is compounded when trying to compare the complex features of multiple interchanges. This task developed a spreadsheet-based decision tool as a method of quantifying and comparing the complexity of freeway interchanges in the United States. Efforts within the task included initially developing the tool by the research team, reviewing the preliminary tool by a set of experts, and modifying the spreadsheet tool based on a review of the characteristics of 28 existing interchanges in 11 States. These study sites ranged from relatively simple to very complex, and results indicate that the spreadsheet generated scores that were generally consistent with researchers' qualitative estimation of the sites' relative complexity.

During the initial stages of developing the spreadsheet, researchers discussed a variety of methods to apply a consistent set of criteria to measure complexity, and many potential variables were considered (e.g., geometric design variables, traffic control device variables, driver workload variables, etc.). Researchers also discussed the basis on which the following variables would be considered:

Researchers also discussed how to determine the complexity of each variable. That is, what quantity of a particular variable is considered to add complexity, and how does that compare to the complexity contributed by other variables? Researchers assigned threshold values to each variable, such that if the variable exceeded that value at a given interchange, it was deemed to have greater complexity. For example, if an interchange had more than three left-hand exits, the complexity of that interchange would be greater than an interchange with three or fewer left-hand exits. Threshold values could also be compared to values in commonly used guidelines, such as the 2011 American Association of State and Highway Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book), and they can be weighted to reflect their contribution to the overall complexity relative to other variables.(2)

Given all of these considerations, researchers compiled a list of noteworthy variables, assigned proposed values and weights to them, and incorporated this list into the initial version of the spreadsheet tool. In January 2011, the researchers conducted an expert panel discussion to present the initial spreadsheet tool for feedback and enlist the experts' help in identifying factors that contribute to the driving complexity of an interchange. This discussion was limited to design and geometric variables and did not address existing signing or other traffic control devices. In addition to the four members of the research team, the panel was composed of six practitioners: three from State transportation departments, two from the Federal Highway Administration (FHWA), and one from a State turnpike authority.

Overall, the panel thought the three categories of variables were helpful for addressing interchange complexity, but they noted that the workload and expectancy categories were very interrelated and could be combined into a single category. The example provided was that if the decision points for several major destinations were within the interchange area, the workload would be significantly increased without violating any expectations regarding traffic movements. Panelists stated that workload can be reduced through interchange design by spreading the decision points along the corridor and that addressing variables within the design category could eliminate complexity from both workload and expectancy violations. This point emphasized the need for early coordination of geometric design and signing needs.

Based on the panel's feedback, researchers revised the spreadsheet into its current version. Some of the key features of the spreadsheet tool are as follows:

The effect of each of these variables on the complexity of an interchange is calculated separately, but it is weighted to provide an indication of that variable's complexity compared to the others in the spreadsheet.

To determine how well the spreadsheet tool would evaluate interchanges, the research team issued a request to State transportation departments for locations of the most complex interchanges in their respective states. The research team received responses from 11 States, documenting 35 interchanges. After reviewing the information provided by the transportation departments, the research team used 28 of the interchanges for processing in the spreadsheet. Six of the remaining interchanges contained more than four approaches, which is the capacity of the spreadsheet, while the last site was not used because of poor aerial image quality. The 28 interchanges had a variety of characteristics, providing a unique opportunity to use the spreadsheet tool to compare interchange complexity. Researchers tested multiple combinations of weights to develop scores for the 28 sites based on the effects of characteristics from each site. The results generated by the final version of the spreadsheet produced a set of interchange complexity scores that were nearly identical to the research team's subjective ranking of the sites' complexity. This suggests that for the characteristics included in this spreadsheet, the spreadsheet tool provides a practical means of quantifying and comparing the relative complexity of interchanges in different locations with different characteristics.

     

 

ResearchFHWA
FHWA
United States Department of Transportation - Federal Highway Administration