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Intersection Safety Issue Briefs

Issue Brief 12

Human Factors Issues in Intersection Safety

November 2009

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Research indicates that driver error may be involved in 90 percent of all crashes.While advances in automotive safety and highway design continue to improve, the one component that engineers and designers have very little control over is the driver. Understanding how drivers and all roadway users interact within an intersection environment is fundamental to improving roadway safety and saving lives.

The Driving Task

The overall driving task consists of many smaller tasks that a driver must perform at the same time. The three major subtasks include control, guidance, and navigation, as shown in Figure 1. Control relates to the physical operation of a vehicle. Guidance refers to interacting with other vehicles, for example following, passing, merging, and other actions such as following traffic control devices. Navigation refers to the driver choosing a route from origin to destination by reading guide signs and using landmarks. When a driver is overloaded with information, the driver actively sheds the information load by ignoring the navigational level in order to maintain the physical control of the vehicle and keep from colliding with another vehicle or other hazard when negotiating an intersection.

Figure 1. Levels of the Driving Task
The three major driving subtasks include control, guidance, and navigation.
Source: Lunenfeld, H. and Alexander, G.J. A User's Guide to Positive Guidance, 3rd Edition. Washington, DC: FHWA, 1990.

Driver Attention and Decision Making

Negotiating intersections is one of the most complex and demanding tasks a driver faces. To successfully execute a vehicle maneuver through an intersection, the driver must receive and recognize available information, make a decision, and execute the desired action. One limitation is that humans are serial processors, and the cognitive task load at intersections can be quite large. Common items a driver must consider when approaching an intersection include the following:

Given the short time drivers have to process a large amount of information, it is imperative that designers and engineers provide clear and accurate information to drivers to help them navigate an intersection. The ability for a driver to successfully accomplish that task is greatly impacted by their expectancies. When expectancies are reinforced, drivers respond quickly and correctly and in predictable ways. However, if they encounter unfamiliar situations when approaching an intersection (e.g., missing or obscured traffic control devices, poor sight distance, unfamiliar lane assignments, lack of advance notice of the intersection, unusual intersection alignment), their responses may take longer to initiate and errors may result. Thus, it is essential to design consistency into intersections to minimize the likelihood of errors.

Vision is the most important information reception characteristic of drivers. Features of human vision are tied to specific roadway design elements as illustrated in Table 1.

Table 1: Human Vision Characteristics as Related to Roadway Elements
Visual Characteristics Related Roadway Element(s)
Visual Acuity. Ability to see small details clearly. Sign size; reading distant traffic signs
Contrast Sensitivity. Seeing objects that are similar in brightness to their background. Pavement markings and delineation; detection of dark clothed pedestrians at night
Color Vision. Discrimination of different colors. Sign, signal, and pavement marking design and retroreflectivity
Visual Field/Peripheral Vision Sign placement, signal placement; seeing a bicycle approaching from the left; seeing pedestrians at the intersection
Scan Patterns Sign and signal placement, delineation treatments
Motion Judgment/Angular Movement. Seeing objects moving across the field of view. School zones and other crosswalks, highway railroad crossings; Judging the speed of cars crossing our path of travel
Movement in Depth. Detecting changes in visual image size. Judging the speed of an approaching vehicle
Visual Illusions Guide signs, pavement markings
Depth Perception. Judgment of the distance of objects. Passing on two-lane roads with oncoming traffic
Eye Movement. Changing the direction of gaze. Scanning the road environment for hazards
Glare Sensitivity. Ability to resist and recover from the effects of glare. Reduction in visual performance due to headlight glare

Driver Error

Perceptual failures account for a large portion of driver errors. These can include such items as "looked but did not see," visual obstructions and clutter, reduced visibility due to environmental factors, poor judgment of speed and/or distance, and low conspicuity of target. Also, drivers are estimated to engage in potentially distracting secondary tasks nearly 30 percent of the time their vehicles are moving. These distractions (e.g., conversation with passengers, eating, cell phone use, texting, and so forth), increase the likelihood of driver error. According to a 2006 study by the National Highway Traffic Safety Administration: "Almost 80 percent of all crashes and 65 percent of all near-crashes involved the driver looking away from the forward roadway just prior to the onset of the conflict.1 Prior estimates related to 'distraction' as a contributing factor have been in the range of 25 percent." Misinterpretation of information and driver impairment are also major contributing factors to driver error. Additionally, intersections themselves present their own unique set of driver errors, depending on the type of intersection at hand.

Signalized Intersections

Common driver errors include the following:

Unsignalized Intersections

Common driver errors include the following:


Common driver errors include the following:

Design Considerations

Design policy implicitly incorporates principles of human factors. The American Association of State Highway and Transportation Officials (AASHTO) Green Book design criteria assume that the "design driver" is alert and in control of physical and mental abilities, has a reasonable ability to see and perceive the roadway environment, and has reasonable motor skills to enable steering, braking, and other operations. Table 2 summarizes key human factor considerations and their relationship to design elements.

Table 2: Human Factors and Their Relationship to Roadway Design Elements
Human Factor Design Value Design Element Affected
Perception-reaction time 1.0-2.5 sec. Stopping Sight Distance
Deceleration rate 11.2 ft./sec.2 Stopping Sight Distance
Pre-maneuver. Distance for driver to detect an unexpected condition 3.0–9.1 sec. Decision Sight Distances
Gap acceptance.
Turning left or right from stop.
Crossing from stop
7.5 sec. (left turn)
6.5 sec. (right turn)
Minimum Stopping Sight Distance
Driver height of eye 1,080 mm Stopping Sight Distance
Pedestrian walk times 3.0–4.5 ft./sec. Pedestrian Facilities

Engineering Solutions

Humans are not perfect when making decisions, and some errors in judgment are inevitable. However, steps can be taken to help reduce the likelihood that driver errors will take place. Therefore, intersection design and features are important and should take the limitations of human performance into account.

Drivers perform best under moderate levels of driver workload, while they make more errors under low- or high-workload environments. To achieve moderate driver workload conditions, apply the two guiding principles of intersection design and operation: Clarify and simplify.

Clarify means that the approaches to intersections are readily visible to the driver as they relate to configuration, lanes, and type of traffic control. Simplify means that the driver workload is at a medium level and never at a high level. This is accomplished by separating the actions of the driver approaching an intersection as follows and as illustrated in Figure 4:

Figure 4. Intersection Layout Illustrating Concepts of Clarify and Simplify
Diagram of the correct design of an intersection, including distances for sign placement.
Image Details

A significant proportion of intersection crashes involve left turns. Older drivers in particular run the greatest risk of being involved in a left-turn crash, due in part to their diminished ability to judge closure rates of oncoming vehicles. Using alternative intersection designs for left-turn lanes can help alleviate this problem. For example, a positive offset design can help improve visibility of oncoming vehicles. Adding protected left-turn phases can also assist drivers in turning movements.

Other elements of the intersection can be added or modified to improve driver performance and reduce the likelihood of errors. Some include the following:

Photo of a crosswalk containing a pedestrian refuge island.
Figure 5: Using pedestrian refuge islands when possible to provide a common and consistent location for drivers to look for pedestrians


National Highway Institute Course: Human Factors for Transportation Engineers. For further information, contact FHWA, Office of Safety, at 202-366-2288. This one-day workshop includes interactive modules on information reception, decision-making, driver responses and human factors principles. Upon completion of the course, participants will be able to perform the following:

Institute of Transportation Engineers

Introduction to Human Factors (Web seminar)

Please contact the Professional Development Division at 202-289-0222 for further information. A summary of the course follows: Provides an introduction to the field of human factors as it relates to human behavior, roadway design, and safety, as well as human characteristics as they relate to the driving task. The Web seminar will review demographic and environmental influences on driver behavior.


American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets, Fourth Edition. Washington, DC: AASHTO, 2004.

Institute of Transportation Engineers. Traffic Engineering Handbook, 5th Edition. Washington, DC: ITE, 1999.

NCHRP Report 500, Volume 5: A Guideline for Addressing Unsignalized Intersection Collisions. Washington, DC: TRB, 2003.

NCHRP Report 500, Volume 9: A Guide for Reducing Collisions Involving Older Drivers. Washington, DC: TRB, 2004.

NCHRP Report 500, Volume 10: A Guide for Reducing Collisions Involving Pedestrians. Washington, DC: TRB, 2004.

NCHRP Report 500, Volume 12: A Guide for Reducing Collisions at Signalized Intersections. Washington, DC: TRB, 2004.

NCHRP Report 500, Volume 18: A Guide for Reducing Collisions Involving Bicycles. Washington, DC: TRB, 2008.

Dewar, R. and Olson, P. Human Factors in Traffic Safety, Second Edition, Lawyers & Judges Publishing, Tucson, AZ, 2007.

Shinar, David. Traffic Safety and Human Behavior, First Edition, Elsevier Science, Amsterdam, 2007.

Highway Safety Manual, First Edition. Washington, DC: TRB, To be published.

Lunenfeld, H. and Alexander, G.J. A User's Guide to Positive Guidance, 3rd Edition. Washington, DC: FHWA, 1990.

NCHRP Report 600A: Human Factors Guidelines for Road Systems, Collection A: Chapters 1, 2, 3, 4, 5, 10, 11, 13, 22, 23, 26. Washington, DC: TRB, 2008.

Driver Distraction: A Review of the Current State-of-Knowledge. Report No. DOT-HS-810-704. Washington, DC: NHTSA, 2008.

NCHRP Report 572: Roundabouts in the United States. Washington, DC: TRB, 2007.

Roundabouts: An Informational Guide. Report No. FHWL-RD-00-67, Washington, DC: 2000.

The 100-Car Naturalistic Driving Study: Phase II – Results of the 100-Car Field Experiment, National Highway Traffic Safety Administration, Report No. DOT-HS-810-593.

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