Multiple Sources of Safety Information

from V2V and V2I: Redundancy, 

Decision Making, and Trust—Safety 

Message Design Report

PUBLICATION NO. FHWA-HRT-15-007 NOVEMBER 2015

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD

Vehicles and roadways of the future will employ advanced communications technologies to 

facilititate applications to make driving safer, more efficient, and more environmentally friendly.

The safety benefits will largely be achieved by communicating relevant safety information to the 

driver. The Human Factors for Connected Vehicles research program is focused on 

understanding, assessing, planning for, and counteracting the effects of signals or systemgenerated messages that take a driver’s eyes off the road (visual distraction), the driver’s mind 

off the driving task (cognitive distraction), and the driver’s hands off the steering wheel (manual 

distraction). The overall goal of this research is to support the introduction of this technology as 

a benefit to all transportation users.

The research described in this report provides some intial design guidance for vehicle-toinfrastructure (V2I) safety messages as well as some limited guidance for vehicle-to-vehicle 

systems. This report primarily uses existing transportation safety research, as well as research 

from related domains. This information can be used by connected-vehicle system designers and 

other State transportation department personnel to develop and implement V2I applications, to 

make sure these systems work effectively and safely within a larger vehicle-to-infrastructure, 

vehicle-to-vehicle, and vehicle-to-device environment. In this way, these findings will help make 

the interaction of roadway and vehicle systems safer, reduce the likelihood of crashes and 

injuries, and increase safety for all roadway users.

Monique R. Evans, P.E.

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.

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.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-15-007

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

4. Title and Subtitle

Multiple Sources of Safety Information from V2V and V2I: Redundancy, 

Decision Making, and Trust—Safety Message Design Report

5. Report Date

November 2015

6. Performing Organization Code

7. Author(s)

Christian M. Richard, Justin F. Morgan, L. Paige Bacon, Justin S. Graving, 

Gautam Divekar, and Monica G. Lichty

8. Performing Organization 

9. Performing Organization Name and Address

Battelle Seattle Research Center

1100 Dexter Avenue North, Suite 400

Seattle, WA 98109

10. Work Unit No.

11. Contract or Grant No.

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

Interim Report; Oct 2012–

May 2014

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract

This report is part of the Human Factors for Connected Vehicles (HFCV) research program, whose goal is to 

minimize driver workload by eliminating connected vehicle (CV) device-related distractions. The research 

described in this document is part of an effort to develop initial design guidance for vehicle-to-infrastructure (V2I) 

safety messages provided using driver-infrastructure interfaces and driver-vehicle interfaces. Existing HFCV 

research, in addition to research from related domains, was used to develop this preliminary design guidance. The 

target audiences for this information are CV system designers and other State transportation department personnel 

involved in the development and implementation of V2I applications that provide safety information. The design 

topics are divided into two sets. The first set is composed of general topics providing background information and 

covers issues that apply across multiple V2I applications. The second set includes topics focusing on specific V2I 

applications. The general objective of the guidance information is to assist in the coordination of V2I and vehicleto-vehicle displays and messages to safely support driver information needs and decision making. This information 

may serve as a resource for the development of guidelines that address V2I system-related human factors design 

issues in future versions of NCHRP 600: Human Factors Guidelines for Road Systems.

17. Key Words

Human Factors Connected Vehicle Program, Connected 

Vehicles, Design Guidance, Vehicle-to-Infrastructure, 

Vehicle-to-Vehicle, Driver-Infrastructure Interfaces, 

Driver-Vehicle Interfaces, Human Factors, Driver 

Behavior, Safety, V2X

18. Distribution Statement

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

79

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
ii

* SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380.

(Revised March 2003)

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius o

C 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit o

F 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

 
iii

TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION................................................................................................ 1

BACKGROUND ........................................................................................................................1

OVERVIEW OF THE REPORT.............................................................................................3

CHAPTER 2. OVERVIEW OF CV APPLICATIONS............................................................. 4

CV COMMUNICATION ARCHITECTURE........................................................................4

V2V..........................................................................................................................................4

V2I ...........................................................................................................................................4

Vehicle-to-Infrastructure, Vehicle-to-Vehicle, and Vehicle-to-Device (V2X) .......................4

DII............................................................................................................................................4

DVI ..........................................................................................................................................5

RSE ..........................................................................................................................................5

SUMMARIES OF SAFETY APPLICATIONS DISCUSSED IN THIS REPORT.............5

SSA ..........................................................................................................................................5

SLTA........................................................................................................................................6

RLVW......................................................................................................................................7

CSW.........................................................................................................................................8

SWIW-RS ................................................................................................................................9

CHAPTER 3. DII SAFETY MESSAGE GUIDELINE INFORMATION............................ 11

TOPIC 1. CONSIDERATIONS FOR ADDING A DII TO A CV RSE..............................12

Introduction............................................................................................................................12

Guideline................................................................................................................................12

Discussion ..............................................................................................................................13

Design Issues .........................................................................................................................15

TOPIC 2. SYSTEM-LEVEL CONFLICTS ..........................................................................16

Introduction............................................................................................................................16

Guideline................................................................................................................................16

Discussion ..............................................................................................................................17

Design Issues .........................................................................................................................19

TOPIC 3. MESSAGE-LEVEL INCONGRUENCE BETWEEN DII AND DVI 

SYSTEMS...........................................................................................................................19

Introduction............................................................................................................................19

Guidelines..............................................................................................................................19

Discussion ..............................................................................................................................20

TOPIC 4. MESSAGE CONTENT: MESSAGE ELEMENTS ............................................22

Introduction............................................................................................................................22

Guideline................................................................................................................................22

Discussion ..............................................................................................................................24

Design Issues .........................................................................................................................25

TOPIC 5. MESSAGE CONTENT: USE OF SYMBOLS AND TEXT...............................25

Introduction............................................................................................................................25

Guideline................................................................................................................................26

Discussion ..............................................................................................................................27

Design Issues .........................................................................................................................29
iv

TOPIC 6. SUPPORTING DRIVER TRUST OF SAFETY SYSTEMS .............................29

Introduction............................................................................................................................29

Guideline................................................................................................................................29

Discussion ..............................................................................................................................30

TOPIC 7. FACTORS THAT AFFECT GAP CHOICE.......................................................32

Introduction............................................................................................................................32

Guidelines..............................................................................................................................32

Discussion ..............................................................................................................................35

Design Issues .........................................................................................................................36

TOPIC 8. INTERSECTION DECISION SUPPORT FOR STOP-CONTROLLED 

MINOR AND MAJOR ROAD JUNCTIONS................................................................37

Introduction............................................................................................................................37

Guidelines..............................................................................................................................37

Discussion ..............................................................................................................................39

Design Issues .........................................................................................................................42

TOPIC 9. SLTA .......................................................................................................................42

Introduction............................................................................................................................42

Guideline................................................................................................................................42

Discussion ..............................................................................................................................43

TOPIC 10. RLVW ...................................................................................................................47

Introduction............................................................................................................................47

Guideline................................................................................................................................47

Discussion ..............................................................................................................................48

Design Issues .........................................................................................................................51

TOPIC 11. CSW.......................................................................................................................52

Introduction............................................................................................................................52

Guideline................................................................................................................................52

Discussion ..............................................................................................................................53

Design Issues .........................................................................................................................55

TOPIC 12. ROAD WEATHER: GENERAL HUMAN FACTORS CONSIDERATIONS55

Introduction............................................................................................................................55

Design Guidance....................................................................................................................56

Discussion ..............................................................................................................................56

Design Issues .........................................................................................................................58

CHAPTER 4. SUMMARY AND CONCLUSIONS................................................................. 60

REFERENCES............................................................................................................................ 63
v

LIST OF FIGURES

Figure 1. Photo. CICAS-SSA (DII) prototype implemented by Minnesota ................................... 6

Figure 2. Photo. CICAS-SLTA implemented at a signalized intersection ..................................... 7

Figure 3. Photo. Example of a CICAS-V DII................................................................................. 8

Figure 4. Photo. DCWS .................................................................................................................. 9

Figure 5. Illustration. Two-phase CMS display showing weather advisory................................. 10

Figure 6. Illustration. Example sign locations that consider ISD ................................................. 38

Figure 7. Diagram. Comparison of graphical displays for intersection decision support 

systems that send messages to drivers on the major road, minor road, or both...................... 39

Figure 8. Photo. General region drivers scan when making an LTAP of oncoming traffic ......... 43

Figure 9. Illustration. MUTCD symbol for “No Left Turn” that was presented on the DII 

to examine drivers’ understanding of the SLTA system ........................................................ 46

Figure 10. Illustration. MUTCD symbol for “Left Turn Yield” that was presented on the 

DII to examine drivers’ understanding of the SLTA system.................................................. 47

LIST OF TABLES

Table 1. DII safety message guideline information........................................................................ 3

Table 2. Operational and situational factors related to DII installation........................................ 13

Table 3. System activation overlap matrix ................................................................................... 17

Table 4. Definitions of message characteristics related to congruence ........................................ 20

Table 5. Message elements for consideration............................................................................... 23

Table 6. Enumerated combinations of message elements by warning system ............................. 24

Table 7. General considerations for implementing symbols and text elements in signs.............. 26

Table 8. Design considerations that affect driver trust ................................................................. 30

Table 9. Situational/site factors that can affect gap choice........................................................... 34

Table 10. Driver/vehicle characteristics that can affect gap acceptance decisions....................... 35

Table 11. General design considerations for intersection characteristics..................................... 44

Table 12. Timing factors that affect RLVW compliance ............................................................. 48

Table 13. Example DIIs for the RLVW as a function of condition/utilization............................. 48

Table 14. DII Elements to support horizontal curves................................................................... 53

Table 15. Criteria for legibility distances by seconds of visibility ............................................... 54

Table 16. Differences in presenting weather information via DII and DVI................................. 56

Table 17. Presenting weather information via DII and DVI......................................................... 56
vi

LIST OF ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials

CICAS-SLTA Cooperative Intersection Collision Avoidance System-Signalized Left 

Turn Assist

CICAS-SSA Cooperative Intersection Collision Avoidance System-Stop Sign Assist

CICAS-V Cooperative Intersection Collision Avoidance System to Prevent 

Violations

CMS Changeable message sign

ConOps Concept of operations

CSW Curve speed warning

CV Connected vehicle

DCWS Dynamic curve warning system

DII Driver-infrastructure interface

DSRC Dedicated short-range communication

DVI Driver-vehicle interface

HFCV Human Factors for Connected Vehicles

HFG Human Factors Guidelines for Road Systems

ISD Intersection sight distance

ITE Institute of Transportation Engineers

ITS Intelligent transportation systems

LCV Large combination vehicle

LED Light-emitting diode

LTAP Left turn across path

LTAP-OD Left turn across path—opposite direction

MUTCD Manual on Uniform Traffic Devices

NASS-GES National Automotive Sampling System-General Estimates System

OVW Overweight vehicle warning

PCI Pedestrian crossing intersection

POV Principal other vehicle

PRT Perception reaction time

RLVI Red light violator indication

RLVW Red light violation warning

RSE Roadside equipment

SLTA Signalized left turn assist

SPaT Signal phase and timing

SSA Stop sign assist

SWIW Spot weather information warning

SWIW-RS Spot weather information warning-reduced speed

TCD Traffic control device

V2I Vehicle-to-infrastructure

V2V Vehicle-to-vehicle

V2X Vehicle-to-infrastructure, vehicle-to-vehicle, and vehicle-to-device
1

CHAPTER 1. INTRODUCTION

BACKGROUND

Vehicle-to-vehicle (V2V) communications and vehicle-to-infrastructure (V2I) communications 

involve the wireless exchange of data among and between infrastructure and vehicles traveling in 

the same vicinity with the goal of realizing significant safety, mobility, and environmental 

benefits. This communications capability will enable a host of vehicle- and infrastructure-based 

safety systems and applications. The vision is that all vehicles on the roadway, (e.g., 

automobiles, trucks, transit vehicles, and motorcycles), will be able to communicate with other 

vehicles and the infrastructure to enable active safety applications, as well as improvements in 

mobility and environmental benefits.The Human Factors for Connected Vehicles (HFCV) 

research program seeks to understand, assess, plan for, and counteract the effects of signals or 

system-generated messages that take drivers’ eyes off the road (visual distraction), drivers’

minds off the driving task (cognitive distraction), and drivers’ hands off the steering wheel 

(manual distraction).

This Federal research investment is a critical factor in developing the knowledge to enable 

connected vehicle (CV) technologies to save lives and reduce injuries without unintended 

consequences. The ability to establish the basic principles of attention and distraction within the 

context of specific advanced communication and messaging technologies (used in vehicles and 

in roadway infrastructure components) is a challenging effort whose outcomes will form the 

parameters for and guide consistent development of safer systems and interfaces for countless 

new applications. When developing new applications, consistency and adherence to basic 

countermeasures for distraction are paramount to ensuring the ultimate safety of the driver. 

Human factors research allows the development of more robust algorithms for prioritizing safety 

and messages that assist the driver, as opposed to providing greater distraction or workload.

From a higher level transportation planning perspective, the National ITS (Intelligent 

Transportation Systems) Architecture was created to provide a common framework for planning, 

defining, and integrating ITS. The idea of V2V and V2I communications that enable active 

safety applications fits into this National ITS Architecture. For example, there are a number of 

relevant National ITS Architecture Service Packages, including AVSS10-Intersection Collision 

Avoidance and AVSS05-Intersection Safety Warning1

. The Intersection Collision Avoidance

Service Package describes a system that determines the probability of an intersection collision 

and provides timely warnings to approaching vehicles so that avoidance actions can be taken.

The Intersection Safety Warning Service Package describes a related system that monitors 

vehicles approaching an intersection and warns drivers when hazardous conditions are detected.

Such a system could detect impending violations (e.g., red light violations) and potential 

conflicts between vehicles occupying or approaching the intersection (e.g., situations where a left 

turn would be unsafe because of approaching traffic). When a potentially hazardous condition is 

detected, a warning is communicated to the involved vehicles using short range communications 

and/or signs/signals in the intersection1.

 1

Intelligent Transportation Systems Joint Program Office (2015). Web site: http://www.iteris.com/itsarch/html/mp/mpindex.htm, accessed 

on 7/13/15.
2

The scenario described above fits in very closely with the research covered in the current report.

In particular, the primary objective of this report is to investigate how drivers handle critical 

safety information from multiple sources, including V2V and V2I sources. This safety-critical 

information could include potential conflicts between vehicles occupying or approaching the 

intersection (e.g., situations where a left turn would be unsafe because of approaching traffic). To 

meet the objectives of this project, the following specific tasks were completed:

• Task 1: Literature Review and Gap Analysis.

• Task 2: Develop Research Plan.

• Task 3: Execute Research Plan.

• Task 4: Document Design Guidance.

This report documents the design guidance that comprises task 4. The objective of task 4 is to 

integrate the findings from previous project tasks into a safety message design guidance 

document that can be readily converted into more formal design guidelines by developers of 

future versions of NCHRP 600: Human Factors Guidelines for Road Systems.

Because this design reference focuses on human factors design issues related to infrastructure 

and other roadway elements, the current project activities have a similar focus. The target 

audiences for this information are CV system designers and other State transportation department

personnel involved in the development and implementation of V2I applications that provide 

safety information. Accordingly, the safety messages and design information provided in this 

report primarily address the V2I component of CV technologies. Also, while V2I communication 

with drivers can involve both a driver-infrastructure interface (DII) and a driver-vehicle interface 

(DVI) to provide CV information, this report focuses on providing design guidance related to 

DIIs, which is the aspect of V2I systems that would be the responsibility of roadway engineers 

and designers. Table 1 lists the safety message topics and their page locations in this document.
3

Table 1. DII safety message guideline information.

Topic

Page 

Number

Topic 1. Considerations for Adding a DII to RSE 12

Topic 2. System-Level Conflicts 16

Topic 3. Message-Level Incongruence Between DII and DVI Systems 19

Topic 4. Message Content: Message Elements 22

Topic 5. Message Content: Use of Symbols and Text 26

Topic 6. Supporting Driver Trust of Safety Systems 29

Topic 7. Factors That Affect Gap Choice 32

Topic 8. Intersection Decision Support for Stop-Controlled Minor and 

Major Road Junctions

36

Topic 9. Signalized Left Turn Assist 41

Topic 10. RLVW 46

Topic 11. Curve Speed Warning 51

Topic 12. Road Weather: General Human Factors Considerations 54

RLVW = Red light violation warning.

RSE = Roadside equipment.

OVERVIEW OF THE REPORT

The remainder of this report is composed of the following three chapters:

• Chapter 2. Overview of CV Applications: This section provides the definitions for 

the key terms used to describe the CV communication architecture, in addition to 

summary descriptions of the safety applications addressed in this report.

• Chapter 3. DII Safety Message Guideline Information: This section provides a set 

of safety message guidance topics for designers to use as a reference—including 

specific design parameters, identified design problem(s), and/or information—in 

developing safety applications.

• Chapter 4. Summary and Conclusions: This section provides a summary of the 

guideline effort as a whole, and discusses the key project conclusions .
4

CHAPTER 2. OVERVIEW OF CV APPLICATIONS

CV COMMUNICATION ARCHITECTURE

This chapter provides definitions of key terms used in the current report to describe the CV 

communication architecture. This chapter also provides an overview of the operation, collision 

types addressed, and target-site characteristics for the CV safety applications discussed in this 

report. The following applications are described in this chapter:

• Stop sign assist (SSA).

• Signalized left turn assist (SLTA).

• Red-light violation warning (RLVW).

• Curve speed warning (CSW).

• Spot weather information warning—reduced speed (SWIW-RS).

Additional CV safety applications have been developed under the CV program. These include 

applications such as overweight vehicle warning (OVW) and railroad crossing warning. These 

applications, however, are not discussed in this report.

V2V

V2V communication is the dynamic wireless exchange of anonymous, vehicle-based data using 

dedicated short-range communication (DSRC) protocols. The minimum transmitted data package 

from a vehicle is referred to as the “basic safety message” and contains information regarding the 

vehicle’s current position, speed, heading, acceleration, braking status, and vehicle size.(1) This 

information is broadcast to and received from surrounding vehicles. This communication enables 

a vehicle to sense the position of other vehicles and the threat or hazard they present with a 

360-degree awareness, calculate risk, issue driver advisories or warnings, or take preemptive 

actions to avoid and mitigate crashes.

V2I

V2I communication is the wireless exchange of safety and operational data between vehicles and 

the highway infrastructure (via roadside equipment (RSE)) using DSRC protocols. V2I 

communication is intended to prevent or reduce the severity of vehicle crashes; however, it can 

also provide system mobility and environmental benefits by supporting applications such as 

speed harmonization and traffic optimization.

(2)

Vehicle-to-Infrastructure, Vehicle-to-Vehicle, and Vehicle-to-Device (V2X)

V2X communication is a term collectively referring to any type of CV communication, including 

V2V, V2I, or vehicle-to-device communication.

DII

DII is a changeable information display located externally to the vehicle that provides 

information to drivers and other road users. CV safety application DIIs use V2X information to 
5

identify roadway conditions and provide appropriate safety messages, which are typically visible 

to multiple roadway users, as long as the DII display is pointing towards them. DIIs include a 

range of equipment, such as changeable message signs (CMSs), blank-out signs, triggered 

beacons accompanying static signs, dynamic signals, traffic control devices (TCDs), and inroad/in-path lighting. 

DVI

DVI is an in-vehicle display (or set of displays) and controls that a driver uses to obtain 

information from a vehicle.

(3) Messages provided via DVI can be targeted to the individual 

vehicle. A variety of DVIs can support CV safety applications, including dash- or instrument 

panel-mounted screens, head-up displays, auditory displays, and vibrotactile or haptic displays 

(i.e., seat pan vibrations or steering wheel torques). Throughout this report, the display element is 

typically the focus of discussions regarding DVIs for CV safety applications. CV safety 

application DVIs provide warning information to the driver using information acquired through 

V2X communication. Not all vehicles will support all CV safety applications; some vehicles may 

only support specific CV safety applications.

RSE

RSE is roadside-installed hardware used to relay messages using DSRC protocols. RSE may 

receive messages from vehicles, other RSEs, or from back offices that monitor traffic system 

performance (i.e., a traffic management center). RSEs may be permanently or temporarily 

installed, allowing their use for either long-term or short-term (e.g., work zones) applications. 

SUMMARIES OF SAFETY APPLICATIONS DISCUSSED IN THIS REPORT

The following section presents a brief summary of each of the five CV safety applications listed 

in the previous section, along with an overview of the application’s operation, collision types it 

may address, and its target-site characteristics.

SSA

SSA is a system that supports drivers on minor roads who are attempting to either cross or enter 

the intersecting major road. SSA provides drivers with information about oncoming vehicles 

traveling on the major road. The objective of this system is to help drivers safely travel through 

or turn onto a highway from a stop-controlled intersection. This system is related to the 

Cooperative Intersection Collision Avoidance System-Stop Sign Assist (CICAS-SSA).

SSA has the following attributes:

• Operation: Identifies the location and speed of vehicles traveling on the major road 

and provides the driver on the minor road with information via DII to assist in 

selecting an adequate gap when turning or going through the intersection.

• Collision Types: Includes left/right turn into path (rear-end), sideswipe (same 

direction), and right-angle crashes with vehicles travelling on the major road. 
6

• Target-Site Characteristics: Includes rural stop-controlled/through-stop 

intersections where lower speed/volume minor roads intersect higher speed/volume 

median divided highways. The DII is positioned where it can be easily seen by 

stopped drivers on the minor road.

Figure 1 provides an example of a CICAS-SSA (DII) protype.

Source: National RITS Conference.

Figure 1. Photo. CICAS-SSA (DII) prototype implemented

by Minnesota.

(4)

SLTA

The SLTA system supports drivers who are making permissive left turns at signalized 

intersections. The system provides information to left-turning drivers about the presence of 

oncoming vehicles based on proximity or available gap size. The objective of this system is to 

help reduce driver errors related to detecting traffic and judging gaps. This system is related to 

Cooperative Intersection Collision Avoidance System-Signalized Left Turn Assist (CICASSLTA).

SLTA has the following attributes:

• Operation: Identifies the location and speed of approaching vehicles and informs the 

turning vehicle about their presence by displaying a message on a DII and/or DVI to 

discourage drivers from making left turns when a gap is inadequate. The SLTA 

system can also be combined with other systems to provide information about the

presence of pedestrians and bicyclists in the left turning driver’s path. However, the 

design information in the current document provides information about SLTA 

systems for oncoming vehicles only—pedestrian detection/warning is not addressed.

• Collision Types: Includes vehicle head-on and sideswipe with on-coming traffic from 

opposite direction (left turn across path—opposite direction (LTAP-OD)). 
7

• Target-Site Characteristics: Includes intersections with a high volume of oncoming 

traffic and limited sight-distance/visibility for oncoming traffic. The DII may be 

collocated with existing TCDs to support drivers seeing the display.

Figure 2 provides an example of a CICAS-SLTA at a signalized intersection.

Source: University of California, Berkeley.

Figure 2. Photo. CICAS-SLTA implemented at a signalized intersection.

(5)

RLVW

The RLVW system supports drivers in safely traveling through signalized intersections. The 

system provides a warning to drivers who may potentially enter the intersection in violation of 

the TCD. The objective of this system is to reduce the frequency of red light violations. This 

system is related to Cooperative Intersection Collision Avoidance System to Prevent Violations 

(CICAS-V).

RLVW has the following attributes:

• Operation: Identifies vehicles that will enter an intersection in violation of the TCD 

based on vehicle speed and heading (contained in the vehicle’s basic safety message) 

and the TCD signal phase and timing (SPaT) information. Warnings are provided 

using DVI and/or DII.

• Collision Types: Inclues single and multivehicle collisions occurring within the 

intersection, including sideswipe (angle), broadside, and rear-end crashes. 

• Target-Site Characteristics: Includes signalized intersections, especially when sight

distances are limited or red light violation crashes are problematic. The DII may 

include displays collocated with the TCD, signs at the intersection, or in-pavement 

lighting.

Figure 3 provides an example of a CICAS-V DII.
8

Figure 3. Photo. Example of a CICAS-V DII. Adapted from previous research 

implementation.(6)

CSW

The CSW application supports drivers in traversing a roadway curve at a safe speed. The system 

provides an alert/warning to drivers if their current travel speeds exceeds a safe/advisory speed 

for the curve. The objective of this system is to reduce the occurrence of rollover or run-off-road 

crashes owing to unsafe speed in curves.

CSW has the following attributes:

• Operation: Identifies vehicles approaching a curve traveling at a speed above the 

safe or advisory speed and provides a warning to the driver via DVI and/or DII and 

advisory speed information.

• Collision Types: Includes single-vehicle rollover and run-off-road at road curvatures. 

• Target-Site Characteristics: Includes horizontal and complex curves. The DII 

placement should be in accordance with the Manual on Uniform Traffic Control 

Devices (MUTCD) standards and far enough in advance of the curve to allow drivers 

to make appropriate speed corrections prior to encountering the curve.(7)

Figure 4 provides an example of a dynamic curve warning system (DCWS) on a freeway.
9

Source: Oregon Department of Transportation.

Figure 4. Photo. DCWS.

(8)

SWIW-RS

The SWIW-RS system supports drivers who may encounter adverse weather conditions on their 

travel route. The SWIS-RS system provides drivers with information about potential weatherrelated hazards and appropriate precautions, such as reduced travel speed. The objective of the 

system is to reduce the risk of crash under adverse weather conditions.

SWIW-RS has the following attributes:

• Operation: Identifies vehicles traveling on roadways that will be or are affected by 

near-term, adverse weather conditions and provides an alert message via DVI and/or 

DII with information on the weather event and necessary actions (i.e., reducing 

speed).

• Collision Types: Includes single and multivehicle crashes owing to weather, 

including pre-crash control loss caused by high winds, flooding, adverse road surface 

conditions, and reduced visibility. 

• Target-Site Characteristics: Includes roadways that encounter frequent adverse 

weather conditions and localized road-weather hazards (e.g., icy bridge).

Figure 5 provides an example of a two-phase CMS message.
10

Figure 5. Illustration. Two-phase CMS display showing weather advisory.

(9)
11

CHAPTER 3. DII SAFETY MESSAGE GUIDELINE INFORMATION

This report is part of a broader effort to develop initial design guidance for V2I safety messages 

provided to drivers using DIIs. This work primarily involved empirical data collection, which 

used a part-task driving simulator to examine the simultaneous presentation of V2I and V2V 

messages in left turn across path (LTAP) scenarios.

(10) As part of this work, a variety of 

analytical activities was conducted to develop basic requirements for presenting messages from 

V2I and V2V sources simultaneously. In particular, many existing research sources that provided 

information about human factors considerations related to V2I and V2V communication were 

indentified, in addition to other useful information pertaining to the design and implementation 

of DIIs. There was sufficient information to support the development of initial annotated outlines 

that cover basic human factors design topics related to DII implementation.

The remainder of this report provides the initial annotated outlines for DII safety message 

guidelines. The annotated outlines were developed with the objective of including them in future 

versions of the Human Factors Guidelines for Road Systems (HFG) as part of a single chapter 

that covers the use of DIIs to communicate V2I information to drivers.(11) The format of the 

safety messages provided in this report is similar to annotated outlines developed as part of 

earlier HFG projects; however, they contain a more developed narrative.(11,12,13) Another notable 

difference is that the topics also do not conform to the two-page format of the HFG, although 

most of the key sub-sections (e.g., Discussion and/or Design Issues) for individual guidelines are 

included. Also, the narrative in the Discussion section for each safety message is more detailed 

and lengthy. This approach was a deliberate attempt to provide a broader starting point for the 

development of more focused HFG topics in the future. At that time, decisions can be made to 

eliminate less important information, divide some topics into multiple related topics, or move 

some information into tutorials.

Note that while every attempt was made to be thorough and comprehensive in the development 

of the current safety message topics, the scope of the current project prevented application of the 

same rigor and repeated review cycles that are typically required to develop formal design 

guidelines.

(11,14) Moreover, the information provided in each topic is intended to serve as a 

starting point for more formal guideline development efforts in the future, rather than as an 

authoritative source of design guidance that can be used now. Nevertheless, the topics presented 

in this report identify key sources of existing information, and they provide discussion of human 

factors design considerations that can be applied to the design and implementation of V2I safety 

systems and corresponding safety messages.

The remainder of the report provides the annotated outlines for the safety message topics. 

Twelve topics are organized into two sets. The first set is composed of general topics that 

provide background information and cover issues that apply across multiple V2I applications. 

The second section includes topics that are focused on specific V2I applications.
12

TOPIC 1. CONSIDERATIONS FOR ADDING A DII TO A CV RSE

Introduction

This topic covers considerations for determining whether a DII should be added to a CV RSE to 

communicate information directly to drivers. The decision to add a DII is not a simple one 

because the long-term vision of the CV program is that most vehicles will have a DVI to 

communicate RSE information to drivers through in-vehicle applications. Moreover, adding a 

DII will increase the overall cost of an infrastructure-based safety system, yet under certain 

circumstances, it may provide no benefit above the in-vehicle systems. The decision to add a DII 

may arise when an RSE is being installed at a new location, an RSE is being updated, or an 

engineering study has suggested the addition of an RSE system. Several factors should be 

considered before adding a DII to an existing or newly installed RSE.

Guideline

A key prerequisite for implementing a DII is the existence of a traffic safety problem at that 

location that can be addressed by providing drivers with additional information that is not easily 

obtained from the environment. Table 2 identifies operational and situational factors, along with 

corresponding advantages and disadvantages, that should be considered in the decision to add a 

DII at a location with RSE.
13

Table 2. Operational and situational factors related to DII installation.

Factor Advantages Disadvantages Considerations

Visual 

Interaction 

at Location

Allows drivers to keep 

looking at the roadway 

environment and is 

usually easily noticed.

No in-vehicle alerting 

component (i.e., auditory 

message). Consequently, DII 

may be less effective for 

drawing a distracted driver’s 

attention back to roadway.

The conspicuity of the DII 

is important. Drivers may 

look for but not notice a 

DII.

Driver 

Workload

Location of the DII can 

provide context and 

permit a simplified 

message with little 

workload impact.

The DII may add visual 

clutter that interferes with 

the driving task and 

increases workload.

Location complexity and 

task difficulty may 

contribute to driver 

workload. 

Targeted 

Messaging

DII information is 

available to all drivers 

that can see the 

display, so all drivers 

can benefit from the 

information.

Non-targeted drivers may 

receive incorrect or 

unnecessary information that 

causes confusion and 

inappropriate responses.

Research postulates safety 

concerns about providing 

non-target drivers with 

irrelevant information.

Driver 

Decisionmaking

A DII can support safe 

and efficient decisions. 

Drivers may prefer 

messages via a DII 

rather than a DVI.

Generalizability of DII 

information may make its 

information unsuitable for 

some drivers.

Information should be 

relevant and actionable. 

Interaction 

With Other 

Systems

Interaction with other 

connected DII- and 

DVI-based systems can 

be managed.

Limited ability to manage 

interactions with 

nonconnected systems.

This interaction is 

primarily a concern with 

conflicting messages 

across different systems. 

V2X 

Market 

Saturation

A DII can be effective 

at all levels of market 

saturation.

Until market saturation of 

CVs is high, the DII will 

require separate 

infrastructure-based sensors 

to provide data on vehicle 

movements, etc.

This issue relates to 

targeted messaging and 

system interactions; it will 

be less of a concern over 

time.

Discussion

The decision to include a DII for new or existing RSE depends on factors besides cost (table 2).

This guideline presents some high-level information about these factors that should be 

considered prior to implementation. The information in this guideline is not specific to a 

particular V2X application.

Before discussing these factors, however, location-specific prerequisites should be defined. As 

previously stated, a key location-specific prerequisite is the existence of a traffic safety problem 

that can be addressed by providing drivers with unambiguous and actionable information that is 
14

otherwise not easily obtained from the environment. For example, a study may recommend the 

addition of a CSW for a horizontal or compound curve that has sight distance limitations. A 

likely operational factor that should be considered is the DII potentially being implemented in a 

way that provides information in an ambiguous and convoluted manner. For example, if a CSW 

system can only be implemented at a location ahead of a gore point when it only applies to a 

single branch, then the message may be ambiguous, and other measures should be examined

instead.

The following subsections describe the other factors that should be considered when determining 

whether it is appropriate to include a DII with RSE.

Visual Demand: The location of a DII must be salient and positioned where drivers will be 

paying close visual attention (e.g., locations where drivers expect TCDs, such as intersections 

and roadway edges). A DII that is not implemented in a salient manner may not be as effective in 

conveying the intended message. This same benefit of providing easily noticed information, 

however, means that distracted drivers may not see the DII message unless an attention-getting 

device (e.g., flashing beacons) is employed. V2X communication supports providing these 

messages inside the vehicle through the DVI, although a majority of vehicles will not have this 

technology for some years. 

Driver Workload: The inherent workload of a given location and situation, as well as the 

workload imposed by the DII, should be considered. In areas where workload is already 

increased, such as traveling through a complex interchange with high traffic volumes, the 

additional workload imposed by the DII may not justify the benefits of providing additional 

information. In cases when crashes are occurring because the task at hand requires a specifically 

difficult maneuver, the DII may reduce those demands.

(15) In contrast, for low workload areas,

such as rural intersections with low traffic, the additional workload imposed by a DII is not 

likely to have a deleterious effect.

In general, a well-implemented DII will likely have a minimal effect on driver workload, 

especially if the DII supplies the driver with information that is integrated with normal driving 

activities. For example, a DII placed alongside other TCDs, such as an SLTA located alongside a 

signal, allows drivers to gain information from the DII while performing normal visual scanning. 

However, whether it provides useful information or not, a DII can be a source of information 

drivers feel compelled to pay attention to, especially if it resembles a regulatory sign or device.

(5)

A DII that does not directly address a safety concern can be a source of visual clutter, which may 

reduce the effectiveness of the DII and may needlessly increase a driver’s workload.

(16)

Targeted Messaging: An important factor when deciding to implement a DII is whether the 

information will target all road users or only specific road users. Information presented via DII 

that is intended for specific road users may be unintentionally responded to by nontargeted road 

users.(17) For instance, a driver in a multilane approach to a signalized intersection could receive 

a DII-based RLVW message intended for a slower driver in the adjacent lane, which could result 

in the nontargeted driver unnecessarily braking or stopping in the intersection and creating a 

safety concern. In similar situations, the DVI approach may be the optimal method for providing 

targeted information.
15

Decision Making: A DII may support driver decision making if: (1) driver information needs are 

not being addressed via DVI or through the existing infrastructure, (2) the DII can provide 

information in a timely manner, and (3) the information provided about the required maneuver is 

actionable by the driver. Ideally, a DII would provide information to drivers that could eliminate 

uncertainty about immediate driving conditions, thus facilitating their decision making. 

However, this process requires drivers to trust the system enough to rely on the information 

provided. (See topic 6 for more information.)

Interactions With Other Systems: The decision to install a DII should include the potential for 

interactions with other DII safety systems, as well as non-V2X vehicle-based safety systems. 

Initial requirements for V2X-based applications rely on the infrastructure RSE for timing (both 

of the DII and the DVI).(18) While timing is not expected to be a problem in these systems, there 

may be exceptions regarding vehicles that take into account driver and vehicle performance. 

While system-level conflicts between DII elements are unlikely, some combinations of systems 

are more likely to produce conflicts at either the system or message level. (See topics 2 and 3.)

V2X Considerations: Little cost or benefits data exist regarding adding a DII when all vehicles 

are equipped with V2X DVIs. This issue is a key consideration until CV technology market 

penetration is at a level in which a majority of drivers may be expected to receive V2X 

information. Therefore, if some communication with the driver is required, then a DII may be a 

reasonable approach. If drivers are unlikely to be provided with safety-related information via a 

DVI, then DII warnings about imminent hazards (e.g., CSW, spot weather information warning 

(SWIW), or RLVW) are potentially beneficial.

Even if drivers can be reasonably expected to have a DVI available, an additional DII may still 

be useful. Research suggests that drivers are more inclined to use information provided by a 

DII.

(10,17) If the driver’s task is complicated or time constrained, such as making vehicle gap 

judgments, then a well-placed DII may be easier to integrate into the driving task than a DVI 

message because it can be located near visual targets that drivers look at while performing the 

gap judgment task. Also, the placement of the sign can permit messages to be simplified because 

relevant information is provided by the driving context. For example, an LTAP DII positioned to 

a left-turn signal may not need to indicate the nature of the hazard (oncoming vehicle) because it 

is implied by the DII’s association with the left turn.

Design Issues

Before implementing a DII, ensure that the available data suggest the intended benefit is likely. 

Some novel DIIs, especially V2X DIIs, have limited evidence for their safety benefits and cost 

effectiveness. For example, a field test of a CICAS-SSA demonstrated an increase in likelihood 

of making a complete stop; however, it demonstrated no improvements in accepted gaps or timeto-collision.(19) It may be beneficial to carefully examine the empirical support for a system 

before implementing a novel DII.
16

TOPIC 2. SYSTEM-LEVEL CONFLICTS

Introduction

This guideline covers scenarios in which information presented via both DII and DVI could 

result in conflicting information being presented to the driver. There are two general forms of 

conflicts: (1) system-level conflicts involving functionally different applications that attempt to 

communicate to the driver at the same time, and (2) message-level conflicts involving 

incongruent information presented by two different sources to the driver. This guideline provides 

information on system-level conflicts. Message-level conflicts are covered by topic 3. If multiple 

infrastructure and/or vehicle-based safety systems are active at locations such as intersections, 

then system conflicts may occur if they are triggered simultaneously. (See table 3.) As the 

number of available infrastructure-based safety systems increases, especially when multiple 

systems are attempting to address the same basic hazard, the engineer must be aware of the 

potential for conflicting messages coming from uncoordinated systems. Presenting 

uncoordinated messages can burden drivers with unneeded or conflicting information when 

driver workload is already elevated because of the driving situation.

Guideline

The following design guidance is applicable for the use of DII and DVI:

• In general, DII displays should conform to driver expectations and observations.

• DVIs and DIIs that assess the same hazard should provide consistent instruction or 

information, and should be coordinated if possible. 

• System timing and activation algorithms that are compatible and congruent across 

systems should be used if they assess the same hazard.

• DIIs should incorporate available V2X information to reduce false and 

nuisance alerts. 
17

Table 3. System activation overlap matrix.

DVI

DII

SSA

SLTA

RLVI

PCI

CSW

RLVW

SWIW

IMA

LTAP

FCW

DNPW

BSW + 

LCW

OVW

CICASSSA 1 1*

CICASSLTA 1 2* 1 1*

RLVI 2* 1

PCI 2 2 1 2 2

CSW 1 2 2** 2***

RLVW 1 1 2**

Blank cell = Simultaneous system activation is unlikely.

1 = Systems are active for the same hazard.

2 = Systems active for different hazards.

*Possible conflict resulting from warning activation parameters. 

**Forward traffic must be present.

***It is remotely possible to have an OVW and a CSW if there is an entrance to a 

tunnel or overpass just before a curve, as the OVW alert or warning should be given 

well in advance of the actual hazard to the driver can exit and take an alternate route.

BSW + LCW = Blind spot warning and lane change warning.

DNPW = Do not pass warning.

FCW = Forward collision warning.

IMA = Intersection movement assist.

PCI = Pedestrian crossing intersection.

RLVI = Red light violator indication.

Discussion

While vehicle-based communication applications (i.e., V2X systems such as V2V and V2I) have 

the potential to greatly reduce the likelihood and severity of certain crashes, the market 

penetration necessary for reliable V2X communication is still many years away. Even after this 

level of V2X market penetration is reached, many vehicles on the road will still not have this 

capability. Infrastructure-based solutions (i.e., a DII) can offer safety benefits in the absence of 

V2X communication. However, a DII in the presence of other V2X communication raises the 

potential for information conflicts, incongruences, and similar concerns when multiple systems 

that are functionally similar are present and attempting to provide information to the driver. If 

left uncoordinated, different systems that activate for the same hazard can present incongruent 

information, potentially leading to driver confusion that may result in delayed reaction times 

and/or mistrust of the system. The information in this guideline is meant to provide engineers 

with guidance regarding the types of incongruencies that may occur across systems and some 

general approaches for reducing the potential negative impacts of such incongruencies.

Congruence With Driver Expectations: At the highest level, system implementation should be 

congruent with driver expectations and observations. An important prerequisite for managing 

information from multiple sources is for each system to be itself compatible with drivers’

expectations of the situation-based cues provided by the situation and the maneuver. Drivers rely 
18

on information from the infrastructure to provide guidance and decision-making support.(20) The 

DIIs and DVIs should cue the driver to a sequence of behaviors that support safe driving. 

Information that conflicts with what drivers expect or observe may lead to confusion and 

mistrust of the warnings.

(21) (See topic 6.) The following subsections discuss the types of systemlevel conflicts that can occur and how their occurrence may be mitigated.

Hazard Conflicts: Conflicts can arise when a DII and a DVI detect the same hazard using 

different information or activate simultaneously for different hazards based on the same scenario. 

These conflicts are not concerns for most combinations of DII and DVI elements. Table 3 shows 

conflicts that may arise from the use of uncoordinated systems (e.g., the hazard identification and 

timing algorithms between systems are not coordinated). Even with this uncoordinated 

environment, however, the majority of system overlaps do not present a potential conflict. 

Further, many CV safety applications are DVI only or are highly unlikely to be active in the 

roadway environments where the DII elements explored in this report are located (e.g., do not 

pass warning or blind spot warning and lane change warning). The potential for system-level 

conflicts may be determined through comparison of current or planned CV safety applications. 

This process may be done by comparing which DII elements are present (shown in the rows of 

table 3) against which DVI components (shown in the columns of table 3) may be active at the 

same time. 

When conflicts occur, drivers may receive multiple messages or may not understand the intent of 

the warning. An example of this issue (see table 3) is red light violator indication (RLVI) and 

CICAS-SLTA. The CICAS-SLTA can inform a driver waiting to turn left that no oncoming 

vehicle is present, implying that the driver can enter the intersection to turn; however, a red light 

runner approaching from the right would trigger an RLVI intended to warn the driver to not enter 

the intersection. In this scenario, the CICAS-SLTA system indicates that the roadway is clear,

while the RLVI system warns of unsafe conditions. This type of incongruence could potentially 

cause driver confusion and have a negative effect on the driver’s use of the system. Another 

concern is that with multiple hazards, it may be necessary to arbitrate message presentation so 

drivers only receive the most safety-critical message. This discussion is beyond the scope of this 

report but is covered in detail in Campbell et al.(22) These scenario types require special

consideration because any number of different DVIs may be present or become available during 

the lifespan of a particular DII element. However, understanding which CV safety applications 

may present conflicts or coordinating between CV safety applications that are present can largely 

eliminate this potential problem.

Another consideration is to avoid system-level conflicts by coordinating hazard-detection 

algorithms and message timing across systems that assess the same hazard. Incongruent 

information presented across two different systems represents a conflict at the algorithm or 

implementation level. The presentation of information should always be consistent between DVI 

and DII to avoid driver confusion. Recent requirement specifications provide for the DII to act as 

the coordinating timing source, as well as the source of safety messages to DVIs.

(18) One way to 

reduce system-level conflicts is to use only one application that calculates hazards across 

systems.

When feedback from a V2X-based system is available, information provided to the driver via 

DII should reflect that information. The use of V2X information should reduce the potential for 
19

providing incongruent information and can serve as a way to increase the accuracy of a DII 

message. Accordingly, information provided to the driver via the DII should reflect available 

vehicle safety data recorded by the in-vehicle system. In scenarios involving a conflicting

principal other vehicle (POV), DII information should also incorporate relevant data available 

from the POV, especially when there is a high probability that the driver may execute a 

maneuver that the DII is warning against because it is safe for that individual driver to do so 

based on the driver’s capabilities. In this case, if the V2X application determines that the conflict 

will be completely avoided through the current behavior of the vehicle and driver, then the DII 

and DVI warning presentation can be suppressed. However, if the alert is presented when the 

driver sees no threat or hazard, then it can be viewed as a false or nuisance alert, which can have 

a negative impact on the driver’s trust in DIIs.

Design Issues

There is a tradeoff between using a DVI or a DII for alerting a driver to a hazard. The DVI has 

an advantage for salience in that it is able to present a message in a multimodal manner using 

visual and auditory elements, which can make it generally easier for the driver to detect. In 

contrast, the DII may be viewed as more reliable by drivers and is able to benefit all drivers 

regardless of the presence of CV technology within the vehicle.(23) These issues, and those

discussed in topic 3, should be considered when implementing a new safety application. 

It is unknown to what extent drivers will rely on the DII compared with the DVI when either 

analogous or incongruent messages are provided. Competing messages from a DVI can be more 

salient and attention-getting if they are accompanined by auditory signals. In addition, driver 

preference for DIIs and DVIs can vary on a number of factors, such as driver characteristics 

(e.g., age and experience with the location), their perception of the scenario, and the presence of 

a DVI.(23) Therefore, the DII should provide direct and authoritative messages in support of 

drivers’ visual sampling of the environment for relevant information.

TOPIC 3. MESSAGE-LEVEL INCONGRUENCE BETWEEN DII AND DVI SYSTEMS

Introduction

This guideline addresses conflicts between safety applications that occur at the level of 

individual messages, primarily because of incompatible or incongruent message characteristics. 

When messages are incongruent, drivers may have to visually attend to multiple locations in the 

environment to help resolve the conflict. This process can lead to a delayed response to the 

warning and may reduce the warning’s overall effectiveness. The engineer implementing a DIIbased system should be aware of the different types of incongruence that can occur and should

work to minimize their impact on drivers while remembering that clear, actionable messages lead 

to the most rapid and accurate driver responses.

Guidelines

The following design information describes strategies for avoiding incongruent messages:
20

• Message Timing: Warning messages pertaining to the same hazard should be 

presented at the same time. Timing support the coordinated presentation of DVI and 

DII messages when V2X information is available. 

• Message Content: The information and wording in separate messages should be 

consistent across systems with respect to the basic hazard being communicated.

• Message Format: Standard message formats should be used, such as those specified 

in the MUTCD or generally recognizable icons and wording. 

• Message Frame: Multiple DIIs related to a single hazard should provide the same 

level of warning or advice. Ensure that messages broadcast via V2X communications 

use the same message framing as the DII. 

• Message Stages: Multiple DIIs related to a single hazard should be coordinated 

regarding the number of message stages implemented and how and when messages 

move through different stages. Messages broadcast via V2X communications should 

include message staging information.

Table 4 provides definitions of message characteristics related to congruence.

Table 4. Definitions of message characteristics related to congruence.

Message Characteristic Definition

Message Timing The time frame in which the message is provided to the driver 

relative to the identified hazard.

Message Content The information within the message identifying the referent 

hazard and/or any driver actions that should be considered.

Message Format The manner in which information is presented to the driver. For 

DIIs, this manner is typically visual and can include text, icons, 

lights, and other typical DII presentation means. 

Message Frame The instruction level of the message provided to the driver. 

Possible instruction levels include “permissive,” “advise/inform,”

“alert/warn,” and “prohibit.”

Message Stages Stages represent different hazard severity levels, such as 

“inform/advisory” and “warn” messages. 

Discussion

An increasing number of safety applications that use the DII are available or under development, 

and it is likely that future systems will be able to communicate via V2X to take advantage of the 

DVI within the vehicle. However, multiple DII- or DVI-based safety systems simultaneously 

presenting different messages can result in incongruent information reaching the driver. This

conflict can produce a range of undesirable outcomes, such as increased reaction times to alerts, 

lower trust in the system, and drivers ignoring alerts.

(21,24) (See topic 6.) When an infrastructurebased system is implemented, it is important to consider how the DII message will be presented, 

what other messages might be presented in the roadway environment, and what other messages
21

may be presented within the vehicle. Being cognizant of these potential information sources can 

help engineers minimize incongruent messages. This guideline covers different ways that 

messages presented via DII may be incongruent. 

Message incongruence can result when multiple conflicts are derived from any of the following 

five message characteristics: timing, content, format, framing, and staging. It is also possible that 

multiple hazards may be portrayed within a single DII message (e.g., integrated PCI and SLTA). 

While engineers have the greatest influence on reducing incongruence when multiple DIIs are 

present, the design of connected DIIs can help reduce the incongruence between a DII and a DVI 

as well.

Message Timing: The timing of multiple related DII messages should be coordinated. If 

multiple warnings about the same hazard are presented in an uncoordinated manner, then the 

driver may not understand that all of the messages are referring to a single hazard. For example, 

if one DII message is delayed relative to other messages for the same hazard, then drivers may 

believe that the warning encountered later pertains to a different hazard. Similarly, nontargeted 

drivers who observe a DII message targeting another driver may unnecessarily respond if the 

message appears to be relevant to their situation.(25) Likewise, with more advanced vehicles or 

CVs, the driver may receive messages pertaining to the same hazard via the DVI. Thus, the 

infrastructure should support the coordinated presentation of DVI and DII messages when V2X 

information is available. Resources are available that explore the prioritization of multiple safety 

messages and the integration of multiple V2X messages.(26,27)

Message Content: Applicable standards, such as National ITS Architecture market packages,

should be used when the system operational characteristics are compatible. For new messages, 

consistent message content should be used across multiple systems when referring to the same 

hazard, which can serve as an unambiguous warning. Message consistency can be achieved by 

ensuring that DII elements agree in their identification of a hazard and that they all signify the 

same hazard. Coordinated messages between DII and DVI are also important because it has been 

shown that coordinated DII-DVI messaging is more effective than either DVI or DII alone.(28) If 

different DII elements are not consistent, then drivers may not clearly understand the intended 

message or may have delayed reactions as a result of having to interpret the intended meaning of 

the sign.

(29) For example, the integrated PCI/SLTA example alluded to earlier should be designed 

so the driver perceives the information as flowing from one system and not multiple systems. 

The perception of multiple systems can result in a scenario in which drivers look to two or more 

different locations and must make a comparison between the information received from each 

before finalizing a reaction. 

Message Format: Standard message formats as identified in the MUTCD (or generally 

recognized formats) should be used to avoid message incongruence when multiple interfaces are 

present. In particular, different DII elements may present information using different formats, 

such as a CMS using text and a blank-out sign using an icon. It is therefore important that the 

representation used across different DII elements is consistent with driver expectations because 

drivers may not understand nonstandard symbols or messages.

(10,30) If multiple DIIs of the same 

type and configuration are referring to the same hazard, then they should use the same message 

format. When V2X communication is available, this information can be provided to the DVI as 

well to provide a greater level of standardization.
22

Message Frame: The following frame incongruencies can result when multiple DIIs related to a 

single hazard present conflicting types of information:

• Permissive (stating that the action is allowable).

• Advise/inform (providing neutral information about a traffic condition, such as time 

to arrival of cross traffic, and leaving the hazard judgment up to the driver).

• Alert/warn (providing information about an active hazard).

• Prohibit (stating that an action is not allowed). 

These messages used on different displays that pertain to the same hazard should be carefully 

crafted to ensure that they use the same message frame. Failure to do so can lead the driver to 

misinterpret the intended message. For instance, the presentation of an advisory message about 

pedestrians along with an activated “pedestrian present” warning from a PCI system could lead 

the driver to see the PCI as another advisory sign and to consequently ignore the message. This 

form of incongruence can also occur between a DII and DVI. When available, broadcast 

messages via V2X communications at the same level as those provided by the DII. 

Message Stages: The number of stages across DII elements that refer to the same hazard should 

be the same. Multiple-stage messages are those that move through different frame levels (e.g., 

advise/inform, alert/warn, prohibit). When possible, multiple DIIs related to a single hazard 

should have the same number of message stages, and changes in stages should be coordinated in 

time. Otherwise, drivers may not be able to accurately interpret messages received from multiple 

DIIs that present information using different stages, because the status of the hazard and/or 

corresponding required driver actions would be unclear.

TOPIC 4. MESSAGE CONTENT: MESSAGE ELEMENTS

Introduction

This guideline discusses the message elements that could be included as part of a message 

provided by a V2I safety system. Message elements are components of the message that each 

provide a specific piece of information that describes the situation to drivers. Regardless of the 

message format (e.g., text and/or symbols), the information elements in the message are 

important to consider because the driver’s interpretation of the message will depend on these 

elements. The basic message elements discussed in this topic were adapted from existing 

guidance for messages displayed on CMSs.

(31)

Guideline

The following design guideance is applicable to the development of message content.

• The message provided to drivers should include some combination of the message 

elements described in table 5. The required elements vary across systems and depend 

on the situation or context.
23

• Unnecessary elements may be excluded from the message.

• In general, the message elements to include are those required to accurately convey 

the pertinent aspects of the situation (especially when a hazard is not visible or if it 

affects only a subset of the drivers) and to motivate the desired driver behavior.

Table 5. Message elements for consideration.

Message 

Element Definition When to Include

Problem Provides information about what 

the driver will encounter.

When the hazard is neither visible nor 

implied by the situational context.

Location Describes the location of or distance 

to the situation.

When the hazard is not ahead on the same 

route on which the driver is traveling, or 

when the message could be misinterpreted to 

apply to a relevant hazard from another 

direction.

Action Provides a recommendation to the 

driver in response to the problem 

and location information.

In all system messages.

Audience 

for Action

Includes the subset of roadway 

users who are the intended 

recipients of the message.

When the hazard is not on the current route 

or when the hazard only affects a subset of 

drivers and the audience cannot be implied 

by DII placement.

Good 

Reason for 

Following 

the Action

Provides additional justification 

intended to give a driver confidence 

that following the message will 

result in safety or time benefits.

When it is necessary to motivate drivers to 

take an action (e.g., they should follow it for 

a reason other than fulfilling a normal driving 

objective such as avoiding a collision).

Using these message elements as a structure, the information elements were enumerated for each 

of five different V2X safety systems. Table 6 includes a row for each of the warning systems and 

a column for each message element. In each cell is the specific form that the message element 

would take for that warning system. Note that the form of the message element is not meant to 

suggest specific wording but rather to describe the nature of the content that should be conveyed 

by the message. The font style denotes whether the element needs to be included in the message 

for that warning system. Italicized font elements are otherwise apparent to the driver and 

therefore do not need to be included. Font elements in roman may or may not be necessary to the 

driver. Bold font elements definitely should be conveyed to the driver in some way.
24

Table 6. Enumerated combinations of message elements by warning system.

Warning 

System Problem Location Action Audience Reason

SSA Crossing or 

merging 

vehicle(s)

Left, then right Yield Left-turners, 

right-turners, 

straight traffic

Avoid crash 

with vehicle

SLTA Oncoming 

vehicle

Forward Yield Left-turners Avoid crash 

with vehicle

RLVW Red light Ahead Stop Approaching 

traffic

Avoid crash 

with vehicle

CSW Sharp curve, 

blind curve, 

compound

curve, 

unexpected 

curve

Ahead Slow 

down

Approaching 

traffic

Avoid losing 

vehicle 

control

Road Weather Unexpected 

road weather

Ahead (possibly 

on another 

route)

Varying Approaching 

traffic (or other 

route traffic)

Safety/time 

savings

Rail-Grade 

Highway 

Crossing

Oncoming 

train*

Left or right Yield Straight traffic Avoid crash 

with train

Note: Bold indicates esstential, roman indicates possibly necessary, and italic indicates not necessary.

*May be situationally implied.

Discussion

The following discussion provides information about considerations for including each of the 

message elements. By looking at the columns in table 6, it is possible to determine trends in 

situational characteristics that make each message element necessary (i.e., by looking across the 

warning systems and using information about situations when those systems would activate).

These characteristics are discussed in the following subsections.

Problem: The problem element is important to state either when the hazard is not visible or 

when it is not implied by the situational context. For road weather (e.g., icy roads), it is likely 

necessary to state the exact problem because drivers often cannot see it because it is miles ahead 

or imperceptible, such as black ice. In addition, in some hazard scenarios, oncoming vehicles or 

upcoming geometry may not be visible to the driver, so the message may require a direct 

statement of the hazard to make sense to the driver. It is also possible that the placement and 

content of the DII might help imply the presence of the hazard (e.g., placing an SLTA DII near 

the oncoming traffic stream) such that a direct statement of the problem may not be necessary. 

For a situation in which multiple hazards are present (e.g., oncoming vehicles and crossing 

pedestrians in SLTA), however, it may not be clear which hazards are accounted for by the 

system and which are not. This uncertainty may affect driver reaction times once the display 

returns to the inactive state if drivers are unsure whether they need to consider other hazards in 
25

the environment. For example, if an SLTA system is blank because an oncoming vehicle is not 

approaching, will drivers understand whether they still need to look for pedestrian hazards?

Location: The location element should be included when the hazard is not ahead on the same 

route that the driver is traveling or when the message could be misinterpreted to apply to a 

relevant hazard from another direction. Dudek states that generally, if drivers observe a 

changeable message, then they will assume that the hazard is ahead on the same roadway unless 

they are told otherwise.(31) Therefore, the location ahead does not need to be stated, but 

conversely, the location is important to include if the hazard is on a different roadway. The 

location element should also be included when the warning message could be interpreted to 

apply to a hazard from another direction (e.g., cross traffic for SSA). The location of the hazard, 

however, might be implied by the physical location of the DII (e.g., near the oncoming hazard).

Action: The action element should always be included. The main objective of messaging is to 

change or influence driver behavior. Therefore, the desired behavior needs to be conveyed to 

the driver.

Audience for Action: The audience for action should be included when the hazard does not 

affect the current route or when the hazard only affects a subset of drivers and the audience 

cannot be implied by DII placement. For example, in an SLTA scenario, it may be possible to 

imply that the audience for the message is left-turning drivers, either by the message content or 

the DII placement. However, if the DII is placed overhead and the message content is more 

ambiguous (e.g., yield), then there may be confusion regarding which drivers need to follow the 

message. Similarly, if the message is for drivers who will be transferring to another route (e.g., 

road weather on a different route), then it is important to specify which drivers are affected.

Good Reason to Follow the Action: This element should be included when it is necessary to 

motivate drivers to take an action (e.g., they should follow it for a reason other than fulfilling a 

normal driving objective such as avoiding a collision). This requirement is particularly important 

when drivers cannot perceive the hazard (e.g., hidden curve) or they underestimate the severity 

of the hazard and, therefore, need a reason to change their behavior.

Design Issues

This information applies to symbolic messages, text messages, and combination symbol-text 

messages. Symbolic messages may not directly state the elements but instead convey them 

visually. In addition, some of the message elements may be implied by other display aspects, 

such as placement of the display within the context of the roadway geometry.

TOPIC 5. MESSAGE CONTENT: USE OF SYMBOLS AND TEXT

Introduction

This guideline discusses considerations for designing DII and DVI sign messages using symbols, 

text, or a combination of both. These messages most likely use a combination of existing TCD 

message elements. Some V2X-based messages will likely differ from existing messages in 

important ways (i.e., the inclusion of temporal information), which may require the modification 

of existing messages or the design of new ones. Given the type of safety applications involved, 
26

these messages are typically intended to be viewed in time-limited situations, which requires 

them to be simple and quickly understood. There is substantial existing research and guidance on 

message elements. The current guideline only provides a summary of key design considerations 

and identifies key references that should be used to obtain more comprehensive information 

regarding these considerations. 

Guideline

Table 7 provides general considerations for implementing symbols and text elements in signs to 

produce messages that are clear and easy for drivers to understand. Key information sources are 

provided next to each consideration.

Table 7. General considerations for implementing symbols and text elements in signs.

General Sign Design Guidance Key Reference(s)

• Maintain spatial, conceptual, and representational 

compatibility among the sign text, symbols, and the 

message they represent. These elements are all related to 

how the sign is perceived by the driver. See Discussion 

for a more detailed description. 

Shinar et al.

(30)

Ben-Bassat and Shinar(32)

Ng and Chang(29)

• Use familiar sign elements and messages. MUTCD(7)

• Use standard colors, shapes, symbols, and text. Campbell et al. (33)

Ng and Chan(29)

MUTCD(7)

HFG(11)

• Ensure that all message elements required to understand a 

message are included in the sign. Avoid requiring drivers 

to infer additional meaning to the message. 

Shinar et al.(30)

Symbol Design Guidance

• Use symbols on roadway signs to communicate 

information to drivers in the following situations: 

o To provide safety and warning information.

o To indicate prohibited actions.

o In visually degraded conditions.

o In areas with higher posted speeds.

• Use a clear and simple font. 

• For simple messages, limit text to no more than two to 

three words. 

• Use precise symbol descriptions.

• Use nontechnical, common vocabulary.

HFG(11)

Icon Guidelines(33)

• Minimize use of abbreviations. MUTCD(7)

Shinar and Vogelzang(34)
27

• Use text on roadway signs to communicate information to 

drivers in the following instances:

o For highly complex messages.

o When indicating a hazard in or near the roadway to 

drivers.

o For destination information. 

o In areas requiring unexpected or unique driver actions 

(e.g., frequent lane changes). 

HFG(11)

Symbol and Text Design Guidance Key Reference(s)

• If a symbol could potentially be unfamiliar to a number 

of drivers, the addition of text on the sign can help them 

more easily understand the meaning of the sign. 

Shinar and Vogelzang(35)

• When the sign communicates a particularly important 

message or warns of a particularly hazardous situation, 

text can be used to give drivers another method to receive 

the information in addition to the use of a symbol. 

HFG(11)

• When symbols are abstract and have no conventional or 

broadly understood meaning, text can be used to 

supplement the symbol.

Shinar and Vogelzang(35)

Discussion

DII and DVI displays for safety applications are unique in that drivers will likely encounter them 

infrequently during the early stages of general adoption—particularly DIIs. Because of these low 

levels of exposure, drivers may require more support to understand the intent of a message. Thus, 

it is important that messages be clear and easy for drivers to understand, especially when drivers 

first encounter them. 

General Sign Guidance: In general, the design of roadway signs should closely follow the three 

ergonomic design principles of sign-content compatibility, familiarity, and standardization. Signcontent compatibility is a combination of the following three principles:

• Spatial compatibility is the physical arrangement of objects in the environment 

relative to the position of information and directions (i.e., the direction in which the 

road is turning is presented by an arrow bending the same direction on the sign). 

• Conceptual compatibility is the extent to which symbols and/or text match 

associations that drivers have with a concept. An example of this concept is using an 

airplane symbol (MUTCD sign I-5) on a sign to indicate that an airport is nearby.

• Representational compatibility refers to the similarity between the content (the 

symbol and/or text on the sign) and the object or situation it is intended to 

communicate to the driver. An example of good representation on a sign is using a 

symbol of a person using a shovel (MUTCD sign W21-1) to indicate road 

construction or road work nearby.
28

In a study that asked participants to rate road signs on how closely their design followed these 

ergonomic design principles, the signs that most closely followed ergonomic design principles 

were better understood by both local and nonlocal drivers.

(32) While one cannot design for 

familiarity as one can for compatibility or standardization, designing new signs according to the 

compatibility and standardized design principles can influence drivers to better understand these 

new signs. Using familiar sign elements can also influence drivers’ understanding of signs. 

Symbol Guidance: Symbols are often used on road signs because they can communicate 

information to a wider range of drivers because they are mostly language-neutral. Under normal 

driving conditions, drivers can identify signs using symbols from greater distances compared 

with text signs.

(36) Common symbols have also been shown to have generally shorter 

comprehension times compared with text comprehension times.

(37) In a more recent study by 

Shinar and Vogelzang, however, the use of symbols on signs was shown to be advantageous only 

when the symbol was familiar to drivers.(35)

Text Guidance: Text messages do not have to be interpreted to the same extent as symbols, 

which is an advantage for messages that are challenging to communicate with symbols. The 

guidance for text information addresses text legibility, which, when implemented, helps drivers 

read and comprehend sign information. Another potential concern is that text is languagespecific, meaning that the use of text on a sign may be less effective than symbols for 

communicating with drivers who do not speak the local language. Text should be used for highly 

complex messages, destination names, and other similar types of information. The references in 

the table provide more detailed information about the use of text on signs.

Symbol and Text Guidance: If there is uncertainty about whether to use symbols or text in a 

sign, then one option is to use both. In a recent study, Shinar and Vogelzang compared three sign 

conditions (text only, symbol only, and symbol and text) for level of comprehension and 

comprehension time.(35) Results indicated that level of comprehension was highest for signs that 

used both symbol and text, followed by text-only signs and was worst for signs that used 

symbols only. For symbols that may be unfamiliar to some drivers, the addition of text to the 

sign gives drivers the opportunity to pair the meaning of the sign with the symbol used to 

portray it.

One issue that may be problematic for DVI/DII sign messages is the use of symbols that do not 

physically or conceptually resemble an object or scenario or have a well-learned association with 

the context. In this case, the association of these symbols with their meaning must be learned 

over time. However, if drivers are infrequently exposed to these symbols, at least initially, then 

learning the symbol meaning will be more difficult. Consequently, symbols should only be used 

if they can provide a clear representation of the situation or required action. If this clarity cannot 

be achieved, then it may be better to use a text sign(30)

.

A logical solution would be to use symbols with which drivers are already familiar. If the 

symbols are used in a way that differs from their typical meaning, however, then drivers may 

have difficulty interpreting the intended meaning. For example, one simulator study examining 

an SLTA system compared driver understanding during first exposure of signs using two 

familiar symbols to communicate the presence of an oncoming vehicle (i.e., that it was unsafe to 

turn).(10) When a left turn prohibited symbol was used, many drivers thought that the sign 
29

indicated a permanent left-turn restriction at this intersection, which is consistent with how this 

symbol is typically used. In contrast, almost all drivers correctly interpreted a different sign 

based on a yield symbol. This fact suggests that designers should be cautious when using 

symbols to communicate messages that differ from the symbol’s typical meaning.

Design Issues

Given that new DVI/DII systems may require new types or designs of signs, MUTCD signs 

should always be used when possible. Any new sign designs should not contradict or be 

inconsistent with the guidance provided in the MUTCD. 

TOPIC 6. SUPPORTING DRIVER TRUST OF SAFETY SYSTEMS

Introduction

This guideline covers general factors related to providing drivers with information from safety 

systems via DII and DVI in a way that supports driver trust of the system. Trust is an 

individual’s subjective belief pertaining to his or her willingness to rely on or comply with 

information provided by a safety system, including those that provide DII-, DVI-, or combined 

DII/DVI-based messages. Trust in a system affects an individual’s likelihood of using the 

system, which is important because drivers who do not trust a safety system will not benefit from 

the safety messages provided by a DII or DVI.(38) While little research specifically examines

trust in roadway safety system information, the guidance presented here is based on general best 

practices related to supporting trust in systems and research in trust of automated systems. 

Guideline

Trust can be affected directly and indirectly by multiple factors. Design considerations for key 

factors are listed in table 8.
30

Table 8. Design considerations that affect driver trust.

Factor Design Considerations for Driver Trust Factors

Accuracy Minimize occurrence of false alarms. False alarms are detrimental to driver 

trust. Warnings provided when no visible hazards are present may be 

perceived as inaccurate.

Reliability Ensure that system performance remains stable over time. Systems that are 

perceived as unreliable may not be accepted by drivers.

Understandability Provide clear and useful information. Information that is easy to understand 

encourages driver trust. Active elements on DIIs may also help promote 

driver trust.

Message Framing Use a prohibitive message frame for DII messages. Permissive, 

advise/inform, and warning messages may not be perceived as accurate 

when presented via DII unless a hazard is visible or the message is direct 

and understandable.

Message 

Coordination

Coordinate presentation between the DII and the DVI. Driver trust may be 

lowered by incongruent presentation of warnings between the DII and the 

DVI. 

Familiarity Ensure that the system functions in a consistent manner when compared 

with other systems with which drivers would be familiar within a region. 

Familiar systems that are consistent are likely to produce faster driver 

responses.

Discussion

Trust can be defined as an “attitude that an agent (i.e., a DII or DVI) will help achieve an 

individual’s goals in a situation characterized by uncertainty and vulnerability.”(39) In the case of 

safety messages, DIIs or DVIs provide information to reduce uncertainty or vulnerability 

regarding driving maneuvers. Driver acceptance of these messages is related to their trust of the 

system. Trust has a direct relationship with the likelihood of using a system, and research from a 

variety of fields has demonstrated that operators who distrust a system will underutilize that 

system.

(38,40) Moreover, Lee and Moray suggest that trust is determined by both the performance 

of the system and the operators’ confidence in their own decisions.(41)

Research on driver trust with regard to V2X and DII systems is in a nascent stage. Because of the 

relative lack of research examining this topic, much of what is known about supporting driver 

trust in roadway safety systems is drawn from studies examining trust in automation and vehiclebased active safety systems. While these two topics overlap in many areas, further examination 

of how connected safety systems can best use DII- and DVI-based presentations to support driver 

trust is clearly needed. This guideline provides information on the determinants of trust and how 

a system can ensure that driver trust is supported. 

Accuracy and Reliability: Information accuracy and reliability are the most influential

determining factors in drivers’ trust of information coming from a system.

(41) Accuracy for a 

system is its performance at identifying a safety issue at a given point in time. Research 

examining the presentation of traffic information suggests that 70-percent accuracy is a 
31

minimally acceptable value, below which drivers are unlikely to accept or use information.

(42)

However, it is recommended that engineers target higher levels of system accuracy in their 

designs. 

Drivers may also question the accuracy of a system when it presents a safety message in the 

absence of a detectable threat. An evaluation of a wildlife crossing warning system found that 

drivers did not slow for a warning presented without a visible animal, while speeds were reduced 

when a decoy animal was located along with the warning sign.

(43) Similarly, an examination of

flooded road warning signs found that the visible threat posed by water on the road surface was a 

factor in driver compliance with a warning sign.

(44)

The timing accuracy of the safety message may affect driver trust. The timing of a safety 

message represents a tradeoff between presenting a message early enough to allow a comfortable 

driver response and avoiding false alarms from alerting to a scenario that will not develop into a 

hazard.

(14) Providing false alarms can have an extremely detrimental effect on driver trust in a 

system.

(45) In addition, drivers may ignore messages from a system when a maneuver appears to 

be safe.

(10) Therefore, the timing of the system should incorporate considerations for minimizing 

false alarms. 

Reliability is the ability for a system to maintain a level of accuracy over time. Reliable systems 

increase trust, whereas unreliable systems decrease trust and can lead to underutilization or 

ignoring the system.

(38,46) Research on automated systems suggests that reliability can have an 

effect on both driver satisfaction as well as drivers’ willingness to use a system.

(47) In addition, 

preconceptions of system reliability can have an effect on how individuals use a system.

(48)

Understandability: Understandability refers to the ability of the DII or DVI to provide legible 

and easily understood information to the driver. When a system is easily understood, drivers’

trust is supported. This includes aspects such as ensuring that messages provided by DIIs using 

CMSs are concise, convey the hazard clearly, and provide the driver with enough information to 

respond.

(34) For other types of DIIs, providing active warning elements (e.g., a flashing beacon to 

indicate an ongoing hazard) can assist drivers in understanding the presence of a threat.

(44) An 

examination of an SSA system for rural highways examined both countdown and icon displays 

compared with a hazard sign.

(49) DIIs with active elements (i.e., the countdown and icon 

displays) were rated as providing more trustworthy information by participants. 

Message Framing: Message framing refers to the instruction level of the message. (See topic 3.)

A message may have one of multiple types of message frames, including the following: 

• Permissive: Stating that the action is allowable.

• Advise/Inform: Providing neutral information about a traffic condition, such as time 

to arrival of cross traffic, and leaving the hazard judgment up to the driver.

• Alert/Warn: Providing information about an active hazard.

• Prohibit: Stating that an action is not allowed. 
32

There is a lack of research examining how message frame relates to trust; however, it is 

reasonable to assume that driver trust in a DII or DVI safety message may depend on a 

message’s framing. For example, a permissive DII stating a left turn is safe may be viewed as 

less trustworthy than a prohibitive DII stating a left turn is unsafe because it leaves drivers in a 

more vulnerable position. 

Message Coordination: Coordination between DII and DVI can support driver trust. Research 

suggests that drivers prefer information presented via DII and view it as more authoritative than 

DVI messages.

(10,23) It is unclear whether this preference for DII information will be a stable 

effect over time, especially as the prevalence of CV applications in vehicles increases. As safety 

messages provided via DVI become more common, drivers may begin to view them as equally

authoritative as DII messages. Therefore, coordination between DII and DVI is an important 

factor to consider in establishing drivers’ trust in a safety system. (See topic 2.)

Familiarity: Familiarity with a DII may have an effect on the driver’s trust of the system; drivers 

who are familiar with a system message may respond faster.

(34) The faster responses result from 

drivers being able to make assumptions about what message the system is trying to convey, 

based on their understanding of other familiar systems or messages (rather than interpreting and 

applying the information). Consequently, a DII or DVI that has familiar elements should 

function consistently with the key operational aspects of the original systems. Otherwise, those 

driver assumptions will be incorrect, leading to reduced trust. A drawback of familiarity is that a 

message that drivers view repeatedly may be attended to less frequently by the driver.

(46)

Ensuring that safety-critical messages appear in a salient manner can help reduce this 

potential issue.

TOPIC 7. FACTORS THAT AFFECT GAP CHOICE

Introduction

This topic covers human factors considerations for the use of driver gap choice for a safety

system activation algorithm (e.g., intersection decision support systems). Gap choice is a

measure that represents gap sizes within a traffic stream that most drivers will reject or accept, 

depending on the measurement approach. It reflects a strategy for selecting a warning system 

activation parameter that is based on driver behavior at specific locations. The use of gap choice 

as an activation parameter may serve to enhance driver acceptance of a system if the message 

activation is tuned appropriately using pertinent factors that affect gap choice. There are many 

options for assessing driver gap acceptance. The purpose of this guideline is to illustrate the 

importance of considering the factors that affect gap acceptance. The content applies to conflicts 

at both two-way stop controlled intersections and left turn situations at four-way intersection 

(e.g., LTAP, straight crossing path, and left/right turn in path crash types).

Guidelines

The following design guideance is applicable to when considering factors that affect gap choice:

• Consider situational factors that may apply at a location when developing timing 

algorithms. (See table 9.) 
33

• Assess lag and gap independently because lag is qualitatively different from gap. 

There is considerably higher variation in lag compared with gap sizes.

Table 9 provides a list of situational/site factors that can affect gap choice.
34

Table 9. Situational/site factors that can affect gap choice.

Situational/

Site Factors Finding

Data 

Quality

Design 

Relevance

Distribution of 

Gap Sizes

Gaps in traffic that are large enough to allow a driver to 

pass through must be present. The distribution of gaps 

may be skewed toward smaller durations in areas where 

traffic is denser.

High High

Gap and Lag Drivers show a higher degree of variation (three times 

greater variance) in lag acceptance compared with gap 

acceptance when turning left; drivers have a tendency to 

reject lag and then accept the first gap.

High Moderate

Maneuver Average gap acceptance can be the same for right turns 

and left turns (from a median) but tends to be longer for 

through maneuvers (e.g., 6.7 s for turn and 7.9 s for 

through maneuvers; gap differences per maneuver may 

vary depending on the geographic location of the 

intersection.

**

High High

Number of 

Lanes to Cross 

The accepted gap becomes larger as the number of lanes 

of opposing traffic increases.

***

High High

Number Of 

Rejected Gaps 

The quantity of rejected gaps can reduce critical gap size 

(e.g., from 6.24 to 2.25 s after rejecting four+ gaps.

**

Moderate High

Point of 

Departure 

Gap behavior tends to be more aggressive during median 

departures compared with stop-sign departures; left turns 

from a major road have shorter gaps than left turns from a

minor road.

**

High High

Queue 

Presence

The presence of a queue behind a driver can reduce 

accepted gap size to 4.5 s, from 6 s.

**

Moderate Moderate

Rain Intensity Rain intensity leads to longer gap acceptance, from 6.5 to 

13 s as rainfall increased from 0 to 0.39 inches/h.

*

High High

Sight Blockage 

Caused by 

Waiting Traffic

Sight blockage from a left turning vehicle during an LTAP 

maneuver can double the amount of time drivers use to 

decide whether they want to turn (e.g., from 1.7 to 3.1 s).*

High High

Time of Day Drivers’ acceptable gap may be larger in the morning

compared with midday and evenings; peak periods affect 

gap acceptance relative to the constancy and availability of 

gaps.

**

Moderate Moderate

Wait Time Increasing wait time can lead to smaller gap acceptance 

(e.g., 8 to 2 s gap after 100 s of waiting at a stop bar to 

turn left; longer wait times lead drivers to accept gaps that 

they previously thought were too short.

***

High High

*Data collected at a signalized intersection.

**Data collected at a stop or yield controlled intersection.

***Data collected at a stop/yield controlled intersection and a signalized intersection.
35

Discussion

Gap choice pertains to drivers deciding to accept or reject lags and gaps in traffic. The difference 

between the decision to reject or accept is straightforward, but the differences between gap and 

lag require explanation. Specifically, lag is the time interval between the arrival of a vehicle at 

the TCD on the minor road of an intersection (e.g., stop or yield sign) and the arrival of the first 

vehicle on the major road. Gap is the time interval between the passage of one vehicle and the 

arrival of the next, yet there is still uncertainty about the definition of gap because some 

practitioners include headway.

(59,60) In addition, lag can be thought of as the remainder of a gap 

when a driver on a minor road arrives at the intersection. Lag and gap are measured to 

characterize how drivers coordinate their movements relative to their proximity to other drivers 

on the road. The size of a gap that drivers find acceptable to enter is probabilistic, and drivers 

change what they find acceptable depending on a variety of factors. (See table 9 and table 10.) 

Table 10. Driver/vehicle characteristics that can affect gap acceptance decisions.

Driver/Vehicle 

Characteristics Finding

Data 

Quality

Design 

Relevance

Driver Gender There is some evidence of gender differences;

there is also evidence that gender is not a 

significant factor.

*

Low Low

Driver Age Older drivers tend to reject more usable gaps and 

accept longer wait times than younger drivers;

teens display more aggressive gap acceptance than 

adults.

*

High Moderate

Vehicle Type Motorized two-wheelers’ gap acceptance is 

shorter than for passenger cars; drivers of older 

cars accept larger gaps than drivers of newer 

cars.

*

High High

*

Data collected at a stop or yield controlled intersection.

Factors That Affect Gap Size Acceptance: The process by which drivers decide to accept any 

one gap is complex and has multiple considerations—some are observable (e.g., wait time), 

while others are not (e.g., internalized perception of risk that influences drivers’ assessment of 

the value of accepting a gap).(61) The size of gaps and lags that are accepted by drivers vary as a 

result of many factors. While several published predictive factors at a population level (i.e., 

macroscopic gap selection factors) and at an individual level (i.e., microscopic gap selection 

factors) are available, it is prudent to assume that an individual’s gap acceptance behavior will 

show widely varying differences. V2X, however, may provide an opportunity in the future to 

incorporate historical driver behavior (e.g., aggressive versus conservative gap acceptance) into 

system activation parameters. Another consideration is that some factors, such as the presence of 

queues and number of rejected gaps, are well established but may not always manifest in the 

same manner at every location.(56,61,62) For example, the data from Gorjestani et al. showed that 

aggregate measures reflecting 80th percentile rejected gap were different depending on the point 

of departure (i.e., from TCD versus from median) and geographic location.

(52)
36

The distribution of gap sizes affects gap acceptance, and these distributions change throughout 

the day.

(55) This dynamic nature of traffic flow can cause drivers to be less able to assess traffic 

for acceptable gaps. Logically, for drivers who are less familiar with the traffic flow of the 

opposing or cross traffic, any unstable and unpredictable changes in traffic flow may cause 

drivers to unwittingly wait until larger gaps become available. Drivers who are more familiar

with (and thus better able to judge the utility of) the available gaps may accept comparatively 

smaller gaps (i.e., familiarity with the traffic distribution may allow drivers to be better able to

identify passable gaps). Learning the operation of a traffic distribution allows drivers to change 

their perceptions of the value and utility of taking any one gap, and can occur while drivers wait 

for an acceptable gap the first time they drive through an intersection.(61,62)

Lag and Gap: Gap should be considered as fundamentally different from lag. Data from 

Gorjestani et al. show a tendency for drivers at two-way stop-controlled intersections to reject 

the initial lag and then immediately accept the first gap.

(52) (In a sample of more than

16,000 intersection maneuvers, drivers on minor roads accept a following gap 78 percent of the 

time after rejecting the initial lag.

(52)) In addition, Devarasetty, Zhang, and Fitzpatrick describe 

significant differences between accepted lags and gaps at 30 different sites located in Texas, 

New York, and Arizona.(51) They measured gap size when vehicles turned left and passed 

through traffic approaching from the opposite direction. They found that accepted critical lag 

times varied between 2.2 and 7.6 s, which was a much greater range compared with their 

measurements of critical gap (which is an estimate of the point at which rejecting or accepting a 

gap is equiprobable.

(63) Critical gap varied between 5 to 6.8 s. This finding illustrates that gap 

and lag should be evaluated independently rather than unequivocally combined. 

In addition to the situational factors described in the previous paragraph, factors related to 

drivers and vehicle types may also affect gap judgments. These factors are more difficult to 

accommodate through design, but the aspects warrant consideration if these factors will be 

overrepresented at a location (i.e., heavy truck traffic).

Design Issues

In most situations, the size of an accepted gap will actually change once a driver on a minor road

enters an intersection. Specifically, observational data indicate that mainline traffic reduces 

speed and that there can be large reductions in mainline traffic speed (e.g., 31-percent speed 

reduction) at a very gentle rate (e.g., 2.2 ft/s2

) when drivers on minor roads are making 

maneuvers.

(51,66) For left-turning drivers at a signalized intersection, approaching traffic may also 

change their speed, thus changing the gap.

(5)

Note that there are multiple methods for measuring gap-acceptance behavior of drivers, and 

many of the methods result in different conclusions. Ashalatha and Chandra found that different 

estimation methods can produce critical gap values that vary up to 37 percent, even when using 

the same traffic data.(60) Therefore, practicioners should confirm the reliability of their gapacceptance calculations using different estimation approaches.
37

TOPIC 8. INTERSECTION DECISION SUPPORT FOR STOP-CONTROLLED MINOR

AND MAJOR ROAD JUNCTIONS

Introduction

This topic covers human factors considerations for intersection decision support systems 

installed at junctions between major and minor roadways. This topic primarily addresses SSA, 

which provides information for drivers on minor roads; however, pertinent information about 

communicating with drivers on major roads using freeway junction signs is also included 

because they can affect travel speeds on major roads in a complementary manner.

(67,68) A few 

pilot systems are designed to assist drivers who are approaching a stop sign-controlled 

intersection, and while the function of each system is slightly different, the general purpose is the 

same.(67,68,69) Specifically, the objective of these systems is to reduce conflicts between higherspeed, major road traffic and slower-speed, minor road traffic. 

Guidelines

Information Presented to Drivers on Minor and Major Roads: When possible, inform drivers 

who are approaching a limited sight distance intersection of potential hazards from cross traffic. 

The following list includes possible message information items for drivers on minor and major 

roads:

• Driver on minor road: 

o Presence of traffic on major road.

o Proximity of traffic on major roads to intersection, or size of gap in traffic 

flow on major roads.

• Driver on major road:

o Intersection ahead.

o Presence of traffic on minor road.

Intersection, Maneuver, and Traffic-Flow Considerations: The SSA system should 

accommodate intersection- and maneuver-specific factors that affect driver gap acceptance, as 

well as aspects of traffic flow on minor and major roads (e.g., average daily traffic and vehicles 

per hour, distribution of available gaps). 

System Activation Timing: When appropriate, the activation timing for messages to inform 

driver gap choices should incorporate a decision-making time interval (e.g., an activation 

parameter with an additional time component for message reading and comprehension).

DII Placement: DIIs should be located outside of approaching drivers’ intersection sight 

distance (ISD) to avoid occluding portions of their line of sight of cross traffic.
38

Number of Stages in a Message: Limit the number of conditional stages or phases of a message 

(e.g., advise/inform, alert/warn, prohibit).

Figure 6 depicts examples of sign locations that consider ISD. Grey sight triangles are based on 

the location of the driver’s eyes.

(70) The slanted green line represents a location that does not 

negatively affect the driver’s line of sight, and the vertical red line represents a sign position that 

occludes the driver’s view of oncoming traffic.

Figure 6. Illustration. Example sign locations that consider ISD.

Figure 7 provides comparison examples of graphical displays for intersection decision support 

systems.
39

Sources: University of Minnesota, Minnesota Department of Transportation, Transportation Research 

Board, Virginia Transportation Research Council.

*Example sign states are shown, the full set of sign states is not well documented in the research but there are more 

than a dozen elements that change as per descriptions of the systems.

**Example design states are shown for an in-vehicle version of CICAS-SSA. Arrows are used to show which CICASSSA graphics are intended to be identical.

Figure 7. Diagram. Comparison of graphical displays for intersection decision support 

systems that send messages to drivers on the major road, minor road, or both.(19,49,68,67)

Discussion

Only a few efforts have examined systems that provide information to drivers at both minor and 

major road junctions, specifically two-way stop-controlled intersections.

(67,68) However, there is 

extensive documentation from a test implementation of CICAS-SSA, which was designed to 

assist drivers on minor roads in rejecting unsafe gaps. This system displays information about the 

proximity of traffic on the major road to the intersection. The available design information about 

CICAS-SSA includes documentation on initial design efforts and prototype testing. 
40

Unfortunately, field test reports do not provide information about the CICAS-SSA’s 

effectiveness.

Information Presented to Drivers on Minor Roads: The CICAS-SSA attempted to allow 

mainline traffic to remain largely unaffected by cross-traffic, which was meant to aid drivers on 

the minor road with their gap choices. (See references 49, 52, 69, 71, and 72.) This decision 

support was provided using DIIs that delivered the following information about vehicles on 

major roads: (1) notification of vehicles that were arriving at the intersection soon, (2) 

information about the direction of the approaching vehicles, and (3) notification if crossing 

traffic was close enough that drivers on minor roads should avoid crossing. (See figure 7.) The 

behavioral objective of the system was to encourage drivers to reject short gaps, and the warning 

level was based on 80th percentile rejected gap size for all maneuvers measured, with a time 

component added to accommodate reading and comprehension of the DII message.(52)

The major road in this field test was a divided roadway with sufficient space for vehicles on 

minor roads to stop at the median. As shown in figure 7, the CICAS-SSA DII provided 

information about each direction of travel on the major road, which resulted in a display that was 

information dense. However, during initial testing, participants typically indicated that the 

CICAS sign was easy to understand, and they were able to describe its purpose to tell them about 

the presence of approaching vehicles.

(19,49,69) Unfortunately, field test data do not provide clear 

results regarding the effectiveness of this system. However, a related driving simulator study 

from Becic et al., showed that there was a benefit in gap acceptance for a CICAS-SSA in-vehicle 

display that used a display that operated in a similar manner, but with a simplified visual 

design.(19)

Information Presented to Drivers on Major Roads: In contrast to the inconclusive data 

regarding what information to present to drivers on minor roads, the findings related to 

presenting information to drivers on major roads are more definitive. Specifically, systems that 

inform drivers on the major road about the presence of cross traffic on the minor road have been 

shown to be effective at reducing driver speed on the major road and possibly crashes.

(67,68) One 

type of message sign that has some documented benefit is a static sign on the major road that 

simply indicates the presence of a crossroad (e.g., freeway-style junction signs). A study that 

examined this type of signage found a reduction in crash rates.

(73) Similarly, the CICAS-SSA 

study by Gorjestani et al. found that there were overall reductions in mainline speed because of

cross traffic, but only for large trucks entering from the minor road.(52) The reduction was 

moderate, between 0.5 and 3.5 mi/h for small and large gaps There was no reduction in mainline 

speed when smaller vehicles entered from the minor road. Visibility may have been a factor at 

this location, however, because smaller vehicles could be occluded by the undulant topography 

of the mainline roadway, whereas larger vehicles could still be seen.

Intersection, Maneuver, and Traffic-Flow Considerations: SSA systems that only inform 

drivers of the presence of other vehicles can be simple to implement for mainline traffic.

(67,68) For 

example, at two-way stop-controlled intersections, the presence of a driver at the stop sign can be 

used to activate the message for drivers on the major road. Informing drivers on the minor road 

about approaching traffic, however, is more complex because criteria related to proximity on 

crossing traffic are required to determine relevant distance from the intersection to activate the 
41

message. Hanscom activated their message on the minor road when drivers on the major road 

were within the ISD.

(67)

Alternatively, field measurements of gap choice have also been used to determine when to 

activate SSA messages to drivers on minor roads. The activation parameters for CICAS-SSA 

were determined using field data on drivers’ 80th percentile rejected gap on minor roads.

(52) The 

resultant 6.5-s value was used as the system timing for all stop-controlled intersections tested. 

Note that the observed rejected gap size was different across locations and for different 

maneuvers. It is possible that using the same timing for these different scenarios may have 

undermined the effectiveness of this system by making the system timing seem unreliable or 

inaccurate.

(10) Consequently, developing system timing based on aggregate driver behavior for a 

specific maneuver and at a specific location should be considered during implementation.

It is important to recognize that gap choice is highly variable and may depend on many factors.

(See topic 7.) Accordingly, using gap choice as a basis for selecting a system activation 

parameter should be considered to be a complex approach. It requires significant effort to 

measure, assess, and determine what elements of gap choice are relevant to include as activation 

parameters.

(52) Available research on factors that affect gap choice suggests that no universally

applicable value for gap acceptance or rejection exists. In particular, gap choice has been shown 

to be different for left, right, and straight-through maneuvers.

66,53) The number of lanes a driver 

must cross is also a factor. Drivers tend to wait for larger gaps when crossing or entering 

multilane roadways.

(54,55) The distribution of available gaps is also an important factor to 

consider because some intersections have dense traffic with fewer passable gaps.

(50) When traffic 

is dense, drivers may reject more gaps before accepting a gap, but the gap they accept may be 

smaller than many of the rejected gaps. Thus, situational factors associated with specific 

locations and movements should be considered when selecting system activation parameters.

System Activation Timing: The CICAS-SSA activation timing parameter was based on field 

data collected on 80th percentile rejected gap size across multiple test sites.

(52) This value was 

subsequently modified by adding an extra 1-s delay to account for the perceptual processing time 

needed to read and comprehend the message before initiating a crossing maneuver.

(49,69)

Laboratory research shows that prohibitory signs (e.g., left-turn arrow with prohibitory overlay 

that indicates left turns are not allowed) are associated with high error rates and can take more 

than a half-second for people to understand, although there is no estimate for real-world 

situations.(74,75) Note that an assessment of the amount of time required to comprehend the full 

array of images across the various CICAS-SSA states has yet to be done.

DII Placement: DIIs should not block the minor-road driver’s line of sight of traffic on the 

major road. Early efforts to determine placement of the DIIs for CICAS-SSA found that test 

subjects reported that the DII message boards blocked their view of major road traffic when 

placed adjacent to the minor road.

(76) Placing the DIIs outside of ISD triangles (see figure 6)

allows drivers to see approaching traffic on the major road.

Number of Stages in a Message: The messages that were designed to inform drivers on the 

major road about the presence of drivers on the minor road were two-stage systems (e.g., 

“vehicles entering when flashing”; see figure 7). There has been considerable research on how to 

graphically present CICAS-SSA information to drivers on minor roads. Earlier efforts 
42

culminated in a multistaged message that consisted of over a dozen changing elements.

(49,69)

Later efforts found that less complex graphics and fewer stages may be practical options.(19,71)

Design Issues

There is a lack of research on how the presence of traffic in the same driving lane (e.g., lead 

vehicles) affects the comprehension of messages and whether drivers will understand if messages 

are intended for a lead vehicle or themselves.

(76) This fact is relevant for systems that activate 

messages contingent on where the driver on the minor road is positioned. In the case of a 

CICAS-SSA type system on a median divided highway, a driver located at the stop sign could 

misinterpret a message intended for a driver in the median.

TOPIC 9. SLTA

Introduction

This topic discusses the factors that should to be considered when presenting left-turn gap-assist 

information to drivers making unprotected left turns at signalized intersections. The primary goal 

of SLTA systems is to provide decision support to drivers making permissive left-turn 

movements with oncoming traffic.

(5) LTAP-OD crashes are primarily caused by errors made by 

the left-turning driver. The SLTA system has the potential to reduce the frequency and severity 

of LTAP-OD crashes by providing drivers with information about the presence of oncoming 

vehicles, which can address driver errors related to detecting traffic and judging gaps.(5) An 

SLTA system is an alternative to other left-turn safety countermeasure at intersections such as 

protected left turns, which typically lead to reductions in traffic flow and which sometimes also 

require dedicated lanes to implement.

(5)

Guideline

The following design guideance is applicable when designing systems to provide SLTA 

information to drivers:

• The decision to provide SLTA information should consider intersection 

characteristics and the safety record of the specific intersection. SLTA is most useful 

at a location where there is a high volume of oncoming traffic, high pedestrian and 

bicyclist density, and persistent limitations on sight-distance/visibility. 

• The timing algorithm of the SLTA alert/warning should incorporate factors related to 

intersections, such as geometry, SPaT, traffic volume/density, and speed.

• The SLTA information can be presented either using a DII, a DVI, or both. 

Implementing a DII display for SLTA has advantages because the display is available 

to all drivers making a left turn (i.e., the subject vehicle does not require CV 

technology), and it permits drivers to maintain their gaze on the roadway 

environment.

• The DII and DVI display locations should be integrated with locations where drivers 

would typically be scanning while performing the gap judgment task. 
43

• If SLTA information is presented on multiple displays (DII and DVI) simultaneously, 

then the activation timing of the both displays should be the same.

• The meaning of the message elements and symbols used on SLTA DII/DVI displays 

should be consistent with the allowable movements.

Source: Knodler and Noyce

Figure 8 depicts a typical area that drivers scan when they are making a left turn. The two small 

ovals indicate drivers’ preference for the SLTA-DII display location in their driving 

environment.

(10)

Source: Knodler and Noyce

Figure 8. Photo. General region drivers scan when making an LTAP

of oncoming traffic.

(77)

Discussion

The most frequent causal factor in LTAP-OD crashes is a left-turning driver who inadequately 

perceives the gap required to make a safe left turn.

(78) Other factors that also contribute to LTAPOD crashes include failure to judge speed of closing vehicles, failure to see oncoming traffic, 

and obstruction of the driver’s view by an oncoming opposing vehicle.

(5) From 2005 to 2008, 

there were an estimated 200,212 LTAP-OD crashes at signalized intersections based on weighted 

National Automotive Sampling System-General Estimates System (NASS-GES) data, which 

accounted for an estimated annual cost of more than $10.3 billion.

(79) The objective of the SLTA 

system is to reduce the frequency and severity of these crashes by providing drivers with 

information that will help them better assess oncoming traffic and discourage them from making 

left turns with inadequate gaps in opposing traffic.

(5)

Intersection Characteristics: An earlier SLTA concept of operations (ConOps) by Misener 

et al. investigated a large set of intersection factors to identify the following characteristics that 

should be considered when implementing an SLTA system:

(5)
44

• Left-turn pockets.

• Left-turn lane.

• Intersection geometry.

• Line of sight.

• Pedestrian levels.

• POV volume and speed.

• POV number of lanes.

• Percentage of left-turning vehicles.

• Percentage of heavy-duty vehicles.

• Clustered/dispersed traffic flow.

This ConOps did not provide specific criteria for these intersection characteristics, but it did 

identify a shortlist of design consideration for SLTA systems based on these factors. (See 

table 11.) However, no data indicate which characteristics are most critical for implementing the 

SLTA system, or which observed levels constitute a “high” indicator. Accordingly, these 

considerations should only be used as a general reference. Note that design consideration for the 

systems will not be uniform across intersections, and adjustments would need to be made based 

on the characteristics of the specific intersections. More information is available in 

Misener et al.

(5)

Table 11. General design considerations for intersection characteristics.

Intersection Characteristics Design Considerations

• High traffic volume in the primary 

direction.

• High concentration of LTAP crashes 

involving a vehicle turning from primary 

corridor onto cross streets.

• High speeds.

CICAS–SLTA systems require capability to 

monitor a large number of vehicles at 

relatively high speeds.

• Multiple lanes of oncoming traffic.

• No left-turn pockets.

CICAS-SLTA systems need to consider the 

turning time needed for completing left turns, 

based on the size and/or number of crossing 

lanes. This variable is essential for 

determining the warning threshold. 

• High pedestrian presence. CICAS-SLTA systems need to consider/

design needs to include pedestrian-sensing 

capabilities. 

Line of sight, geometry, and visibility represent other considerations when implementing SLTA. 

Divekar et al. found that drivers did not use an SLTA system when they had a clear view of 

oncoming traffic.(10) In this situation, drivers preferred to rely on their own judgment based on 

observing oncoming traffic; however, drivers used the system when their view of oncoming 

traffic was obscured by a truck in the opposing left-turn lane.(10,80) This fact suggests that the 

system may be more appropriate at intersections where the drivers’ view of the traffic is 

frequently limited by sightline restrictions or other vehicles.
45

System Timing: Determining the timing for the SLTA message alert is challenging because no 

guidance or documentation about the message timing requirements or about suitable approaches 

for determining message timing exists. Misener et al. suggest that factors such as intersection 

geometry, traffic volume and speed, and signal phasing should be considered when determining 

system timing, and that the timing of the message activation should be calibrated for the specific 

intersection where the system is deployed.(5) For example, the timing of the message will be 

longer if the driver must cross multiple lanes of traffic compared with crossing a single lane. 

The unequivocal priority of the SLTA system is to improve safety by reducing LTAP-OD 

crashes. Therefore, the message timing of the system must be designed to provide all drivers 

with sufficient time to safely execute a turning movement. This approach leads to conservative 

system timing parameters biased toward larger gaps in traffic. However, there is a tradeoff 

between larger gap sizes and drivers’ perception of credibility of the system. In particular, 

drivers tend to ignore SLTA information if it provides a “don’t go” (no left turn) indication for 

gap sizes that drivers are normally comfortable turning through.

(10) Moreover, drivers seem to 

rely on the system more and perceive a system to be more accurate, if the message alert timing 

matches their comfortable gap size. For example, in a simulator study by Divekar et al., drivers 

ignored the SLTA system when the alert was presented for gaps that were 2 s longer than the 

gaps that drivers frequently accepted.(10) In contrast, when the alert timing matched the gap sizes 

drivers frequently accepted, a higher proportion of drivers used the system. While it is critical 

that SLTA systems do not encourage potentially unsafe gap acceptance, it is still important to 

recognize that designing a system that is highly conservative runs the risk of being underutilized.

Display Type: The SLTA message can be presented either on a DII, a DVI, or both. The type of 

display does not seem to affect driver use of the SLTA information in meaningful ways. Misener 

et al. found that there was no difference in the drivers’ turning rate and braking response as 

function of the location of SLTA display type (DII or DVI).(5) Divekar et al. found that drivers 

used whichever SLTA display was available in the driving environment to inform their left-turn 

gap judgment and that the performance across the two display conditions (DII versus DVI) was 

the same.

(10) In both studies, however, drivers indicated a general preference for the DII because 

it had a distinct advantage of being located on the roadway and was perceived as more 

authoritative than the DVI. This preference is consistent with a report by Rephlo in which 

participants consistently demonstrated a preference for information presented on the DII in 

responses to questions about reliability of the SSA system.(80)

Display Location: Another factor that needs to be considered when implementing the SLTA

system is the location of the displays in the environment. Specifically, the displays should be 

integrated with drivers’ normal visual scanning behavior during left turns. Incorporating the 

displays within or near the drivers’ typical scanning zone will make it easier for the drivers to 

acquire the information and to respond appropriately.

(10) Drivers making a left turn maneuver 

tend to scan the intersection from the right to left, and they generally take a first look at the 

relevant traffic light and then focus on oncoming traffic.

(77) Drivers also use the stream of 

oncoming traffic as their base point from which they then distribute their glances between any 

objects in their path of travel (pedestrians, bicyclists, etc.), other sources of information at the 

intersection, and the traffic light.

(77,81)
46

DII-DVI Coordination: When presenting information on both DII and DVI displays 

simultaneously, it is important that the timing of the message between the two systems is the 

same. (See topic 3.) Divekar et al. examined the effect of the DII and DVI having different 

timing when presenting SLTA gap-assist information.(10) In one condition, when drivers used just 

one display and were essentially able to ignore the other, there was no evidence of interference. 

In another condition, however, when it was more difficult to simply ignore a display that had 

longer timing (i.e., DVI was near the drivers’ line of sight), there was evidence that interference 

might have occurred because this display showed conflicting information as a result of lag in 

display timing. From a simple human factors perspective, this situation should be avoided 

because providing conflicting information when drivers are making a decision about whether or 

not to turn is likely to interfere with their decision-making process. 

Message Elements and Symbols: Finally, there needs to be consideration of which message 

symbols are used on the DII and/or DVI displays. Existing symbols have a distinct advantage of 

familiarity (see topic 5), but drivers also have a learned meaning for these symbols. If the 

symbol’s commonly accepted or understood meaning does not match the function of the system, 

then drivers’ understanding of the message may be reduced. For example, Divekar et al.

examined drivers’ understanding of two MUTCD symbols in an SLTA scenario: “No Left Turn”

and “Left Turn Yield” symbols presented on a DII.

(10,7) (See figure 9 and figure 10.) Almost half 

of drivers misinterpreted the “No Left-Turn” message on the DII as a permanent restriction 

rather than a temporary restriction based on immediate traffic conditions. On the other hand, the 

“Left Turn Yield” sign in the same context had a much higher initial comprehension rate. These 

findings suggest that it is important that message elements and the symbols used on system 

displays should be consistent with the purpose of the system.

Source: MUTCD.

Figure 9. Illustration. MUTCD symbol for “No Left Turn” that was presented on the DII to 

examine drivers’ understanding of the SLTA system.

(7,10)
47

Source: MUTCD.

Figure 10. Illustration. MUTCD symbol for “Left Turn Yield” that was

presented on the DII to examine drivers’ understanding of the SLTA system.

(7,10)

TOPIC 10. RLVW

Introduction

This guideline discusses the presentation of notifications and warnings for RLVW systems. The 

objective of this application is to warn potential red light runners so they can take action to avoid 

entry into the intersection. Violators are drivers who are on a trajectory that will result in them 

crossing the stop bar and entering an intersection when they do not have the right of way. 

Violator causal factors include both unintentional acts such as missing the signal because of low 

conspicuity or inattention and intentional violations such as trying to “beat the yellow.” RLVW 

holds the potential to reduce the frequency of unintentional violations. This information is 

presented with the assumption that other means of reducing right-of-way violations have been 

undertaken, such as reviewing the signal’s visibility and timing based on MUTCD requirements 

and investigating ways of increasing conspicuity (e.g., optimizing signal placement).(7)

Guideline

The following design guidance applies when determining the presentation of notifications and 

warnings for RLVW systems:

• Use an RLVW to address red light running due to inattention or poor signal visibility. 

• Present the RLVW in a period that is detectable and actionable by the driver. 

• Avoid activating the RLVW while the driver is in the dilemma zone. When a traffic 

signal turns yellow, a dilemma zone region is created upstream of the intersection, 

encompassing distances in which drivers can neither safely stop at the stop line nor 

completely pass through the intersection before the light turns red.

• Base RLVW timing on a consideration of several factors, including driver perception

reaction time (PRT), current travel speed, the distance to the intersection, deceleration 

required to perform a nonviolating stop, and road surface conditions.
48

Table 12 provides a list of timing factors with their respective effects on driver responses

to RLVW.

Table 12. Timing factors that affect RLVW compliance. 

Factor Effect on Driver Response to RLVW

PRT Distracted or inattentive drivers generally display longer reaction times.

Current travel speed RLVW compliance may increase if the driver can decelerate at a 

comfortable level.

Distance to 

intersection

RLVW provided earlier, at greater distances, typically results in greater 

compliance.

Deceleration required Required decelerations should be at or below 0.5 g to facilitate RLVW 

compliance.

Road surface 

condition

Inclement weather and broken/uneven road surfaces may negatively 

affect RLVW compliance.

The choice of DII for the RLVW depends on the salience of the signal and associated visibility 

conditions, as well as the cause of red light violations. Table 13 provides a list of technologies 

that have been examined for or are applicable to RLVW. All are of low salience in their nominal 

state and activate only when providing the RLVW.

Table 13. Example DIIs for the RLVW as a function of condition/utilization.

Condition/Utilization DII Description

Low signal visibility 

and/or contrast

Signal Head, alternating 

flashing beacon

Lights placed on either side of the red signal 

lens that flash in an alternating pattern.

Low signal visibility 

and/or contrast

LED backplate Illuminated backplate for the signal head. 

Not MUTCD compliant, and wind/weight 

load must be considered when adding to 

existing span wires or mast arms.

Low signal visibility 

and/or contrast, or 

cluttered visual scene 

around signal head

Blank-out sign, strobe 

lights

Blank-out sign that displays an illuminated 

octagon with STOP written inside. May 

include strobe lighting at the corners of the 

sign.

Signal head potentially

obscured (e.g., obscured 

by a leading large truck)

In-pavement lighting Conventional or LED lighting installed inpavement at the stop bar. Illuminates when 

triggered to provide the appearance of a 

lighted line across the road.

LED = light-emitting diode.

Discussion

Right-of-way violations at intersections extract a great toll in regard to cost and well-being. 

Eccles et al. estimated that 234,881 crashes led to a total annual cost of $13 billion (using NASSGES data, 2005 to 2008).(79) Furthermore, approximately 45 percent of these crashes resulted in 

fatalities or injuries. Therefore, intersection applications such as RLVW have potentially large 
49

safety benefits concerning crash involvement. Red light violations may result from intentional 

and unintentional factors. Hendricks, Fell, and Freedman characterized three different causal 

factors for these types of collisions: (1) looked and did not see, (2) driver inattention, and 

(3) crossed intersection with an obstructed view.(82) Other causal factors that have been identified 

include trying to beat the yellow, mistaking the phase of the signal, and intentionally violating 

the signal. Of these factors, RLVW holds the potential to reduce the frequency of violations from 

all but those arising from intentional violations.

A number of engineering countermeasures for reducing the frequency of right-of-way violations 

at signals have been identified and tested. Summaries of these are provided by the Institute of 

Transportation Engineers (ITE) and by Antonucci, Hardy, Slack, Pfefer, and Neuman.

(83,84)

Methods such as improving the signal visibility and conspicuity, providing advance notice of the 

upcoming intersection to increase the likelihood of stopping, and eliminating unnecessary 

signalized intersections are all potential countermeasures to right-of-way violations at 

intersections. The information presented in this section assumes that other engineering 

countermeasures were considered prior to the implementation of an RLVW.

Considerations Regarding Driving Inattention or Signal Visibility: The RLVW presentation 

can involve several different DII elements.

(85) DII evaluations have examined both RLVW and 

DII intersection warnings oriented to the potential victims of red light violators. Although 

targeted at different drivers (with different assumptions about driver state), both are discussed 

here because all have the same intended effect of preventing entrance into the intersection. 

Furthermore, many of these countermeasures were not designed to target violators; instead they 

were designed to augment the visibility of the existing TCD and infrastructure. While these 

countermeasures were implemented as a supplement to existing DII, their use as a warning 

(i.e., not activated until a potential violation is identified) may be supported.

Beacons are used to draw attention to the signal head in low visibility or contrast scenarios. For 

notifications to an intersection, these have typically been examined in the form of alternating 

flashing lights on either side of the signal head. A blank-out sign can provide additional 

information when the signal may be of low visibility. These signs can be mounted with the signal 

heads. They are blank in the nominal state and provide a replication of a stop sign when 

activated. Strobe lights may be included on the sign that, when activated, can assist in attracting 

drivers’ attention to a signal or sign in cluttered visual scenes. These elements have been 

examined (largely in conjunction with each other, preventing any detailed discussion of 

individual DII element effects) by both Neale et al. (in conjunction with a blank-out sign and 

strobe lights) as well as Inman, Davis, El-Shawarby, and Rakha.

(6,86) Results from both 

examinations found generally high but not complete compliance with these warnings. A

distracted driver, however, may still miss these signals, and Inman et al. note that higher 

compliance was found with additional and more conspicuous DII elements (such as in-pavement 

lighting).(86)

A light-emitting diode (LED) backplate can serve as an attention attracting element to a signal. 

The MUTCD specifies that signal backplates should have a dull black finish to enhance contrast 

or, optionally, a yellow retroreflective strip for enhanced nighttime appearance.(7) While not in 

compliance with the MUTCD, an LED surround used instead of the retroreflective strip can 
50

allow a more conspicuous illuminated outline of the signal head. Tydlacka, Voight, and Langford

reported that LED backplates are associated with a reduction in red light running violations.(87)

In-pavement lighting is generally described as being used to enhance, or attract attention to, lane 

markings. This type of lighting can also be useful when the signal head may be obscured, for 

instance if a driver is following a large leading vehicle. Inman et al. describe a simulator 

experiment that included in-pavement lighting along the travel lane and the stop line; however, 

these were examined in conjunction with intelligent rumble strips that could be triggered, so the 

effectiveness of the lighting alone cannot be assessed.(86) Tydlacka et al. reported on the 

installation of in-road lights installed with the stop bar at an intersection.(87) The stop bar lights 

provided an improvement in right turn on red violations (a prohibited maneuver at that particular 

intersection) but had no effect on red light running violations. In-pavement lighting should also 

be considered in relation to the cost of installation and maintenance. Carson, Tydlacka, Gray, and 

Voight describe many of the installation and maintenance costs associated with this DII, as well 

as report on some jurisdictions’ operation experiences.(88) In addition, depending on the 

installation, in-pavement lighting may be visible to nonviolating drivers and could possibly lead 

to unintended responses from nonviolating drivers.

Message Presentation Period: Determining the timing of the RLVW is difficult because of the 

direct relationship among the warning time, driver response time, speed, and deceleration 

required to achieve a stop. Identifying a violator can only be done very close to the intersection 

entrance, yet presentation of violation warning must be actionable (beyond the PRT and physical 

limits of stopping the vehicle). Thus, the RLVW must be presented within a period that is 

detectable and actionable by the driver. For instance, if a driver can only be identified as a 

violator 180 ft from an intersection, then a driver approaching at 45 mi/h will only receive a 

warning 2.75 s prior to violation. Assuming an immediate reaction from the driver, this action

would require an average deceleration of approximately 0.75 g, a value that some drivers may 

not be able to sustain. In this case, the driver may pass through the intersection in violation or 

come to a stop beyond the stop bar if he or she attempts to comply with the RLVW. Further, 

factors such as driver distraction can increase the PRT, increasing the required RLVW timing.

Avoid Dilemma-Zone Activation: Because an RLVW system has not been extensively tested in 

highway conditions, some potential unintended consequences should be noted. First is the 

potential for driver confusion (and thus delayed response) if RLVW is presented in the dilemma 

zone. Neale et al. note that this confusion may result in a conflict if the potential violator is given

the warning while the signal is yellow, stating that “...while the warnings were meant to elicit a 

stopping response, the [yellow] light provides a choice to drivers, who must decide whether they 

feel more comfortable stopping or trying to make it.”(6) (p. 162) Drivers’ individual resolutions to 

this conflict could result in a number of different, potentially undesirable actions that may negate 

the benefits of the RLVW.

Timing Considerations: In this topic, warning timing is discussed in regard to the required 

deceleration and what deceleration levels a driver may accept. This discussion is necessary 

because of the relationship among speed, distance, time, and the resultant deceleration, as 

described by the American Association of State Highway and Transportation Officials

(AASHTO).

(89) AASHTO presents design values for stopping sight distances of a “comfortable 

deceleration” level ranging from approximately 0.27 to 0.34 g. Gates, Noyce, and Laracuente 
51

examined drivers passing through different intersections and found that the travel time to the 

intersection following activation of the yellow light was the greatest single predictor in 

determining whether a driver would continue through the intersection.(90) For drivers 

approaching the light at a faster speed, Gates et al. identified decelerations ranging from 

approximately 0.29 g (15th percentile) to 0.42 g (85th percentile), suggesting the AASHTO 

values may be slightly conservative.(89)

Compliance is generally greater when warning times are longer and thus lower decelerations are 

required of the driver. A simulator examination by Inman et al. used timing between 2.75 and 

3.3 s; these values resulted in decelerations between 0.65 and 0.85 g.(86) In a complementary 

closed-course test track experiment, Inman found that slightly lower values of 0.78 g were 

typical. Compliance with a DII-provided warning was higher when longer warning periods were 

provided (thus involving lower required decelerations). However, these findings, when viewed 

alongside collision avoidance research reported by Keifer et al. and the AASHTO standards, 

suggest that drivers may not perform decelerations above 0.5 g.(91,89)

Another factor that may influence RLVW compliance is the road surface condition. Although the 

effect of road surface condition on RLVW compliance has not been systematically evaluated, 

other efforts to reduce right-of-way violations at signalized intersections have identified 

pavement surface conditions as a concern.

(83,84) ITE notes that lower friction forces arise from 

issues such as older or damaged pavement, certain road surface types, and the presence of 

surface water. These may all lead to intersection violations by making a driver less likely or able 

to comply with a yellow signal and should be accounted for in an RLVW. 

Design Issues

Research has suggested potential benefits to presenting RLVW information through both the 

DVI and DII. Both Neale et al. and Inman and Davis concluded that simultaneous DII and DVI 

presentation of an intersection collision warning message was more effective than individual 

presentation through either individual method of communication.(6,92) Therefore, the coordinated 

presentation of RLVW messages should be considered when possible.

Beyond the benefits of presentation via DVI, obtaining information from V2I communication 

can help improve the performance of the RLVW prediction algorithm by providing the 

information contained within the basic safety message. Combining the current vehicle 

parameters (as contained in the basic safety message) with SPaT information could allow more 

accurate, precise, and directly targeted presentation of RLVWs. These V2I-enhanced RLVW 

messages could also benefit from other vehicle information, such as the braking ability of vehicle 

or load status of commercial vehicles.

The effect of surrounding traffic on RLVW compliance is unknown. Inman and Davis examined 

compliance with a DII-based warning of a red light violator (i.e., warning the potential victim 

prior to entering the intersection) in a driving simulator task with no surrounding traffic, lead 

vehicle, and following vehicles.(92) The presence or absence of a following vehicle was found to 

have no effect, and the presence of a leading vehicle affected response time to the yellow light 

but not compliance with the signal. In an examination of DIIs for intersection collision 

avoidance, Neale et al. noted that some participants commented on the presence of a following 
52

vehicle in their responses to a warning and that the following vehicle presence may have had an 

effect on the driver’s comfort with performing the stopping maneuver.(6) Pending further 

investigation, implementation of an RLVW should consider the likelihood of surrounding traffic 

modifying the likelihood of a driver response. 

A final potential issue is the possibility of the RLVW message being visible to nonviolating 

drivers. The driver of a lead vehicle that could enter the intersection legally and exit during the 

red clearance interval could slow in response to the message, thereby becoming a violator. 

Related is the issue of following vehicles. Driving simulator studies have suggested that the 

presence of lead or following vehicles will not affect driver braking likelihood; however, this 

possibility has not been investigated extensively or outside of simulated environments.(92) Some 

evidence does suggest that drivers receiving the RLVW message may hesitate to engage in the 

hard braking required to avoid violating the right of way if another vehicle is following.

(6)

Conversely, the potentially violating driver’s braking may lead to a rear-end collision from 

inattentive drivers in following vehicles.

TOPIC 11. CSW

Introduction

This guideline addresses the use of DIIs in CSW applications, including advanced warning signs, 

vehicle-activated signs, and DCWSs. The corresponding DII elements advise drivers of an 

upcoming horizontal curve and provide a warning and information regarding the safe approach 

and traverse of the curve. Eccles et al. estimated that almost 169,000 annual crashes occur as a 

result of unsafe speed at curves or ramps, with 44 percent resulting in injuries or fatalities and a 

cost estimated to exceed $29 billion.(79) Horizontal curves can also be a challenge for large 

combination vehicles (LCVs), which must approach curves at different speeds than automobiles. 

LCV crashes in curves typically involve improper speed and inattention.

(93) This guideline is 

intended to supplement MUTCD standards for problematic horizontal curves and V2I 

applications in addressing certain vehicle types with higher risk of crash involvement.(7)

Guideline

The following design guideance applies to the use of DIIs in CSW applications:

• Use a combination of advance warning signs and DCWSs to target at-risk drivers 

with information about the safe traversal of a horizontal curve.

• Locate advance warning signs based on table 2C-4 of the MUTCD.

(7) Present CMS 

information in accordance with MUTCD legibility recommendations (for speeds 

above 55 mi/h, 600 ft in nighttime conditions (7.4 s at 55 mi/h) and 800 ft (9.9 s 

at 55 mi/h) in daytime conditions). Advanced warning sign placement should 

consider passenger vehicles and LCVs separately. Consider using vehicle-specific or 

separate DIIs if a single DII cannot accommodate the kinematics of these different 

vehicle types.
53

• Include the advisory speed for the vehicle in DCWS message elements. DCWS 

messages may include the vehicle’s current speed, the risk type, and any driver 

corrective action needed. 

• Follow guidance from section 2L.05 of the MUTCD regarding how to format 

information presented on a CMS that is part of a DCWS.(7)

• Permit the advance warning window for an LCV to overlap with that for a passenger 

automobile; however, the time and distance for some types of LCVs (e.g., empty 

tractor-trailers, vehicles pulling loaded liquid tanker trailers, etc.) may be 

considerably greater than that of a passenger car.

• Enhance a CSW with V2I communication. Include vehicle characteristics in CSW 

algorithms to reduce false alarms. Use the DVI to provide a salient audio or visual 

CSW notification.

Table 14 provides a list of DII elements which support horizontal curves.

Table 14. DII Elements to support horizontal curves.

DII Element Description

Advanced warning 

signs

Notification sign, including an indication as to the type and direction of 

the forthcoming horizontal curve. These indications can include an 

advisory speed and/or warnings specific to LCVs.

Vehicle-activated 

advanced warning 

signs

An advanced warning sign that activates based on an approaching vehicle 

traveling above a predetermined threshold. Signs can include blank-out 

signs or a static sign with flashing beacons.

DCWSs CMS triggered by approaching vehicle. These signs typically include a 

vehicle detection system and can be connected to weigh-in-motion 

systems for LCVs. A DCWS should be used in conjunction with 

advanced warning or vehicle-activated advanced warning signs.

Discussion

CSW DII elements supporting LCVs in horizontal curves should provide information about the 

advisory speed for the curve (relative to the rollover threshold) and be visually salient. LCVs 

have a reduced rollover threshold and must traverse horizontal curves at lower speeds than 

passenger vehicles. Improper speed and driver distraction are contributing factors to many LCV 

crashes in curves.

(93) A CSW can address these factors by targeting specific drivers with 

actionable information about the curve in an attention-getting manner through both DII and DVI 

elements.

Multiple DII Elements: A CSW should be provided via multiple DII elements, including 

advanced warning signs or vehicle-activated advanced warning signs and a DCWS. When used 

alone, static signs may not be an effective method for providing this information. Chowhurdy, 

Warren, Bissell, and Taori noted that many drivers ignore or otherwise do not comply with 

advisory speed signs because of overly conservative advisory speeds compared with the 
54

capabilities of modern vehicles.(94) Assuming that advisory speeds are properly calibrated, 

vehicle-activated signs can help draw attention to an advisory sign by increasing the sign 

salience. The highest salience, however, comes when these are used in conjunction with a 

DCWS.

Location and Presentation: The location and presentation of the CSW DII is dependent on 

factors such as the posted or 85th percentile speed, the difference between posted and advisory 

speed, typical conditions, and the expected driver PRT. The MUTCD provides guidance on the 

placement of advance warning signs.(7) Advanced warning signs may be supplemented by 

beacons or may use blank-out signs activated by approaching vehicles (i.e., vehicle-activated 

advance warning signs) to increase conspicuity. 

The CMS message should be presented for a sufficient duration of time for the driver to read the 

message. The MUTCD provides the following criteria for legibility distances (provided in 

table 15 as seconds of visibility):

(7)

Table 15. Criteria for legibility distances by seconds of visibility.

Design Criteria

55 mi/h

Design Speed

70 mi/h

Design Speed

Daytime condition legibility distance (800 ft) 9.9 s 7.8 s

Nighttime condition legibility distance (600 ft) 7.4 s 5.8 s

A minimum message presentation time of 4 s is recommended.

(95) If the message must involve 

two phases of presentation, then the cycle time should not exceed 8 s. Information targeting LCV 

drivers may be presented earlier than information targeting passenger vehicle drivers; however, a 

distracted driver may not notice the CMS when the message is first presented. Because the

system targets distracted drivers, longer presentations are preferred. In all cases, the placement 

should be based on engineering judgment, balancing the benefits against drivers’ potential to 

forget or ignore the message. 

Driver Workload: DCWS message elements should be provided far enough in advance of the 

curve so as not to increase driver workload. Driver workload at curves becomes elevated at curve 

discovery and is highest at curve entry and negotiation.

(11) CSWs may reduce workload and 

uncertainty by providing information about the curve, no closer than 246 ft or 4 s from the curve.

information includes MUTCD-specified information about the curve such as the horizontal 

curve’s direction and the advisory speed.(7) A DCWS can incorporate information from 

speed/height sensors, machine vision, or weigh-in-motion systems to provide more targeted 

information and reduce instances of false alarms.

Message Comprehension: Format information of a DCWS CMS to facilitate message 

comprehension. When presented on a CMS, messages should be presented with a limited number 

of information units to ensure rapid extraction of the message by the driver. MUTCD provides 

guidance for phrasing messages on CMS in section 2L.05; the simplest message that provides the 

needed information is the best.(7)
55

Presentation for LCVs: The advance warning window for an LCV may overlap with that of a 

passenger vehicle. However, the time and distance required for some configurations of LCVs to 

slow to an appropriate speed (e.g., when the LCV is either empty or heavily loaded, pulling a 

liquid tanker trailer, etc.) may be considerably greater than that of a passenger car. DCWS signs 

can be implemented at greater distances when specifically targeting LCVs. Therefore, the 

information from a DCWS CMS may be more beneficial for LCV drivers when 

presented earlier.

Advisory Speed Calculation: Use V2I communication to enhance CSW applications by 

determining optimal approach speeds based on vehicle characteristics and horizontal curve 

characteristics. For instance, the rollover threshold for an LCV with a high center of gravity will 

be much lower compared with an automobile. Thus, the safe entry speed of the two vehicles will 

likely be different for a given horizontal curve. Calculating an appropriate advisory speed based 

on both sources may provide a more accurate value. V2I also allows coordinated presentation of 

a CSW via DVI and DII, which may provide further benefits to the CSW. The use of the DVI 

will allow auditory presentations of a CSW, which may be very beneficial for distracted drivers 

(who are a large contributing factor to crashes at curves).

(96)

Design Issues

DCWS information should provide accurate advisory speed information because false alarms can 

negatively affect drivers’ trust in the system.

(97) (See topic 6.) CMS information targeted to a 

specific vehicle should be clearly indicated as such to avoid unintended consequences such as 

nontargeted drivers suddenly braking in front of other vehicles. Previous field trials have 

examined the use of transponders and weigh-in-motion systems to identify vehicles for CSW 

messages.

(98) Presenting information targeted specifically to a driver by providing identifying 

information (such as the driver’s given name or the freight carrier), however, is not 

recommended because this public display of personal information may foster driver resentment. 

The information provided in this guideline is primarily oriented to the issue of horizontal curves. 

With appropriate sight distances and proper signage, horizontal curves are unlikely to present a 

problem to most passenger vehicle drivers. With certain types of curves, however, such as 

compound curves or certain sag vertical sections, drivers may improperly judge appropriate entry 

and negotiation speeds.

(99) In these situations, different DII elements (such as those discussed 

within this section) may be advisable.

TOPIC 12. ROAD WEATHER: GENERAL HUMAN FACTORS CONSIDERATIONS

Introduction

This topic discusses guidance for road weather messaging on DIIs and DVIs. The primary 

purpose of these road weather applications is to provide warnings regarding current or near-term 

adverse conditions that require drivers to adjust speed as opposed to providing information about 

forecasted conditions.

(9) The weather conditions of primary interest include high winds, flooding, 

adverse road surface conditions, and reduced visibility. These applications would be most 

relevant in areas that have a history of weather-related crashes.
56

Design Guidance

The following design guideance is applicable when developing DIIs and DVIs with road weather 

messaging:

• Provide relevant weather information that conveys the nature of the weather hazard,

location of the hazard, and appropriate driver responses to maintain safe vehicle 

operations.

• Make the decision to communicate weather information via DII or DVI with the 

consideration of the factors covered in table 16. 

Table 16. Differences in presenting weather information via DII and DVI.

Factor DII DVI

Content Presentation Messages should be primarily 

text on a CMS, dynamic message 

sign, blank-out sign, or a static 

sign with beacon(s).

Multimodal possible, including 

visual graphics, icon, or text.

Targeted Messaging Messages should apply to vehicle 

classes or all road users.

Targeted messaging via DVI is 

appropriate; message can be 

suppressed if not applicable to 

known route.

Location Information Message should be presented in 

terms that road users who are 

unfamiliar with the area will 

comprehend.

More detailed information or 

additional context may be 

provided.

Message Framing Both warnings and prohibitions 

are appropriate.

Both warnings and prohibitions are 

appropriate; any information 

should be mirrored on the DII.

Advisory/Regulatory 

Speed

Both advisory and regulatory 

speeds are appropriate.

Advisory speeds appropriate; 

regulatory speed updates should be 

mirrored on the DII.

Table 17 provides examples of weather information presented via DII and DVI.

Table 17. Presenting weather information via DII and DVI.

DII Message Targeting All Road Users DVI Message Targeting Individual User

“Flooding at US-301”

“Washington DC Use Route 210”

“Flooding at US-301”

“Road Closed Ahead”

“Turn Left: Berry Road”

Discussion

This topic discusses guidance for road weather messaging on DIIs and DVIs. Analyses of crash 

data have estimated that 211,304 annual crashes may involve weather-related factors.

(79)
57

Applications such as SWIW-RS can provide road weather information and help reduce the 

incidence of these crashes by warning drivers of the need to reduce speed because of hazardous 

road conditions. These applications are mainly intended to provide warnings regarding current or 

near-term adverse conditions rather than forecasted conditions.

(18) The weather conditions of 

primary interest include high winds, flood conditions, adverse road surface conditions (e.g., ice, 

snow, or rain) and reduced visibility (e.g., fog or smoke).

(18) These applications would be most 

relevant in areas that have a history of weather-related crashes, such as icy roadways or areas 

prone to flooding.

Road weather messaging on a DII is most likely to take place on CMSs, dynamic message signs, 

blank-out signs, or static signs with a dynamic element (such as a warning sign with a flashing 

beacon). Information presented on the CMS will likely be primarily text-based, and design 

guidance is available in other sources.

(9,34) The road weather messaging on a DVI is able to 

increase the salience of the message through multimodal presentation (i.e., use multiple sensory 

channels to attract drivers’ attention), and to filter road weather information that is not applicable 

to the vehicle. As V2X market saturation increases, providing coordinated information via DII 

and DVI will likely become of greater importance. (See Design Issues.) 

In addition to these general factors, some specific factors have an effect on presenting road 

weather information via DII or DVI. Table 16 provided a comparison of these factors: content 

presentation, targeted messaging, location information, message framing, and 

advisory/regulatory speeds. These are discussed further in the following paragraphs. 

Content Presentation: The content presentation of a message refers to how the message is 

formatted and displayed to the road user, such as text, icons, other graphics, or (in the case of a 

DVI) audio-visual presentation. Messages should be presented via DII in accordance with best 

practices and available guidelines.

(9,34) A CMS can be an effective DII element for presenting 

hazard warnings regarding road weather concerning speed and headway as well as general 

hazard awareness.

(100,101) However, DIIs are spatially fixed in the environment. Information 

presented should account for this fixed location and the single exposure of limited duration that 

drivers will have with the DII, which can be achieved by providing concise, relevant, and wellformatted information to drivers, with complementary road weather information presented via 

DVI or available through 511 systems or similar means.

Targeted Messaging: Messages may be targeted, meaning that they are intended for specific 

road users or classes of vehicles or may be applicable to all road users. Messages presented via 

DII are typically visible to all road users. Information presented via DII should be applicable to 

either all road users or classes of road users specifically identified on the DII (e.g., high 

crosswind advisories for trucks). The same information may be provided via DVI. If the DVI is 

used, then it may be to provide road weather information to all users or to target individual 

vehicles or classes of vehicles. This level of targeting may be particularly useful for vehicles 

with a known route (i.e., to suppress irrelevant updates or provide updates for a specific 

planned route). 

Location Information: Road weather information provided via DII is visible to all road users at 

that location. Because drivers may not be familiar with the area, however, providing drivers with 

information relevant to a specific landmark, location, or route supports better driver compliance 
58

than general messages (e.g., “Fog Delays to Boston, Use Route I-295” is preferred over “Major 

Delays, Use Other Routes”).

(9) The DVI may be used to provide more specific information. 

Because road weather information involves three locations (the sensor location, the DII location, 

and the current location of the driver/DVI), providing highly specific information for a given 

location is challenging.

(102) Therefore, it may be necessary to convey uncertainty in weather 

likelihood, location, timing, and impact to drivers.

(9) This uncertainty may be provided as a 

percentage or qualitatively (e.g., “possible”) but should avoid being inaccurate, irrelevant, 

obvious, repetitive, or confusing.

(34)

Message Framing: Message framing refers to the type of message provided to the driver. In the 

case of road weather information, messages are typically framed as advise/inform (potential 

weather-related hazard), alert/warn (active weather-related hazard), or prohibit (road closed 

because of a weather event). Providing drivers with weather-related warnings and prohibitions 

via DIIs and DVI is appropriate. Examples of these types of information include hazardous 

conditions (e.g., high winds) and road closures. Presentation using both DII and DVI allows 

redundancy. The DVI can provide additional information in a more salient fashion (e.g., 

multimodal messages that combine visual notifications with auditory and/or haptic presentation);

however, the message should adhere to existing design principles that stress the importance of 

keeping drivers’ visual attention directed to the road.

(14)

Advisory and Regulatory Speeds: Presenting advisory and regulatory speeds related to adverse 

weather conditions is an appropriate use of a DII.

(7) DIIs, especially those that resemble existing 

regulatory signs and devices, may be viewed as regulatory devices by drivers.

(23) Presenting both 

advisory and regulatory information via DII (such as a CMS) can assist drivers in maintaining an 

appropriate speed in challenging conditions.

(103) DII presentation of both advisory and regulatory 

speeds has demonstrated effectiveness.

(104,105) Results from investigations of speed advisory 

systems have demonstrated safety benefits from DVI presentation of speed messages.(106) These 

results suggest that DVI presentation of weather-related speed notices may be effective. 

However, little research has examined this specific use of a speed advisory system. Regardless of 

presentation method, any information presented via DVI that is not vehicle-specific should be 

reflected on the DII as well.

Design Issues

One important aspect of road weather messaging in the CV context is the congruence between 

DVI and DII messages. Although both systems may provide information from the same RSE 

sensors, DIIs are only available at fixed locations, while DVIs are available at any driver 

location. DVIs may provide targeted messages, while DIIs typically present messages suitable 

for all road users. In addition, there may be an update delay because of the weather event 

message verification cycle.

(18) When coupled with the fixed location of the DII, it is possible for 

information presented via DVI to reach the driver before information presented on a DII. This 

incongruence in timing could lead to driver confusion, especially if the messages give distances 

that reference the driver’s location or trajectory (e.g., flooding in 4 mi, 20 min delay).

This issue has been considered for providing travel time information via DII, with the ultimate 

recommendation that information that is highly susceptible to change or that is calculated from 

rapidly changing conditions should provide either a timestamp or a range that captures the 
59

potential variation in conditions.(34) As a new concept, drivers’ tolerance for relatively small 

timing incongruencies between DII and DVI is unknown. Providing targeted messaging to 

vehicles or providing a measure of the certainty of the information on either DII or DVI may 

eliminate this concern
60

CHAPTER 4. SUMMARY AND CONCLUSIONS 

This report is part of the HFCV research program, whose goal is to minimize driver workload by 

eliminating CV device-related distractions. The research described in this document is part of an 

effort to develop initial design guidance for V2I safety messages provided using DIIs and DVIs. 

Emerging CV technologies and applications have the potential to significantly improve safety on 

U.S. roads; however, it will take many years for CV technology to reach sufficient penetration 

levels in the US vehicle fleet for those benefits to be realized. In the meantime, DII displays can 

effectively communicate safety messages even for vehicles that do not have integrated CV 

applications. Thus, DII-based approaches provide an important transition to the CV future, in 

addition to providing additional countermeasure options to address local safety problems. The 

current report provides a starting point for roadway engineers and other DOT personnel 

considering developing and implementing infrastructure-based safety measures that include DII 

components.

Although there is sufficient general and application-specific research related to using DII 

displays to communicate safety messages to drivers, this area of investigation is still in an early 

stage. The current effort drew upon on the best available research information to provide initial 

design guidance for developing effective DIIs. Existing HFCV research, in addition to previous 

research from related domains, was used to develop this preliminary design guidance. A total of 

12 guidelines were developed using this approach.

A clear finding throughout the report was that although there was only a limited body of research 

covering specific CV applications, a substantial amount of research has been conducted on 

intelligent infrastructure-based safety applications in general. Therefore, for most of the 

guideline topics, there was sufficient existing research to develop useful design guidance, 

typically by adapting research findings obtained from non-CV safety systems that had key 

operational characteristics in common with the relevant CV applications. This approach was 

particularly effective for the general design guidance topics presented in the first part of the 

document (topics 1 through 7). 

The application-specific guidelines (topics 8 through 12) also contain more specific design 

information that relates to individual applications; however, these guidelines should generally be 

considered preliminary design references at the time of this publication. While every attempt was 

made to use the most recent information about how these systems are intended to work, their full 

operational specifications are still being developed. Also, it should be noted that the basic 

strategy for guideline development in these topics employed an opportunistic approach with 

regards to the specific type of guidance provided in each topic. That is, the content for each topic 

was largely determined by focusing on the best available research information. This enabled the 

inclusion of at least some design information on each topic, and for all the key systems; however, 

this information was not as uniform across topics. It would be valuable to identify key 

information gaps that currently exist across the topics, so that they could be addressed in future 

research. Moreover, real world implementation and evaluation information about these 

technologies is largely missing.
61

Consequently, an important avenue for future research is to investigate driver behavior and 

responses to infrastructure-based CV applications under more realistic driving conditions. This 

research would be particularly relevant if DII messages were examined in combination with 

other in-vehicle safety information, since this will likely represent future operating environment 

in a CV world. These safety systems must strike a fine balance in terms of augmenting drivers’

understanding of their situation with relevant safety information, while at the same time avoiding 

annoying or distracting drivers with information that is unneeded, presented at the wrong time, or 

in conflict with other more important safety information. Examining how drivers manage 

infrastructure-based safety information in the context of the full range of on-road driving tasks, 

and the priorities that drivers assign to them, represents an important step for ensuring that these 

systems are developed and implemented in a way that maximizes the benefits to drivers.

In the meantime, the current document provides some of the most comprehensive and detailed 

design guidance for infrastructure-based CV systems. While individual guidelines discuss key 

design issues and the associated Human Factors considerations, they also serve as a useful 

starting point for users to find relevant research sources that provide more detailed information 

about specific topics or design issues. Importantly, the guidelines presented in this document 

compliment other Human Factors design references and address key information needs. In 

particular, the NCHRP Report 600: Human Factors Guidelines for Road Systems provides 

human factors design guidance for infrastructure, but it does not address connected infrastructure 

elements.(11) Similarly, the Human Factors for Connected Vehicles Driver-Vehicle Interface 

Design Principles focuses exclusively on in-vehicle safety applications, and do not cover 

infrastructure-based systems in detail. Thus, the current guideline document provides useful and 

complementary information to other recent State transportation department design documents.

63

REFERENCES

1. SAE J2735. (2009). Dedicated short range communications (DSRC) message set 

dictionary, Warrendale, PA: SAE International.

2. United States Department of Transportation. (2014). Connected Vehicle Applications: 

Vehicle-to-Infrastructure (V2I) Communications for Safety, retrieved from 

http://www.its.dot.gov/safety/v2i_comm_safety.htm.

3. Green, P. (2002). “Motor Vehicle Driver Interfaces,” in J.A. Jacko and A. Sears (Eds.), 

The Human-Computer Interaction Handbook, Hillsdale, NJ: Erlbaum, 844–860.

4. Jackels, J. (2010). Cooperative Intersection Collision Avoidance System: Stop Sign Assist 

Field Operational Test, Presentation at the National RITS Conference, retrieved from: 

www.nritsconference.org/downloads/Presentations10/D2_Jackels.pdf.

5. Misener, J. Chan, C.Y., Dickey, S., Kim, Z.W., Kuhn, T., Lian, T., . . . Zhou, K. (2010). 

Cooperative Intersection Collision Avoidance System (CICAS): Signalized Left Turn Assist 

and Traffic Signal Adaptation (No. UCB-ITS-PRR-2010-20), retrieved from the University 

of California, Berkeley PATH Program Web site at 

http://www.path.berkeley.edu/PATH/Publications/PDF/PRR/2010/PRR-2010-20.pdf.

6. Neale, V.L., Perez, M.A., Doerzaph, Z.R., Lee, S.E., Stone, S., and Dingus, T.A. (2006). 

Intersection Decision Support: Evaluation of a Violation Warning System to Mitigate 

Straight Crossing Path Collisions (Report No. FHWA/VRTC 06-CR10), Charlottesville, 

VA: Virginia Transportation Research Council.

7. Federal Highway Administration. (2012). Manual on Uniform Traffic Control Devices,

Washington, DC.

8. Bertini, R.L., Monsere, C.M., Nolan, C., Bosa, P., and El-Seoud, T.A. (2006). Field 

Evaluation of the Myrtle Creek Advanced Curve Warning System (SPR 352; FHWA-ORRD-06-13), Retrieved from 

http://www.oregon.gov/ODOT/HWY/ITS/pdfs/benefitsofits/myrtle_creek_report_publish.

pdf.

9. Lichty, M.G., Richard, C.M., Campbell, J.L., and Bacon, L.P. (2012). Guidelines for 

Disseminating Road Weather Advisory & Control Information (Report No. FHWA-JPO12-046), Washington, DC: Federal Highway Administration.

10. Divekar, G., Richard, C.M., Jackson, J.E. (in press). Multiple Sources of Safety 

Information from V2V and V2I: Redundancy, Decision-making, and Trust, Washington, 

DC: National Highway Traffic Safety Administration.

11. Campbell, J.L., Lichty, M.G., Brown, J.L., Richard, C.M., Graving, J., Graham, J., 

O’Laughlin, M., ... Harwood, D. (2012). NCHRP Report 600: Human Factors Guidelines 

for Road System, Second Edition, Washington, DC: Transportation Research Board.
64

12. Campbell, J.L., Richard, C.M., and Graham, J. (2008). Human Factors Guidelines for 

Road System. Collection A: Chapters 4, 5, 10, 11, 13, 22, 23, 26 (NCHRP Report 600A),

Washington, DC: Transportation Research Board.

13. Campbell, J.L., Brown, J.L., Richard, C.M., and Graham, J. (2008). Human Factors 

Guidelines for Road Systems. Collection B: Chapters 6, 22 (Tutorial 3), 23 (Updated) 

(NCHRP Report 600B), Washington, DC: Transportation Research Board.

14. Campbell, J.L., Richard, C.M., Brown, J.L., and McCallum, M. (2007). Crash Warning 

System Interfaces: Human Factors Insights and Lessons Learned (Report No. DOT HS 

810 697), Washington, DC: National Highway Traffic Safety Administration.

15. Doctor, M., Merritt, G., and Moler, S. (2009). “Designing Complex Interchanges,” Public 

Roads, 73(3), 3–11.

16. Lerner, N.D., Llaneras, R.E., McGee, H.W., and Alexander, G. (2003). NCHRP Report 

488: Additional Investigations on Driver Information Overload, Washington, DC: 

Transportation Research Board.

17. Gugerty, L., McIntyre, S.E., Link, D., Zimmerman, K., Tolani, D., Huang, P., and

Pokorny, R.A. (2014). “Effects of Intelligent Advanced Warnings on Drivers Negotiating 

the Dilemma Zone,” Human Factors, 56(6), 1,021–1,035.

18. Stephens, D.R., Timcho, T.J., Young, E., Klein, R.A., and Schroeder, J.L. (2014). 

Accelerated Vehicle-to-Infrastructure (V2I) Safety Applications Concept of Operations 

Document (Technical Report), Columbus, OH: Battelle Memorial Institute.

19. Becic, E., Manser, M.P., Creaser, J., and Donath, M. (2012). Cooperative Intersection 

Collision Avoidance System—Stop Sign Assist: Experiments to Validate the Use of an InVehicle Interface Design (Report No. CTS 12-09), Minneapolis, MN: University of 

Minnesota Department of Engineering.

20. Saad, F. (2002). “Ergonomics of the Driver’s Interface with the Road Environment: The 

Contribution of Psychological Research,” In R. Fuller and J.A. Santos (Eds.), Human 

Factors for Highway Engineers, Oxford, UK: Elsevier, 23–41.

21. Lees, M.N. and Lee, J.D. (2007). “The Influence of Distraction and Driving Context on 

Driver Response to Imperfect Collision Warning Systems,” Ergonomics, 50, 1,264–1,286.

22. Campbell, J.L., Brown, J.L., Graving, J.S., Richard, C.M., Lichty, M.G., Sanquist, T., 

Bacon, L.P., ... Morgan, J.F. (in press). Human Factors Design Principles for the Driver 

Vehicle Interface (DVI), Version 1.2, Washington, DC: National Highway Traffic Safety 

Administration.

23. Nowakowski, C., Cody, D., and O’Connell, J. (2008). “Comparison of Infrastructure and 

In-Vehicle Driver Interfaces for Left-Turn Warnings,” Transportation Research Record

2069, Transportation Research Board, Washington, DC, 33–40.
65

24. Bliss, J.P. and Acton, S.A. (2003). “Alarm Mistrust in Automobiles: How Collision Alarm 

Reliability Affects Driving,” Applied Ergonomics, 34, 499–509.

25. McIntyre, S., Link, D., Zimmerman, K., Tolani, D., Huang, P., and Pokorny, R. (2012). 

“Lane Specific Dilemma Zone Warnings at Signalized Intersections,” Proceedings of the 

Human Factors & Ergonomics Society, 56, 2,221–2,225.

26. Richard, C.M., Campbell, J.L., and Brown, J.L. (2008). “A Procedure for Developing 

Warning Message Prioritization Rules in Integrated Collision Warning Systems for Heavy 

Trucks,” Proceedings of the Human Factors & Ergonomics Society Annual Meeting, 52, 

1860–1864.

27. Doerzaph, Z., Sullivan, J., Bowman, D., and Angell, L. (2013). Connected Vehicle 

Integration Research and Design Guidelines Development: Integration Architecture, Task 

5, Report to the National Highway Traffic Safety Administration, Blacksburg, VA: 

Virginia Tech Transportation Institute. 

28. Inman, V.W. and Davis, G.W. (2010). “Effects of In-Vehicle and Infrastructure-Based 

Collision Warnings to Nonviolating Drivers at Signalized Intersections,” Transportation 

Research Record 2189, Transportation Research Board, Washington, DC, 17–25.

29. Ng, A.W.Y. and Chan, A.H.S. (2007). “The Guessability of Traffic Signs: Effects of 

Prospective-User Factors and Sign Design Features,” Accident Analysis & Prevention, 39, 

1,245–1,257.

30. Shinar, D., Dewar, R.E., Summala, H., and Zakowska, L. (2003). “Traffic Sign Symbol 

Comprehension: A Cross-Cultural Study,” Ergonomics, 46(15), 1,549–1,565.

31. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. 

(Report No. FHWA-OP-03-070), Washington, DC: Federal Highway Administration.

32. Ben-Bassat, T. and Shinar, D. (2006). “Ergonomic Guidelines for Traffic Sign Design 

Increase Sign Comprehension,” Human Factors, 48(1), 182–195.

33. Campbell, J.L., Richman, J.B., Carney, C., and Lee, J.D. (2004). In-Vehicle Display Icons 

and Other Information Elements, Volume I: Guidelines (Report No. FHWA-RD-03-065),

Washington, DC: Federal Highway Administration, retrieved from 

http://www.fhwa.dot.gov/publications/research/safety/03065/index.cfm.

34. Dudek, C.L. and Ullman, G.L. (2006). Dynamic Message Sign Message Design and 

Display Manual (Report No. FHWA/TX-04/0-4023-P3), Washington, DC: Federal 

Highway Administration.

35. Shinar, D. and Vogelzang, M. (2013). “Comprehension of Traffic Signs with Symbolic 

Versus Text Displays,” Transportation Research Part F: Traffic Psychology and 

Behaviour, 18, 72–82.
66

36. Dewar, R.E. and Ells, J.G. (1974). “A Comparison of Three Methods for Evaluating 

Traffic Signs,” Transportation Research Record, 503, Transportation Research Board, 

Washington, DC, 38–47.

37. Ells, J.G. and Dewar, R.E. (1979). “Rapid Comprehension of Verbal and Symbolic Traffic 

Sign Messages,” Human Factors 21(2), 161–168.

38. Hancock, P.A., Billings, D.R., Schaefer, K.E., Chen, J.Y.C., de Visser, E.J., and 

Parasuraman, R. (2011). “A Meta-Analysis of Factors Affecting Trust in Human-Robot 

Interaction,” Human Factors, 53, 517–527.

39. Lee, J.D. and See, K.A. (2004). “Trust in Automation: Designing for Appropriate 

Reliance.” Human Factors, 46, 50–80.

40. Muir, B.M. and Moray, N. (1996). “Trust in Automation. Part II. Experimental Studies of 

Trust and Human Intervention in a Process Control Simulation,” Ergonomics, 39, 429–

460.

41. Lee, J.D. and Moray, N. (1994). “Trust, Self-Confidence, and Operators’ Adaptation to 

Automation,” International Journal of Human Computer Studies, 40, 153–184.

42. Kantowitz, B.H., Hanowski, R.J., and Kantowitz, S.C. (1997). “Driver Acceptance of 

Unreliable Traffic Information in Familiar and Unfamiliar Settings,” Human Factors, 39, 

164–176.

43. Gordon, K.M., Anderson, S.H., Gribble, B., and Johnson, M. (2001). Evaluation of the 

FLASH (Flashing Light Animal Sensing Host) System in Nugget Canyon, Wyoming

(Report No FHWA-WY-01/03F), Washington, DC: Federal Highway Administration. 

44. Higgins, L., Balke, K., and Chrysler, S.T. (2012). “Driver Responses to Signing 

Treatments for Flooded Roads,” Transportation Research Record 2321, Transportation 

Research Board, Washington, DC, 98–107.

45. Parasuraman, R., Hancock, P.A., and Olofinboba, O. (1997). “Alarm Effectiveness in 

Driver-Centered Collision-Warning Systems,” Ergonomics, 40, 390–399.

46. Parasuraman, R. and Miller, C.A. (2004). “Trust and Etiquette in High-Criticality 

Automated Systems,” Communications of the ACM, 47(4), 51–55.

47. Weinstock A., Oron-Gilad T., and Parmet Y. (2012). “The Effect of System Aesthetics on 

Trust, Cooperation, Satisfaction and Annoyance in an Imperfect Automated System,”

Work, 41(1), 258–265.

48. Madhavan, P. and Wiegmann, D.A. (2007). “Effects of Information Source, Pedigree, and 

Reliability on Operator Interaction with Decision Support Systems,” Human Factors, 49, 

773–785. 
67

49. Creaser, J., Manser, M., Rakauskas, M., and Donath, M. (2010). Sign Comprehension, 

Considering Rotation and Location, Using Random Gap Simulation for Cooperative 

Intersection Collision Avoidance System—Stop Sign Assist: CICAS-SSA Report #4 (Report 

No. CTS 10-34), Minneapolis, MN: University of Minnesota Human FIRST Program. 

50. Ragland, D.R., Arroyo, S., Shladover, S.E., Misener, J.A., and Chan, C. (2006). “Gap 

Acceptance for Vehicles Turning Left Across On-Coming Traffic: Implications for 

Intersection Decision Support Design (Paper 06-2696),” Transportation Research Board 

85th Annual Meeting Compendium of Papers (CD ROM]).

51. Devarasetty, P.C., Zhang, Y., and Fitzpatrick, K. (2012). “Differentiating Between LeftTurn Gap and Lag Acceptance at Unsignalized Intersections as a Function of Site 

Characteristics,” Journal of Transportation Engineering, 138(5), 580–588.

52. Gorjestani, A., Menon, A., Cheng, P.M., Newstrom, B., Shankwitz, C., and Donath, M. 

(2010). Macroscopic Review of Driver Gap Acceptance and Rejection Behavior at Rural 

Thru-Stop Intersections in the U.S.—Data Collection Results in Eight States: CICAS-SSA 

Report #3, Minneapolis, MN: University of Minnesota Intelligent Vehicles Laboratory.

53. Radwan A.E. and Sinha, K.C. (1980). “Gap Acceptance and Delay at Stop Controlled 

Intersections on Multi-Lane Divided Highways,” ITE Journal, 50(3), 38–44.

54. Zhody, I., Sadek, S., and Rakha, H.A. (2010). “Empirical Analysis of Effects of Wait Time 

and Rain Intensity on Driver Left-Turn Gap Acceptance,” Transportation Research Record

2173, Transportation Research Board, Washington, DC, 1–10.

55. Hamed, M., Easa, S., and Batayneh, R. (1997). “Disaggregate Gap-Acceptance Model for 

Unsignalized T-Intersections,” Journal of Transportation Engineering, 123(1), 36–42.

56. Tupper, S., Knodler, M., and Hurwitz, D. (2011). Connecting Gap Acceptance Behavior 

with Crash Experience, paper presented at the 3d International Conference on Road Safety 

and Simulation Conference Compendium, retrieved from 

http://onlinepubs.trb.org/onlinepubs/conferences/2011/RSS/1/Tupper,S.pdf.

57. Yan, X. and Radwan, E. (2008). “Influence of Restricted Sight Distance on Permitted LeftTurn Operation at Signalized Intersections,” Journal of Transportation Engineering, 

134(2), 68–76.

58. Kittleson, W.K. and Vandehey, M.A. (1991). “Delay Effects on Driver Gap Acceptance 

Characteristics and Two-Way Stop Controlled Intersections,” Transportation Research 

Record 1320, Transportation Research Board, Washington, DC, 160–167.

59. Gerlough, D.L. and Huber, M.J. (1976). Traffic Flow Theory—A Monograph (Special 

Report 165), Washington, DC: Transportation Research Board.

60. Ashalatha, R. and Chandra, S. (2011). “Critical Gap through Clearing Behavior of Drivers 

at Unsignalized Intersections,” KSCE Journal of Civil Engineering, 15(8), 1,427–1,434.
68

61. Pollatschek, M.A., Polus, A., and Livneh, M. (2002). “A Decision Model for Gap 

Acceptance and Capacity at Intersections,” Transportation Research Part B, 36, 649–663.

62. Polus, A., Craus, J., and Reshetnik, I. (1996). “Non-Stationary Gap Acceptance Assuming 

Drivers’ Learning and Impatience,” Traffic Engineering & Control, 37(6), 395–402.

63. Brilon, W., Koenig, R., and Troutbeck, R.J.(1999). “Useful Estimation Procedures for 

Critical Gaps,” Transportation Research Part A, 33(3), 161–186.

64. Yan, X., Radwan, E., and Gou, D. (2007). “Effects of Major-Road Vehicle Speed and 

Driver Age and Gender on Left-Turn Gap Acceptance,” Accident Analysis and Prevention, 

39(4), 843–852.

65. Lerner, N., Huey, R.W., McGee, H.W., and Sullivan, A. (1995). Older Driver PerceptionReaction Time for Intersection Sight Distance and Object Detection (Report No. FHWARD-93-168), Washington, DC: Federal Highway Administration.

66. Harwood, D.W., Mason, J.M., and Brydia, R.E. (1999). “Design Policies for Sight 

Distance at Stop-Controlled Intersections Based on Gap Acceptance,” Transportation 

Research Part A, 33, 199–216.

67. Hanscom, F.R. (2001). Evaluation of the Prince William County Collision 

Countermeasure System (Report No. FHWA/VTRC 01-CR5). Washington, DC: Federal 

Highway Administration, retrieved from 

http://www.virginiadot.org/vtrc/main/online_reports/pdf/01-cr5.pdf.

68. Lyles, R.W. (1980). “Evaluation of Signs for Hazardous Rural Intersections.”

Transportation Research Record 782, Transportation Research Board, Washington, DC, 

22–30.

69. Rakauskas, M., Creaser, J., Manser, M., Graving, J., and Donath, M. (2009). Validation 

Study—On-Road Evaluation of the Stop Sign Assist Decision Support Sign: CICAS-SSA 

Report #5, Minneapolis, MN: University of Minnesota Human FIRST Program.

70. Golembiewski, G.A. and Chandler, B. (2011). Intersection Safety: A Manual for Local 

Rural Road Owners (Report No. FHWA-SA-11-08), Washington, DC: Federal Highway 

Administration, retrieved from 

http://safety.fhwa.dot.gov/local_rural/training/fhwasa1108/index.cfm.

71. Hayes, C. and Drew, D. (2011). In-vehicle Decision Support to Reduce Crashes at Rural 

Thru-Stop Intersections, Minneapolis, MN: University of Minnesota Intelligent 

Transportation Systems Institute Center for Transportation Studies.

72. Laberge, J.C., Creaser, J.I., Rakauskas, M.E., and Ward, N.J. (2006). “Design of an 

Intersection Decision Support (IDS) Interface to Reduce Crashes at Rural Stop-Controlled 

Intersections,” Transportation Research Part C, 14, 39–56.
69

73. Maze, T.H., Hochstein, J.L., Souleyrette, R.R., Preston, H., and Storm, R. (2010). NCHRP 

Report 650: Median Intersection Design for Rural High-Speed Divided Highways,

Washington, DC: Transportation Research Board.

74. Castro, C., Moreno-Rios, S., Tornay, F., and Vargas, C. (2008). “Mental Representations 

of Obligatory and Prohibitory Traffic Signs,” Acta Psychologia, 129, 8–17.

75. Roka, J., Castro, C., Bueno, M., and Moreno-Rios, S. (2012). “A Driving-Emulation Task 

to Study the Integration of Goals with Obligatory and Prohibitory Traffic Signs,” Applied 

Ergonomics, 43, 81–88.

76. Creaser, J., Rakauskas, M., Ward, N., and Laberge, J. (2007). A Simulator-Based 

Evaluation of Smart Infrastructure Concepts for Intersection Decision Support for Rural 

Thru-STOP Intersections (Report No. MN/RC-2007-31), St. Paul, MN: Minnesota 

Department of Transportation.

77. Knodler, M.A. and Noyce, D.A. (2005). “Tracking Driver Eye Movements at Permissive 

Left-Turns,” Proceedings of the Third International Driving Symposium on Human 

Factors in Driver Assessment, Training and Vehicle Design, 134–142.

78. White, B. and Eccles, K. A. (2002). “Inexpensive, Infrastructure-Based, Intersection 

Collision-Avoidance System to Prevent Left-Turn Crashes with Opposite-Direction 

Traffic,” Transportation Research Record 1800, Transportation Research Board, 

Washington, DC, 92–99.

79. Eccles, K., Gross, F., Liu, M., and Council, F. (2012). Crash Data Analyses for Vehicle-toInfrastructure Communications for Safety Applications (Report No. FHWA-HRT-11-040),

Washington, DC: Federal Highway Administration. 

80. Rephlo, J.A. (2013). Connected Vehicles for Safety, Mobility, and User Fees: Evaluation 

of the Minnesota Road Fee Test, retrieved from the Minnesota Department of 

Transportation Web site at 

http://www.dot.state.mn.us/mileagebaseduserfee/pdf/EvaluationFinalReport.pdf.

81. Plavšić, M., Bengler, K.J., and Bubb, H. (2010). “Analysis of Glance Movements in 

Critical Intersection Scenario,” In Proceedings of the 3d International Conference on 

Applied Human Factors and Ergonomics, Miami, FL.

82. Hendricks, D.L., Fell, J.C., and Freedman, M. (1999). The Relative Frequency of Unsafe 

Driving Acts in Serious Traffic Crashes (Report No. DTNH22-94-C-05020), Washington, 

DC: National Highway Traffic Safety Administration.

83. Institute of Transportation Engineers. (2003). Making Intersections Safer: A Toolbox of 

Engineering Countermeasures to Reduce Red-Light Running (Publication No. IR-115),

Washington, DC.

84. Antonucci, N.D., Hardy, K.K., Slack, K.L., Pfefer, R., and Neuman, T.R. (2004). NCHRP 

Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan; 
70

Volume 12: A Guide for Reducing Collisions at Signalized Intersections, Washington, DC: 

Transportation Research Board. 

85. Ferlis, R.A. (2001). “Infrastructure Systems for Intersection Collision Avoidance,”

Proceedings of the International Driving Symposium on Human Factors in Driver 

Assessment, Training, and Vehicle Design, 1, 378–382.

86. Inman, V.W., Davis, G.W., El-Shawarby, I., and Rakha, H. (2008). Test Track and Driving 

Simulator Evaluations of Warnings to Prevent Right-Angle Crashes at Signalized 

Intersections (Report No. FHWA-HRT-08-070), Washington, DC: Federal Highway 

Administration. 

87. Tydlacka, J., Voight, A.P., and Langford III, W.C. (2010). “Operational and Crash 

Evaluation of Lighted Pavement Marker Stop Bars and LED Outlined Traffic Signal 

Backplates,” Proceedings of the ITS America 20th Annual Meeting and Exposition (CDROM).

88. Carson, J.L., Tydlacka, J., Gray, L.S., and Voight, A.P. (2008). NCHRP Synthesis 380: 

Applications of Illuminated, Active, In-Pavement Marker Systems, Washington, DC: 

Transportation Research Board.

89. American Association of State Highway and Transportation Officials. (2011). A Policy on 

Geometric Design of Highways and Streets (6th Ed.), Washington, DC.

90. Gates, T.J., Noyce, D.A., and Laracuente, L. (2007). “Analysis of Dilemma Zone Driver 

Behavior at Signalized Intersections (Paper 07-3351),” Transportation Research Board 

Annual Meeting Compendium of Papers (CD-ROM).

91. Kiefer, R.J., LeBlanc, D., Palmer, M., Salinger, J., Deering, R., and Shulman, M. (1999). 

Development and Validation of Functional Definitions and Evaluation Procedures for 

Collision Warning/Avoidance Systems (Report No. DOT HS 808 964), Washington, DC: 

National Highway Traffic Safety Administration.

92. Inman, V.W. and Davis, G.W. (2009). The Effects of In-Vehicle and Infrastructure-Based 

Collision Warnings at Signalized Intersections (Report No. FHWA-HRT-09-049),

Washington, DC: Federal Highway Administration.

93. McKnight, A.J. and Bahouth, G.T. (2008). “Analysis of Large Truck Rollover Crashes,”

Annals of Advances in Automotive Medicine, 52, 281–288.

94. Chowhurdy, M.A., Warren, D.L., Bissell, H., and Taori, S. (1998). “Are the Criteria for 

Setting Advisory Speeds on Curves Still Relevant?” ITE Journal, 68(2), 32–45.

95. Winnett, M.A. and Wheeler, A.H. (2002). Vehicle-Activated Signs: A Large Scale 

Evaluation (Report No. TRL548), Wokingham, UK: Transportation Research Laboratory.

96. Pitale, J.T. and Shankwitz, C. (2009). Benefit:Cost Analysis of In-Vehicle Technologies 

and Infrastructure Modifications as a Means to Prevent Crashes Along Curves and 
71

Shoulders (Report No. MN/RC 2009-39), St. Paul, MN: Minnesota Department of 

Transportation.

97. Torbic, D.J., Harwood, D.W., Gilmore, D.K., Pfefer, R., Neuman, T.R., Slack, K.L., and

Hardy, K.K. (2004). NCHRP Report 500: Guidance for Implementation of the AASHTO 

Strategic Highway Safety Plan Volume 7: A Guide for Reducing Collisions on Horizontal 

Curves, Washington, DC: Transportation Research Board.

98. Janson, B.N. (1999). Evaluation of Downhill Truck Speed Warning System on I-70 West of 

Eisenhower Tunnel, Denver, CO: Colorado Department of Transportation.

99. Appelt, V. (2000). “New Approaches to the Assessment of the Spatial Alignment of Rural 

Roads—Apparent Radii and Visual Distortion,” Proceedings of the 2d International 

Symposium on Highway Geometric Design. Cologne, Germany: Verlag, 620–631.

100. Rämä, P. and Kulmala, R. (2000). “Effects of Variable Message Signs for Slippery Road 

Conditions on Driving Speed and Headways,” Transportation Research Part F, 3, 85–94.

101. Luoma, J., Rämä, P., Penttinen, M., and Anttila, V. (2000). “Effects of Variable Message 

Signs for Slippery Road Conditions on Reported Driver Behaviour,” Transportation 

Research Part F, 3, 75–84.

102. Drobot, S., Mahoney, W.P., Schuler, E., Wiener, G., Chapman, M., Pisano, P.A., Kennedy, 

P.... Stern, A.D. (2009). “Intellidrive Road Weather Research and Development: The 

Vehicle Data Translator,” Proceedings of the Annual Meeting of ITS America, retrieved 

from

http://ops.fhwa.dot.gov/weather/resources/publications/itsapaper9005/itsapaper900.pdf.

103. Garber, N.J. and Srinivasan, S. (1998). Effectiveness of Changeable Message Signs in 

Controlling Vehicle Speeds in Work Zones: Phase II Final Report. (Report No. VTRC 98-

R10), Charlottesville, VA: Virginia Transportation Research Council. 

104. Goodwin, L. (2003). Best Practices for Road Weather Management, Version 2.0 (Report 

No. FHWA-OP-03-081), Washington, DC: Federal Highway Administration. 

105. Al-Kaisy, A., Ewan, L., and Veneziano, D. (2012). Evaluation of a Variable Speed Limit 

System for Wet and Extreme Weather Conditions, Phase 1 Report (Report No. SPR 743),

Salem, OR: Oregon Department of Transportation. 

106. Paine, M., Paine, D., Griffiths, M., and Germanos, G. (2007). “In-Vehicle Intelligent 

Speed Advisory Systems (Paper No. 07-0247),” Proceedings of the 20th International 

Conference on the Enhanced Safety of Vehicles, Lyon, France. 
HRDS-30/11-15(WEB)E