Information As A Source of Distraction

Executive Summary

The overall goal of the Information as a Source of Distraction project was to further the scientific basis for decisions about the types of information that can be safely displayed within the right-of-way without adversely affecting drivers’ attention to their primary task—safe driving. There were two focus areas: electronic changeable message highway signs (CMSs) and guide signs.

This study had the following objectives:

• Determine the distraction potential of non-traffic-related messages.
  
  
• Determine the distraction potential of guide signs that are more closely spaced or more frequent than current guidelines permit.
  
  
• Perform both on-road and driving simulator evaluations of distraction.
  
  
• Create a report and present recommendations that provide a scientific basis for practitioners to assess the informational load imposed on road users by information sources within the highway right-of-way.

Chapter Summary

Chapter 1, the report’s introduction, discusses the definition of distraction, the properties of modern light-emitting diode (LED) based CMSs, and the specific issues addressed in the subsequent chapters.

Chapter 2 presents some laboratory assessments of a full-color, full-matrix, LED display with 0.79-inch (20-mm) pixels. The display represents capabilities of the current generation of CMSs that are compliant with intelligent transportation system (ITS) standards. In one experiment, observers rated the visual similarity of messages on the CMS with the same messages displayed on a 60-inch (152-cm) liquid crystal display. The results of that experiment suggest that it is not necessary to emulate individual pixels of the CMS display to generate images that observers rate as reasonably similar to the CMS images. The results will have significance for future CMS research in either the Federal Highway Administration’s (FHWA) sign laboratory or the FHWA highway driving simulator.

Chapter 3 presents a study of the legibility of the CMS described in chapter 2. These results suggest that the legibility distance of the 0.79-inch (20-mm) pixel-pitch full-color display used in this test provides a longer legibility distance than the previous generation of CMS displays that used a 1.6-inch (40-mm) pixel pitch with amber LEDs.⁽¹⁾ They also suggest that for the display type used and drivers with approximately 20/20 vision, the 90- to 100-percentile legibility distance can be estimated using a factor of 40 to 45 ft/inch (4.8 to 5.4 m/cm) of letter height. This equates to a somewhat lower legibility distance for 0.79-inch (20-mm) pixel pitch CMS display than that provided by the 30 ft/inch (3.6 m/cm) legibility distance criterion in the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD), assuming a 20/40 visual acuity.⁽²⁾ (Because the relationship between letter height and visual acuity is linear, the MUTCD criterion would yield a 60-ft/inch (7.2-m/cm) legibility distance for individuals with 20/20 visual acuity.)

Chapter 4 examines the legibility of CMS messages in a dynamic roadway environment. In this study, observers drove toward the CMS on a winding path at 25 mi/h (40 km/h) and read the messages on the sign as soon as possible. An eye-tracker was used to assess when the observers were looking at the signs. The time and distance for the beginning and ending of response were recorded. The following properties of the messages on the sign were assessed for their effect on gaze behavior and message reading:

• Message length. The amount of time required to read messages of up to five words was fairly constant. Six- and seven-word lists took considerably longer to read. It is recommended that text messages be limited to five or fewer words whenever possible.
  
  
• Flashing. The MUTCD standard is not to flash messages.⁽²⁾ When the first line of a message was flashed, the duration of observers’ responses was longer than for either static messages or all-lines-flashing messages. The delay before participants began responding was longest for all-lines-flashing messages. The findings support the current MUTCD standard.
  
  
• Symbols versus text. The symbol versions of messages (e.g., an electronic version of the signal ahead symbol sign) that filled the same display area as their text equivalents were legible or recognizable from a greater distance than their equivalents. The exception to this finding was the road workers ahead symbol sign, which was unfamiliar to the majority of observers. It is recommended that a familiar symbol be used in place of a text message when: (1) the high-resolution CMS can accurate portray the symbol, and (2) the symbol is recognizable at the same or greater distance as the equivalent text message.
  
  
• Abbreviations. It was found that use of the abbreviations approved in the MUTCD had no adverse effects compared with the spelled out versions of the abbreviated words.⁽²⁾

Chapter 5 presents an experiment conducted in the FHWA highway driving simulator to evaluate whether frequently changing overhead freeway CMS displays with human faces and colorful backgrounds would distract drivers more than static travel-time-related messages or a blank CMS. Another purpose of the experiment was to evaluate whether noncritical information presented on frequently occurring CMSs (i.e., a CMS every 0.5 mi (.8 km)) would cause drivers on a 48-min trip to lose respect for or habituate to the CMS messaging and result in drivers missing a safety-critical CMS message. The results showed that gaze behavior (i.e., number of looks, duration of looks) did not differ between signs with rapidly changing faces and static text messages. Drivers briefly looked at signs with changing faces and travel times about 40 percent of the time (40 percent of the signs) when headway to the car ahead was greater than 1.5 s. When headways were shorter than 1.5 s, the probability of briefly looking at a non-blank sign was about 0.15. Regardless of what was displayed on the CMS, the mean duration of individual looks was 0.2 s or less.

For the simulated trip, in which the drivers passed under 96 noncritical CMS message signs, there was no indication of habituation. The 97th CMS carried the message “ACCIDENT AHEAD ALL LANES BLOCKED USE NEXT EXIT.” Of the 32 participants, 9 failed to exit after passing under that sign. However, only one of the nine participants claimed to be unaware of the critical message. The majority of the remaining participants tried to exit but were unable to find a safe gap to change lanes.

Chapter 6 presents another experiment in the FHWA highway driving simulator similar to the experiment described in chapter 5. However, in this experiment, a spilled load of logs was in the participant’s lane, and the primary dependent measure was whether the participant avoided hitting the spilled load. There were 72 CMSs—1 every 0.5 mi (0.8 km), the 3 sign types (changing faces, travel-time messages, and blank) occurred in cycles of 3. The 72d sign was blank for a third of the participants, had a travel-time message for another third, and had changing faces for the final third. The spilled load came early in the trip (just before the third CMS) for half the participants and late in the trip (just before the last CMS) for the other half. Of 80 participants, 21 hit the logs. There was no significant relationship between the content of the CMS at the spill site and the probability of hitting the spilled load.

Chapter 7 examined the effect of the frequency and spacing of guide signs on navigation, eye glance behavior, and driving performance. The primary focus was the frequency and spacing of specific-service signs and supplemental guide signs. The number of supplemental guide signs varied between zero and three as did the number of specific service signs. The number of destinations on guide signs varied between one and two. The distance between the three types of guide signs also varied. In most conditions, the spacing minimum was 800 ft (244 m), which is the current standard. In the remaining two conditions, the spacing was 400 and 200ft (122 and 61 m). Overall, the results support the current MUTCD standard of 800 ft (244 m) spacing between signs and up to two destinations on advance and supplemental guide signs. There was some evidence that the specific-service six-panel logo signs used in this study required too much visual attention, especially when there was more than one specific-service sign. Participants appeared to scan food and gas service signs in search of a lodging destination and thus did not appear to use the sign legends in their search strategy.

Summary of findings and Recommendations

The overall goal of the project was to further the scientific basis for decisions about the types of information that can be displayed within the right-of-way without adversely affecting drivers’ attention to their primary task—safe driving. There were two focus areas: electronic CMSs and guide signs. Findings and recommendations include the following:

• Research on messaging can be done on laboratory and driving simulator displays without precisely emulating the pixel spacing or color properties of CMS devices. It is sufficient to emulate legibility distance and approximate color and contrast.
  
  
• The daytime legibility distance of 0.79-inch (20-mm), full-color, full-matrix CMS text can be estimated using a letter height of 1 inch (2.4 cm) per 20 ft (6.09 m).
  
  
• Large symbols that motorists easily recognize have greater legibility distance and require less visual attention than equivalent word messages that require the same display area.
  
  
• CMS messages should be limited to five words or fewer whenever possible, especially if the messages contain rarely used elements (e.g., evacuation routing or atypical lane closures).
  
  
• Visually salient CMS images (faces) that change every 3 s attracted no more attention than static travel-time text messages. Blank signs attract less attention than populated signs.
  
  
• Drivers attend to CMSs when driving demands are manageable.
  
  
• The frequently occurring and driving irrelevant CMS messaging used in this study did not appear to cause drivers to rapidly habituate to or lose respect for CMSs such that safety-critical messaging would be ignored. This study did not examine whether long-term habituation might occur over many trips. It is possible that messaging that cannot be easily recognized as driving irrelevant (e.g., messages that must be fully read to determine irrelevance) might result in loss of respect if encountered frequently.
  
  
• CMS messages that were theoretically highly visually salient did not distract drivers from detecting roadway hazards. It appears that drivers have learned to regulate their visual attention in a way that minimizes susceptibility to visual distractions. This finding does not imply that drivers cannot be distracted by CMS messages but rather that visual salience alone is not sufficient to distract.
  
  
• The MUTCD criteria for the frequency and spacing of guide signs, including supplemental and specific-service signs, were supported by the current findings.⁽²⁾
  
  
• Further research is recommended to address why participants in this study searched fuel- and food-specific service signs when their task was to search for a specific lodging business.