Effects of Varying Text Message Length and Driving Speed on the Disruptive Effects of Texting on Driving Simulator Performance: Differential Effects on Eye Glance Measures

Abstract

Eye glance analysis and driving performance during texting while driving: Differential effects of varying driving speed versus text message length. Background and Objective. Texting while driving continues to be a significant public health concern. Eye glances off the roadway are a measure of the visual distraction associated with texting while driving. In the present study, we examined the effects of two ‘real-world’ factors relating to the adverse effects of texting on driving performance and eye glances off the roadway: (1) text message length and (2) driving speed. Methods. Subjects ‘drove’ a fixed-base simulator and read, typed and sent text messages while driving. In study #1, the driving speed was 60 mph and the effects of short (1 word) versus longer (8–10 words) texts were compared. In study #2, the text messages were short only and driving speed was 60 or 80 mph. Driving performance was assessed using the Standard Deviation of Lane Position (SDLP). Video recordings of the drivers’ faces were used to assess eye glances from the road to the phone—and back—during texting. Results. Texting while driving impaired driving performance as measured by SDLP, and both longer text messages and faster drive speeds made driving performance even worse. Analysis of the eye glance data, however, revealed different effects of these two manipulations. Specifically, longer text messages were associated with an increase in the number of eye glances to the phone during a text message episode, an increase in the total time spent with the eyes off the road, and an increase in the single longest eye glance from the road. Moreover, with longer text messages the longest single eye glance away from the road typically occurred at or near the end of the text message episode. In contrast, increasing driving speed to 80 mph did not affect any of these eye glance measures relative to driving at 60 mph. Conclusion and Application. Both text message length and driving speed while texting adversely affect driving performance, but they do so via different mechanisms. These results have implications for how to tailor “don’t text and drive” messaging to better serve the public health.

1. Introduction

Distracted driving is a significant public health concern that can be defined as any activity that diverts a driver’s eyes, hands, feet or mind away from the driving task. Activities included under distracted driving could be anything like phone conversations, texting while driving, changing the radio station, watching videos or even putting on makeup [1].
The increased use of technology-based in-vehicle information systems and cellular connectivity applications, from navigation systems to in-car entertainment systems, has only increased the distractions to which motor vehicle drivers are exposed. Distracted driving is a significant traffic safety hazard. According to the National Highway Traffic Safety Administration (NHTSA), distracted driving accounted for approximately 8.5% of fatal crashes and 14% of all police-reported crashes in the United States in 2019, with over 3100 people killed and about 424,000 people injured in crashes involving a distracted driver in 2019 [2].

One of the more common driving distractions is cell phone use, especially texting while driving [2,3,4]. According to the Centers for Disease Control and Prevention [5], in 2009 about 31% of all adults in the United States had sent or read a text message while driving at least once in the past 30 days, and this figure increased to 39% in 2019 [6].

Texting while driving has been shown to be significantly more dangerous than talking via cell phone while driving [7,8] and is a significant source of motor vehicle crashes. The Automobile Association of America (AAA) has reported that texting is responsible for 12% of all distracted driver accidents [9]. Texting while driving is particularly dangerous because it involves visual, manual and cognitive distractions. First, texting while driving involves taking your eyes off the road. Second, texting requires taking your hands off the car controls in order to operate the cell phone. And third, the concentration required for the driving task is compromised by thinking about the text conversation while also thinking about driving.

Driving simulators are a good tool to study the parameters that influence driving safety and performance. The studies from driving simulation in combination with text messaging have revealed that sending and reading texts negatively influences driving performance [10,11,12,13]; see meta-analysis by Caird et al. [14]. Most of these studies have focused on changes in lateral control, in particular the Standard Deviation of Lane Position (SDLP). The visual distraction associated with texting while driving can be relatively easily measured via the number and duration of eye glances off the road during texting. Commonly-used measures of eye glance analysis include (1) total number of glances to the phone and back to the roadway, (2) the average eye glance duration, (3) the longest single glance to the phone and (4) the total time with the eyes off the roadway [2].

Two ‘real world’ factors that may contribute to the degree/extent of driving impairment associated with texting while driving are text message length and driving speed. Regarding the former, Peng et al. [13] have demonstrated that longer text messages are more disruptive to driving than shorter messages, with longer text messages also associated with an increase in the total number of eye glances to the phone during a texting/driving episode, an increase in the total time with the eyes off the road during texting, and an increase in the duration of the single longest eye glance to the phone. These authors did not examine the relative distribution of eye glances to the phone within the period of the texting-while-driving episode, e.g., where within the series of eye glances does the longest glance occur? With respect to driving speed, although there is ample evidence that increased driving speed increases crash risk in general (see review by Aarts and vanSchagen [15]), to our knowledge there are no published reports on the influence of driving speed on the adverse effects of texting on driving performance. Thus, the present studies were designed to compare and contrast effects of text message length (short versus long texts) and driving speed (60 mph vs. 80 mph) on the disruptive effects of texting on driving performance and to characterize eye glance behavior with these different distraction-increasing conditions.

2. Materials and Methods

Subjects: The subjects were nine (five women, four men) unpaid volunteers 18–25 years of age. These experiments were approved by the Wayne State University IRB Office (IRB #066716BE), and all subjects provided informed consent. All subjects had valid driving licenses, had been licensed for 3–9 years (average: 5.6) and drove 1500–4500 miles/week (average: 3640). Subjects with glasses or contact lenses also wore those devices during the driving simulator tests. All subjects used smart phones and reported texting more than 80 times per day.

EACPHS Driving Simulator: Study participants were seated in a fixed-base driving simulator (DriveSafety, Inc., Draper, UT, USA) that consists of a four-door vehicle (2000 Chevrolet Impala) fully equipped with steering wheel, pedals, ignition switch, gear shift, rear and side view mirrors, headlights, turn signals and a radio (see Figure 1). A fully immersed virtual driving experience was created with six networked computers generating the simulated roadway via three forward projection screens (left, center, right) to provide a 150-degree forward field of view and one rear projection screen. Driving scenarios were created using HyperDrive software (Version 1.6.2), a tile-based scripting tool.

Experimental Design: Subjects drove the EACPHS simulator on a ‘roadway’ characterized by many long straight sections connected by moderate curves. After several ‘drives’ to accommodate to the driving simulator, each subject drove the simulator on two occasions, one in which the text message length varied and a second occasion in which the driving speed was varied. Each ‘drive’ was approximately 10 min in duration. Also, no subject reported experiencing ‘simulator sickness’, largely because this roadway is largely straight. Texting while driving was tested on the straight sections of roadway. These straight roadways had four lanes (two in each direction, divided by a barrier) with no oncoming traffic but with a single pace car (at the appropriate speed). The driver was instructed to (1) drive at the designated speed, (2) follow the pace car at a safe distance and (3) remain in the center of the right-side driving lane.

For the study of text message length, the pace car proceeded at 60 mph for all drives, and the subjects/drivers were instructed to follow the pace car at a consistent and safe distance. Text messages were either (1) one word (“blue”; “impala”) or (2) a short sentence of 8–10 words (“texting while driving is not safe”; “texting while driving is worse than phone calls while driving”). All subjects were exposed to all four text messages over the course of the testing sessions. For the study of driving speed effects, the pace car traveled at 60 or 80 mph and subjects were instructed to follow at a consistent and safe distance. These speeds were selected with the hope that they reflect the 25th and 75th percentile values for speeds that might commonly be encountered in freeway driving. All text messages were short during the driving speed study. Subjects used their own cell phones, and were instructed to not respond to any cell phone calls or texts from callers other than the experimental team members. For standardization, voice-to-text, keyboard swiping and auto-fill procedures were not allowed. Text messages were sent after a brief (5–10 s) baseline period of driving on the straight roadway. In both experiments, the subjects were instructed to read, re-type and send back the text messages while continuing to drive. This approach of ‘read/re-type/send’ text messaging was used because it is more standardized across various subjects than the real-world approach of ‘ask/answer/send’ type of text messaging

Data Collection and Dependent Variables: Both driving performance and eye glance behavior before and during texting were measured. Driving performance was assessed via (1) evaluation of ‘road camera’ videos of the various drives before and during texting, as well as (2) variability in the position of the car on the roadway (Standard Deviation of Lane Position; SDLP). Road camera video clips of driving while texting were rated using a 1–4 scale: 1 = virtually no deviation within the lane; 2 = noticeable deviation but remained in the proper lane; 3 = a single excursion outside the driving lane; 4 = multiple excursions outside the driving lane. This scoring rubric has been found to be highly reliable when ‘calibrated’ against SDLP data [16].

Eye glance behavior was assessed by reviewing video clips of the drivers’ faces during the texting episodes. The primary eye glance measures for each texting and driving episode were (1) the mean (and median) glance duration on the phone (to a temporal resolution of 0.1 s), (2) the total number of eye glances away from the road, (3) the total duration of eyes off the road (TEOR) and (4) the duration of the longest single eye glance off the road during texting.

Analyses of the videos for both driving performance and eye glance measurements were conducted by 3–5 trained scorers who were blinded regarding the treatment condition. Inter-rater reliability was strong, with correlation coefficients of 0.85–0.96 between individual raters.

Statistical Analyses: From the video clips, drive scores and eye glance behavior (number of glances, mean/median glance duration, longest single glance, total time with eyes off road, percent of texting time with eyes off road) for the different conditions (short vs. long text messages; 60 mph vs. 80 mph driving speeds) were compared using a Student’s t-test (means) and Matched-Pairs Wilcoxin test (medians) for paired comparisons. For the driving performance measurement of SDLP, values obtained during the 5 s before initiation of texting (pre-texting baseline) were compared to values obtained during the texting period using Factorial ANOVA with Repeated Measures (factors of pre-texting/texting and either text message length or driving speed). In addition, scatterplots comparing the total number of eye glances during a texting/driving episode versus the relative location (early or later in the texting episode) of the longest single eye glance to the phone were constructed and analyzed via Analysis of Covariance (ANCOVA) to examine the potential relationship between these variables and the influence of text message length and driving speed on this relationship. In all statistical comparisons, p < 0.05 was used as the criterion for statistical significance.

3. Results

Control Driving (i.e., No Texting): In the absence of texting, all subjects were able to remain in the specified driving lane during the straight roadway, and exhibited only occasional and very brief (0.1–0.4 s) eye glances away from the road, typically to check their driving speed. Measures of SDLP (0.10 ± 0.03 m; mean ± SD) were very low during the pre-texting period. This is consistent with previous studies [13,16,17] and is not surprising given the relative ease of driving on the straight roadway.

Texting While Driving—Effects on Driving Performance: In terms of driving performance/behavior, texting significantly disrupted driving performance (Figure 2). This was apparent whether the driving performance was measured via the video scoring of the drive (Figure 2(top panels)) or the change in SDLP compared to pre-texting (Figure 2(lower panels)). All of these behaviors are consistent with previous reports on the effects of texting while driving [7,12,13,16].

Effects of Text Message Length and Driving Speed on Driving Performance: Figure 2 also illustrates that increasing the text message length or increasing the driving speed potentiates the disruptive effects of texting on driving. Neither text message length nor driving speed affected the control (i.e., pre-texting) values, but both longer message lengths and faster driving speeds adversely affected the effects of texting on driving performance. Statistically, text message length significantly affected drive scores (means: t [8] = 8.27, p < 0.05; medians: Z = 2.52, p < 0.05). There was a significant text message length x pre-texting/texting interaction for SDLP (F [1,8] = 8.78, p < 0.05). Similarly, driving speed significantly affected drive scores (means: t [8] = 7.45, p < 0.05; medians: Z = 2.01, p < 0.05), and there was a significant driving speed x pre-texting/texting interaction for SDLP (F [1,8] = 12.95, p < 0.05). Thus, although neither increasing text message length nor driving speed affected control (i.e., non-texting) driving, both factors significantly worsened the effects of texting to disrupt driving.

Increasing the length of the text message increased the time during which the drivers were engaged in texting behavior while driving, from 8.7 ± 3.9 s (mean ± SD) with one-word texts to 30.9 ± 5.7 s for 8–10-word texts. To determine whether the SDLP and driving scores were worse because of the increased texting behavior and not simply because the period of data sampling had increased with longer text messages, in a separate group of subjects we measured SDLP and driving scores during non-texting drives where the sample period was 10 versus 25 s after a 5 s ‘baseline’ period. In the absence of texting, there were no differences in the change in SDLP for sampling periods of 10 s (0.07 ± 0.02 m; mean ± SD) or 25 s (0.09 ± 0.05 m; mean ± SD) compared to baseline (0.10 ± 0.10; mean ± SD). Driving scores also did not vary from baseline values. Thus, the worsening of the drive scores and the increased SDLP observed with longer text messages was not simply the result of the increased period of data sampling.

Effects of Text Message Length and Driving Speed on Eye Glance Behavior. In terms of eye glance behavior, texting while driving was NOT characterized by a single long glance at the phone. Rather, texting while driving typically was characterized by a series of eye glances between the phone and the road. In the present study, the overall average number of glances at 60 mph was 10.7 ± 6.5 (mean ± 95% confidence interval) and the mean glance duration was 1.1 ± 0.4 s (mean ± 95% confidence interval); this is consistent with texting while driving data from previous studies [7,13,16].

Effects of Text Message Length and Driving Speed—Eye Glance Data Analysis: Figure 3 illustrates the effects of increasing the text message length (left panel) or increasing the driving speed (right panel) on the number of eye glance measurements off the road. As can be seen, longer text messages significantly increased this measure (t [8] = 6.46, p < 0.05). Increasing the driving speed did not affect the number of eye glances off the roadway (t [8] = 0.36, ns).

In contrast, as can be seen in Figure 4, neither text message length nor driving speed significantly affected the mean eye glance duration off the roadway (Figure 4(top panel)) or the overall percent of texting time spent with eyes off the roadway (Figure 4(lower panel)).

Thus, although neither increasing message length nor driving speed affected the mean duration of eye glances or the overall percent of time spent with eyes off the road, longer text messages, but not texting at a higher driving speeds, were associated with a dramatic increase in the total number of eye glances off the roadway.

Figure 5 illustrates the effects of text message length and driving speed on two additional measures of eye glance behavior, specifically the total time with the eyes off the road and the longest single eye glance off the road for each subject. These two measures are notable because they have been cited by NHTSA as important criteria/guidelines for assessing the potential for in-car electronic equipment to be deemed a significant driving distraction [18]. As can be seen, increasing the text message length increased both the total time with eyes off the road (t [8] = 8.67, p < 0.05) and the duration of the single longest glance off the roadway (t [8] = 12.64, p < 0.05); in contrast, increasing driving speed did not affect either the longest single eye glance off the roadway (t [8] = 1.25, n.s.) or the total time with the eyes off the road (t [8] = 0.68, ns).

Longest Eye Glance from the Phone Occurs Later in the ‘Text Bout’ for Longer Text Messages. Figure 6 depicts the influence of changing text message length (left panel) or driving speed (right panel) on the relationship between the total number of eye glances away from the road during an episode of texting while driving versus the relative location, i.e., early versus later in the texting episode, of the single longest eye glance to the phone. With respect to text message length, ANCOVA revealed a significant effect for text message length on the duration of the single longest eye glance from the roadway (F [1,34] = 8.516, p < 0.05), as well as a significant positive correlation between the relative location of the longest eye glance to the phone and the total number of eye glances during the texting episode (r = 0.66; F [1,34] = 6.36, p < 0.05). Put another way, the longer the duration of the overall ‘texting bout’, the more likely that the longest single eyeglance would occur at or near the end of that ‘texting bout’.

In contrast, in the driving speed study there was no significant correlation between the total number of glances and the relative location of the longest eye glance when texting while driving was assessed at different driving speeds (Figure 6(right panel)). ANCOVA revealed no significant effect for driving speed on the duration of the single longest eye glance from the roadway (F [1,34] < 1.0, ns), and no significant correlation (r = −0.07; F [1,34] < 1.0, ns) between the relative location of the longest eye glance from the roadway and the total number of eye glances in the texting episode. Thus, the two factors of text message length and driving speed differentially affected the number, duration and relative distribution of potentially harmful eye glances from the road to the phone.

4. Discussion

Texting Adversely Affects Driving Performance, and Text Message Length and Driving Speed Each Potentiated the Effects of Texting to Further Disrupt Driving. Texting while driving impairs driving performance; this is not a surprise and is consistent with several previous reports (see meta-analysis by Caird et al. [14]; see recent review by Voinea et al. [19]. This problem was worse for longer text messages and when driving at faster speeds. The effect of longer text messages to exacerbate the effects of texting on driving performance is consistent with a previous report by Peng et al. [13]. To our knowledge, this is the first report to demonstrate that the detrimental effects of texting on driving are also exacerbated by increased driving speed.

Text Message Length and Driving Speed Exerted Differential Effects on Eye Glances During Texting While Driving. Although longer text messages did not increase the mean (or median) duration of eye glances off the road, longer text messages were associated with a significant increase in the total number eye glances off the road, an increase in the single longest eye glance off the road, and a resultant increase in the total time with eyes off the roadway, suggesting that driving performance got worse with longer texts at least in part because drivers engaged in more and longer duration ‘episodes’ of drive-inappropriate behavior, i.e., eyes off the road. In addition, as the total number of eye glances to the phone increased (e.g., with longer text messages), there was an increased likelihood that the longest single eye glance to the phone would occur later and later into the “texting episode”. These findings suggest that the subjects/drivers (1) know that their texting is adversely affecting their driving performance, yet (2) mistakenly engage in even more risky activity (longer single eye glances, particularly near the end of the testing/driving episode) in order to stop engaging in the dangerous activity of texting while driving. On one occasion, during a longer text message, the subject was overheard urging himself to “...just finish the d*** text and go back to driving….” This testimonial/confession was confirmed and supported by additional subjects in follow-up conversations and represents a human subjects factor that warrants further study. To our knowledge, this is the first study to analyze and demonstrate the relationship between eye glance duration and the relative location of the longest eye glance off the road (and, by logical extension, the highest crash risk) within a texting while driving situation.

In contrast to the effects of changing text message length, increasing the driving speed when texting while driving further disrupted driving performance but did not result in an increase in the number, duration or temporal pattern of eye glances away from the road. This suggests that driving performance got worse during texting at 80 mph simply because it was more difficult to text and simultaneously drive at 80 mph when compared to 60 mph. At higher speeds, even a momentary distraction (brief eye glance to the phone) can lead to a longer distance traveled off course before the driver can initiate a corrective response. Drivers seem to know this, since Caird et al. [14] have reported that drivers who read and type texts tended to decrease their speed and increase headway distance, which would be expected to at least partially compensate for the impairments caused by texting while driving.

In summary, the present findings suggest that although both increasing text message length while driving and increasing driving speed while texting adversely affect driving performance, they do so via different mechanisms. Moreover, these data suggest that driving simulator data, combined with analysis of eye glance data, can be used to characterize more fully the nature of driving distractions such as texting. This approach might be useful in characterizing the effects of other driving distractions such as drugs, administered alone and in combination with texting, on driving performance.

5. Potential Pitfalls and Limitations

• Driving simulator versus the ‘real’ road. The present studies were conducted using a fixed-base driving simulator and not on a real roadway. Driving simulator studies have been shown to be a very safe and effective tool for understanding and predicting future real-world driving experiences. Moreover, in a post-test survey, subjects in the present study reported an average score of 8.6/10 for the ‘reality’ of the experience.
• The roadway was relatively ‘easy’ to navigate and not representative of all ‘real-world’ driving situations. In the present study, texting while driving was studied on a rather easy roadway because this kind of roadway (straight road; no oncoming traffic; single pace car) yields consistent and low baseline measures, which facilitates the identification and study of distractions; indeed, NHTSA has recommended the use of a similar ‘easy’ roadway for the study of distracted driving [18]. 
Future research should consider the impacts of surrounding road infrastructure such as intersections and ambient traffic.
• Only young drivers were studied. Clearly, texting while driving is a problem that is NOT restricted to young drivers; indeed, when compared to younger drivers, older drivers have been shown to be more adversely affected by texting while driving [20]. Future studies examining the variables of text message length and driving speed in older drivers and across various texting experience levels are planned.

6. Key Points

Both increasing text message length and increasing driving speed are associated with greater disruption of driving while texting, as assessed by both (1) evaluation of driving videos and (2) SDLP measures.

Increasing text message length—but not increasing driving speed—was associated with an increase in the total number of eye glances away from the roadway, an increase in the single longest eye glance away from the roadway and an increase in the total time with the eyes off the roadway

Increasing text message length—but not increasing driving speed—was associated with the longest eye glance away from the roadway occurring later in the texting-while-driving bout; an informal comment from one subject, and more structured inquiries with other subjects, indicated that with longer text messages the subject was trying to ‘just finish the *** text’ and get back to safer driving.

These findings have implications for “don’t text and drive” messaging to reduce crashes and therefore better serve the public health. Such messages would need to be carefully prepared, since one would NOT want to create messaging that suggests that texting while driving is safe as long as the messages are short or the driving speed is slower.