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Public Roads - January/February 2013

January/February 2013
Issue No:
Vol. 76 No. 4
Publication Number:
Table of Contents

Why Drivers Do What They Do

by C. Y. David Yang, Jim Shurbutt, and Brian Philips

Read on to find out how researchers at FHWA's Human Factors Laboratory are studying motorists' behaviors. The potential dividends are significant.

Located at FHWA's research center in McLean, VA, this driving simulator is a high-fidelity, state-of-the-art research tool that the Human Factors Laboratory uses to generate multiple driving scenarios for evaluation and analysis.

In recent years, national attention toward the issue of driver distraction has increased among the transportation community, the media, and the public. Texting and cell phone use while driving, for example, are major safety concerns due to their role as contributing factors in a growing number of vehicle crashes. But roadway and roadside infrastructure also can contribute to driver distraction. For example, does the placement of changeable message signs at certain locations cause motorists to look away from the road more than they would otherwise? Do certain sign messages cause driver confusion or misunderstanding?

To answer questions like these, the Federal Highway Administration's (FHWA) Human Factors Laboratory, located at the Turner-Fairbank High-way Research Center (TFHRC) in McLean, VA, helps FHWA and its partners examine strategies for enhancing the operation and safety of the Nation's highways. The laboratory has a long history of carrying out research to further understanding of the needs and limitations of transportation users. Based on the research findings, FHWA has developed guidance, for example, to optimize the design and placement of traffic signs at freeway interchanges and intersections to communicate critical information to drivers more effectively.

Understanding the capabilities and limitations of travelers can help engineers design roadways to minimize human errors and enhance the safety of the traveling public. Research on user characteristics can lead to improvements in roadway design, construction, and maintenance that will enable the transportation system to operate more efficiently and safely.

"The Human Factors Laboratory employs multiple research tools to help transportation practitioners evaluate various roadway designs before building them," says FHWA Associate Administrator Michael Trentacoste, Office of Research, Development, and Technology. "For example, State departments of transportation [DOTs] that plan to implement new interchange designs, such as the diverging diamond interchange [also known as the double crossover diamond] and the restricted crossover U-turn, can create virtual interchanges using the lab's modeling software to conduct detailed analyses of these designs and others like them. FHWA and its partners will continue to use this state-of-the-art research facility to carry out important human factors and driver behaviors studies."

Recent improvements in the various tools employed by the Human Factors Laboratory, along with a sample of projects that have employed those tools, illustrate how the lab's research continues to benefit motorists and other users of the transportation network. But first, a quick look at how the Human Factors Laboratory came about.

The Lab's Back Story

To understand the needs and limitations of transportation users, FHWA began conducting human factors and driver behavior studies in the 1960s when the agency was known as the Bureau of Public Roads, a division of the U.S. Department of Commerce. Two researchers with the Bureau of Public Roads, R. M. Michaels and B. W. Stephens, published a journal article in a 1963 issue of the Highway Research Record (predecessor of Transportation Research Record: Journal of the Transportation Research Board) titled "Driver Characteristics, Night Visibility, and Driving Simulation." The article described the use of a simulation experiment at the Bureau of Public Roads' laboratory to study driver performance by tracking activities such as eye movements.

Another example of FHWA's ongoing support of human factors research related to transportation is a 1981 article by D. A. Gordon. This article, published in Public Roads, described several studies that examined the design of roadway signs using human factors principles, such as applying a correct legibility ratio on sign letters to help drivers read and understand these signs from a distance.

In subsequent years, FHWA's human factors team, with support from onsite technical contractors, have performed numerous studies using a variety of research tools to gain better understanding of the capabilities and limitations of drivers in the context of transportation infrastructure and improvements to enhance travel safety.

Driving Simulator

One of the research tools employed at the Human Factors Laboratory is a driving simulator used for a vari--ety of behavioral studies related to safety and operations conducted for FHWA and other stakeholders. The simulator consists of a full automobile chassis surrounded by a semi-circular projection screen. Five projectors render a seamless 240-degree view (motorists' field-of-view) of high-fidelity, computer-generated roadway scenes.

This image from FHWA's driving simulator shows a sample scenario used in a human factors study of the double crossover diamond interchange.

The lab recently upgraded the simulator to enhance the motion base, from 3 to 6 degrees of freedom. This improvement makes the motion and vestibular (the perception of body position and movement) feedback much more realistic for drivers. In addition, the driving simulator has a 120-hertz (Hz) eye-tracking capability (that is, it takes 120 samples per second) so the researchers can investigate where participants are looking when they drive through the various roadway scenarios.

The driving simulator played an important role in testing human factors issues related to double crossover diamonds when FHWA introduced that interchange design in the United States in 2004 in Springfield, MO. Although France has used the double crossover diamond successfully for 30 years, this freeway interchange design was new to the United States. To assist with U.S. development, Michel Labrousse, director of the Centre d'Etudes Techniques de l'Equipment Normandie-Centre, provided records, signal layouts, and traffic flow and crash data from a groundbreaking installation in Versailles, France.

Many conventional interchanges in urban areas are congested and experience high crash rates. In comparison to a conventional diamond interchange, a double crossover diamond design involves having drivers cross over from the right side of the road to the left side and then back, thus combining left-turning and through traffic movements. Because of this novel design, one human factors concern was that drivers might become confused and make a dangerous maneuver. To evaluate this concern, the FHWA researchers created visualizations in the simulator of various driving scenarios.

The Missouri Department of Transportation (MoDOT) designed and built the first U.S. double crossover diamond interchange in Springfield, MO, and opened it to traffic in June 2009. During the design phase, the Missouri engineers visited the Human Factors Laboratory to virtually drive through a simulated double crossover diamond. At the same time, the lab's researchers provided feedback on the details of the MoDOT design. This visualization and testing in the driving simulator helped to alleviate safety concerns about the new design. The FHWA researchers then created video clips from the simulation scenarios to facilitate outreach to the Missouri public.

"The driving simulator experience boosted MoDOT staff's confidence in the double crossover diamond concept," says Joshua Scott, P.E., senior highway designer with MoDOT. "The simulator provided valuable feedback on our preliminary design, allowing us to make changes to the geometrics, signing, and pavement marking, some of which we were able to test while at [TFHRC]. In addition, we were able to get drive-through videos that helped MoDOT convince a skeptical public -- and public officials -- that the double crossover diamond was truly the best solution for the location."

A current study using the simulator examines issues related to driver distraction. Researchers are investigating whether advertising on changeable message signs is distracting to drivers. Some of the measures used in the study include the number and duration of eye glances to each sign, and whether participants notice a sign telling them to exit the freeway because there is a crash ahead. The researchers would also like to determine whether there is any correlation between potential distraction from advertising on changeable message signs and safety concerns. The study is in the data collection phase, and the results are expected to take another year.

Field Research Vehicle

Another tool in use at the Human Factors Laboratory is a field research vehicle, an instrumented 2007 sport utility vehicle (SUV). The SUV is outfitted with equipment to record GPS position, vehicle speed, and vehicle acceleration.

The vehicle also is equipped with a state-of-the-art eye-tracking system that consists of two infrared light sources and three cameras mounted on the dashboard facing the driver. These cameras and lights are small and are not attached to the driver in any manner. The cameras are synchronized to the light sources and help track the head position and gaze of the driver.

Researchers at FHWA's Human Factors Laboratory use this field research vehicle to conduct onroad experiments to better understand driver behavior and performance.

Other components of this eye-tracking system are three additional cameras mounted on the exterior of the vehicle's roof, directly above the driver's position, for capturing the forward driving scene. The cameras capture the panorama of the driving scene in front of the vehicle, together providing an 80-degree-wide by 40-degree-high field of the forward view. The forward view area reaches from the left side of the windshield to a portion of the right side.

Recently, the laboratory's team used the research vehicle to collect data for a study to examine where drivers look when they are driving past commercial electronic variable message signs and standard billboards. The lab measured the signs and billboards with respect to luminance, location, size, and other relevant variables.

The study assessed two data collection efforts that employed the same methodology in two cities -- Reading, PA, and Richmond, VA. In each city, the researchers examined eye glance behavior toward two variable message signs on arterials and two on freeways. The study used an equal number of signs on the left and right sides of the roads, and selected standard billboard environments that matched as closely as possible those of the variable message signs. The researchers instructed the motorist participants to drive the routes as they would normally, paying attention to other traffic, speed limits, and other elements in the roadway.

In Pennsylvania, 14 participants drove at night and 17 during the day. In Virginia, 10 participants drove at night and 14 during the day. During both scenarios -- variable message signs and standard billboards -- drivers in the study directed the majority of their visual attention to areas of the roadway that were relevant to the task at hand -- driving.

Sign Design and Research Facility

Another facility at the Human Factors Laboratory is known informally as the sign lab and consists of a 60-inch (152-centimeter) light-emitting diode/liquid crystal display (LED/LCD) high-definition television connected to a computer control center. The sign lab enables researchers to present traffic signs to participants in a controlled environment. When developing new traffic signs, researchers need to determine the maximum distance at which participants can recognize and comprehend signs.

An eye-tracking device (the three circled cameras) in this field research vehicle helps researchers study where drivers are looking when they drive through various roadway environments.

To this end, a participant sits at the computer and looks at the TV as a researcher displays a sign as a small distant object and then enlarges it so that its appearance approximates the way it would be viewed as a vehicle approaches the sign at a specified speed. The researcher then uses the size of the image at the moment the participant recognizes it to approximate the sign's recognition sight distance. The computer precisely controls the sign display duration and image size, and measures the participant's reaction time. The researcher generally records sign comprehension using open-ended questions relating to the participant's understanding of the traffic sign. For example, the research might ask, "If you were driving and saw this sign, what action would you take?"

"The Human Factors Laboratory at TFHRC provides a resource for conducting research evaluations not only on general safety and operational activities, but also on topics of specific safety and operational concerns to States involved in pooled-fund studies," says Kevin Sylvester, FHWA Office of Operations, Manual on Uniform Traffic Control Devices (MUTCD) team. "The lab's innovative research on traffic control devices continues to provide progressive, comprehensive, and relevant solutions that meet the needs of the road user."

Recently, FHWA researchers at the sign lab conducted two studies funded by the Traffic Control Device Consortium Pooled Fund Program, which combines States' funds into a pool for Federal research. The first study evaluated identification signs at freeway interchange approaches and the efficacy of the signs at providing motorists with information based on business logos. Currently, the MUTCD limits the number of business logos on a single interchange approach sign to six. Whether increasing or decreasing this number would produce favorable results was one aspect of the study. The research also evaluated the effectiveness of using businesses' logos versus standard highway sign text.

The researchers showed 103 participants multiple combinations of four-, six-, and nine-panel signs. They displayed the signs on the television screen at a simulated distance of 121 feet (37 meters). This distance is approximately half the minimum legibility distance. Results suggested that participants were less able to accurately identify specific business logos compared to standard text on highway signs. (See

Participants also needed more time to identify artistic logos. Across each of the panels, identification accuracy was higher starting at the top of the sign and shifting downward from left to right. In addition, more signs on a panel resulted in more eye glances away from the simulated road. Results from this study showed that any benefit of providing drivers with more service information, such as nine-panel signs, is outweighed by the potential risk of increasing driver distraction.

The second study performed in the sign lab examined the legibility of multiple alternatives of symbols listed in the MUTCD. The alternatives were either currently used internationally, were State specific, or were generated by the lab or elsewhere. Each research participant evaluated each symbol. The team exposed the participants to scenarios containing each of the sign alternatives for each of the sign groups.

For legibility testing, the researchers used software designed to increase the size of the sign gradually, simulating how the sign would appear as a motorist drives toward it at a specified speed. The researchers then measured the legibility distance for each sign. Following each scenario, the team recorded the participants' comprehension using open-ended and multiple choice questions, and by the participants' rankings of how well they thought the signs would work.

Results showed that some alternatives clearly performed better than others, while other comparisons were not as definitive. For instance, under the multiple choice questions, alternative 2 of the WEAVE sign clearly outperformed the three other alternatives, garnering correct responses 95 percent of the time. In the case of the four alternatives for the TRUCK ROLLOVER sign, however, the results revealed no statistically significant differences in performance.

When developing new traffic signs, researchers need to determine the maximum distance at which motorists can recognize and comprehend a sign. The FHWA sign lab, shown here, enables researchers to present traffic signs to participants in a controlled environment and study their responses.

MiniSimTM Driving Simulator

In partnership with the National Highway Traffic Safety Administration, the Human Factors Laboratory recently upgraded its MiniSim, a part-task simulator consisting of a quarter cab setup that includes an adjustable driver's seat, driver controls such as pedals and a steering wheel, and a meter cluster including a speedometer. The upgrade includes three 42-inch (107-centimeter) forward-display LCD televisions, software, and computers for generating driving scenes and controlling vehicle dynamics.

The MiniSim is useful for evaluating driver performance in simple environments, such as various infrastructure-related studies that do not require the full immersion of high-fidelity driving simulation. This tool enables researchers to conduct low-cost studies to answer specific questions or to conduct preliminary research prior to a larger scale simulation or onroad research.

A recent study using the MiniSim examined driver performance on horizontal curves of rural two-lane roadways. According to the Fatality Analysis Reporting System, a total of 23,740 fatalities resulted from run-off-road crashes on the horizontal curve sections of rural two-lane roadways from 2005 to 2009 -- an average of 4,748 fatalities per year. An analysis of the National Motor Vehicle Crash Causation Survey suggests that a driver who is familiar with a roadway is twice as likely to be involved in a run-off-road crash as one who is unfamiliar with it. In addition, a driver who is in a hurry is 3.2 times more likely to be involved in a run-off-road crash than one not in a hurry, and an inattentive driver is 3.7 times more likely to be in a crash than an attentive driver.

The research team examined possible procedures for establishing drivers' familiarity with a roadway, eliciting states of distraction because of being in a hurry, and determining the effect of these factors on driver performance on rural two-lane horizontal curves, as compared to baseline conditions. Measurements included vehicle speed and lane positioning.

Results indicate that the methodological procedures were effective at simulating the precipitating events and might be useful in future experiments by providing realistic driving situations for the development of dynamic traffic control devices using simulation.

In a study that examined sign legibility and drivers' comprehension, researchers asked participants to compare signs listed in the MUTCD to multiple alternatives.

Future Plans for The Laboratory

The Human Factors Laboratory at FHWA has been conducting research to improve the transportation industry's understanding of fundamental aspects of the ways drivers perceive, process, and respond to the roadway environment -- with the ultimate goal of advancing safer roadway designs. The lab also evaluates specific roadway and highway design elements for their suitability for various kinds of drivers and other roadway users such as pedestrians.

Using findings from FHWA's human factors studies, transportation agencies in the United States and abroad are implementing safety countermeasures such as retroreflective raised pavement markers and horizontal curve signage that can help reduce the probability of driver errors and save lives.

"As FHWA's human factors program continues to grow, we want to work with various partners and carry out studies that will enable and expand safe roadway environments for all user groups," says Monique Evans, director of FHWA's Office of Safety Research and Development (R&D). "As an international leader in technologies and methodologies that enable human factors research on roadway systems, FHWA's human factors team will work closely with State and local transportation agencies, share research findings, and provide resources and training to encourage designers and system operators to produce roadways that take users into consideration."


R. M. Michaels and B. W. Stephens, "Driver Characteristics, Night Visibility, and Driving Simulation," Highway Research Record, No. 25, January 1963, pp. 87-94.

D. A. Gordon, "Driver Considerations in Highway Design," Public Roads, Vol. 44, No. 4, March 1981, pp. 148-155.

Federal Highway Administration, "Double Crossover Diamond Interchange," TechBrief, FHWA-HRT-09-054, October 2009.

C. Liu and T. J. Ye, "Run-Off-Road Crashes: An On-Scene Perspective," DOT HS 811 500, National Highway Traffic Safety Administration, July 2011.

The MiniSimSM shown here enables researchers to conduct low-cost studies to answer specific questions or preliminary research prior to a larger scale test.



C. Y. David Yang is the team leader of the human factors program in FHWA's Office of Safety R&D. He joined FHWA in 2008 in the Office of Operations R&D. Yang is the chair of the Transportation Research Board's User Information Systems Committee and serves on the editorial board of the Journal of Intelligent Transportation Systems. He received his B.S., M.S., and Ph.D. in civil engineering at Purdue University. His doctoral dissertation used principles of human information processing and human factors to develop design recommendations for Advanced Traveler Information Systems. 

Jim Shurbutt is a behavioral research psychologist with FHWA's Office of Safety R&D. He received his B.S. and M.S. in psychology from Jacksonville (AL) State University and his Ph.D. in applied behavior analysis from Western Michigan University. His areas of research include pedestrian/bicycle safety and access, rural roadways, complex interchanges, and traffic control devices.

Brian Philips joined FHWA in 2011 and is a senior research psychologist on the human factors team. Philips has been involved in human factors research for more than 20 years and in surface transportation for 8 years. He serves as a reviewer for the journal Human Factors. His areas of research include driver distraction, automation, traffic control devices, and driver navigation and decisionmaking. He received his B.S., M.S., and Ph.D. in human factors/applied experimental psychology from Tufts University and George Mason University.

The authors would like to thank Randall VanGorder of FHWA's Office of Operations R&D for producing several photos used in this article.

For more information, contact David Yang at 202-493-3284 or