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92 ⢠Survey data o Comparison of DMS, overhead signs, and pictograms o Comparison of presentation styles o Preferred ATM medium Road layout The simulated road layout was designed to require drivers to change their speed according to dynamic speed limit information. For each experimental condition, a total of four overhead ATM signs were presented (i.e., four gantries within one block). Distance between signs was set to 0.5 miles. Although speed limits were presented for each lane, the speed limit was the same across all lanes. Similar to Experiment 1, participants experienced the same road layout through the experiment and only the order of experimental condition varied across drivers to avoid order effects. Figure 79 shows the road layout for Experiment 2. In Figure 79, black vertical lines with numerical digits on the top represent overhead gantries and dynamic speed limits. Figure 80 shows a sample image of dynamic speed limit information presented by the overhead gantry signs. Figure 79. Road layout for Experiment 2.
93 Figure 80. Sample image of dynamic speed limit information presented on gantries for Experiment 2. Results The results section consisted of three subsections: (a) driving measurements, (b) glance measurements, and (c) survey responses. Driving measurements focused on participantsâ speed choice behavior across various experimental conditions. Glance measurements focused on glance behavior and potential visual distraction caused the smartphone application. Survey responses interpreted participantsâ responses to various questionnaires regarding ATM media and experimental conditions. Driving measurements Overall, percentage of speed compliance was high across all conditions (M = 94.6%). Among the conditions, the AV-prescriptive mode led to the highest speed compliance (M = 97.6%). The level of speed exceedance was also low across all conditions. The smartphone-only condition led to the highest speed exceedance, but the average speed exceedance was around 1 mph. See Table 21. Table 21. Percentage of speed compliance and average speed exceedance by experimental conditions. Baseline 1 (smartphone only) Baseline 2 (gantry only) Visual information AV information (descriptive) AV information (prescriptive) Percentage of speed compliance Mean (SD) 88.0 (18.2) 95.7 (8.72) 95.5 (7.04) 96.4 (5.50) 97.6 (3.37) Speed exceedance Mean (SD) 0.99 (2.84) 0.23 (1.34) 0.33 (1.80) 0.25 (1.47) 0.35 (1.65)
94 A linear mixed-effect model was applied to examine differences across the experimental conditions. The drivers were set to a random effect and the experimental conditions were set to a fixed effect. Two baseline conditions exist; thus, the two models had the same fixed and random effects, but with different orders of levels of the conditions were tested. The first model compared results to the smartphone-only condition and the second model compared results to the gantry-only condition. ⢠Compared to the smartphone-only condition (baseline 1), all conditions (visual mode, AV mode-descriptive, and AV mode-prescriptive) were associated with a significantly higher speed compliance percentage (p < .001). o For the dynamic speed limit application, disseminating the information from only the smartphone led to the lowest speed compliance. Specifically, the smartphone- only condition led lower speed compliance compared to the current application (i.e., gantry-only). ⢠Compared to the gantry-only condition (baseline 2), only the smartphone-only condition showed a significant difference in speed compliance. Other conditions did not lead any significant differences. o This result may indicate that for the dynamic speed limit application, disseminating the information from the smartphone may not bring any behavioral benefits. ⢠Post-hoc contrast testing showed that there was no significant difference in percentage of speed compliance between modalities (V vs. AV) and information types (descriptive vs. prescriptive). It must be acknowledged that speed cues in the driving simulator are weak, thus speed perception and speed choice in the simulator may have limited validity. Figure 81. Percentage of speed compliance.
95 Figure 82. Speed exceedance. Glance measurements Glance measures for each condition are shown in Table 22. Overall, the smartphone-only mode led to longer total glance time and longer average duration compared to other conditions. However, all of the conditions were associated with relatively shorter average glance durations (around .5 seconds) to the smartphone compared to the AAM distraction criterion (2 seconds). As observed in Experiment 1, this may indicate that checking/receiving ATM information from the smartphone did not distract drivers at unacceptable levels. Also, long-duration glances (> 2 seconds) to the smartphone were seldom observed from our sample, and the average number of long-duration glances per participant was ranged from 0.1 to 0.3 across all conditions.
96 Table 22. Summary glance statistics for Experiment 2. Baseline 1 (smartphone only) Baseline 2 (gantry only) Visual information AV information (descriptive) AV information (prescriptive) Number of glances Mean (SD) 39.20 (20.70) 28.50 (21.30) 33.50 (21.10) 31.00 (20.90) 33.70 (21.10) Number of long glances Mean (SD) 0.27 (0.65) 0.08 (0.28) 0.38 (1.09) 0.19 (0.57) 0.22 (0.89) Total glance time Mean (SD) 22.00 (15.20) 14.30 (12.30) 17.40 (14.00) 15.40 (12.30) 16.50 (13.00) Mean glance duration Mean (SD) 0.54 (0.19) 0.49 (0.16) 0.51 (0.21) 0.49 (0.21) 0.48 (0.18) Maximum glance duration Mean (SD) 1.29 (0.57) 1.18 (0.55) 1.34 (0.74) 1.19 (0.63) 1.20 (0.53) The same set of the linear mixed-effect models was applied to two glance measures: total glance time and average duration of glances to the smartphone. In terms of total glance time (see Figure 83), ⢠Compared to the smartphone-only mode (baseline 1), all other conditions led to shorter total glance time to the smartphone (p < .001) ⢠Compared to the gantry-only mode, visual-descriptive mode led to longer total glance time to the smartphone (p < .05) o The difference between the gantry-only mode and the visual information mode was the addition of ATM information on the smartphone (i.e., in all conditions the smartphone provided at least navigation information). This may indicate that participants used the ATM information on the smartphones since the glance times are longer than when just navigation information was present. o However, when the ATM information was presented in AV modalities, there was no difference compared to the present-day application (i.e., gantry-only condition). This may indicate that the auditory message contributed to decreasing total glance time to the smartphone. ⢠Post-hoc contrast testing showed that there were no significant differences between total glance time in the visual information mode and any of the AV modes (p = .6 and p = .9, respectively) o Through Experiment 1 and Experiment 2, it was observed that visual demand of the ATM smartphone application was not close to the level of distraction in other human-machine interface (HMI) research. Therefore, it can be interpreted that the modality effect is not significant due to the visual demand being low.
97 o In addition, there was no significant difference between the two AV modes (p < .9). Figure 83. Total glance time to the smartphone across experimental conditions. In terms of average glance duration to the smartphone (see Figure 84), ⢠Compared to the smartphone-only mode (baseline 1), both AV modes were associated with shorter average glance duration to the smartphone (p < .05) ⢠Compared to gantry-only mode, none of the conditions led to significant differences in average glance duration ⢠Post-hoc contrast testing showed that there were no significant differences between the visual information mode and any of the AV modes (p = .9 and p = .6, respectively) ⢠In addition, there was no significant difference between the two AV modes (p < .9) Figure 84. Mean glance time to smartphone across experimental conditions.
