Chapter 5 Subjective Rating and Secondary Task Performance
5.5 Secondary Task Performance (STP)
5.5.2 Visual Task Performance
The visual task performance was assessed by several parameters; the Reaction Time (RT) to each image, which is the average of the time lags from the appearance of each new image to the first correct click on the touch screen (i.e. click on the “target”); the Standard Deviation of Reaction Time (SDRT), which is the deviation of RT for each individual visual task; the Error Percentage (ErP) which describes the percentage of mistaken clicks in each task; and the Error Distance (ErD) which is the mean distance from the nearest “target circle” when a
missing-target click occurs. All the visual task performance data were explored first, in order to decide later which statistical test should be used. The data of RT and ErD were heavily skewed, because there is a natural under limit - for example, Reaction Time cannot lower than a certain value for human beings, which is why the RT data distribution is skewed to the right. Figure 5.8 shows the data distribution of RT when performing the visual tasks in Car-Following, where the red line shows the fitness curve for a lognormal distribution (p > 0.05).
Figure 5.8 The distribution of Reaction Time when performing the visual tasks in CF
The RT and SDRT had equal variances across two scenarios, while unequal variances were found for ErP and ErD. Taking the distribution of each parameter into account, for RT and SDRT, the results from Mann-Whitney U Test were reported; while for ErP, ErD and Trans_ErD, the Welch Tests were conducted. The results showed that RT, SDRT and ErP measures differed significantly between the two test scenarios, with longer RT and higher ErP in the Free-Driving Scenario. Figure 5.9 shows RT in the two scenarios. The analysis was then conducted separately in two conditions: Car-Following (CF) and Free-Driving (FD).
Figure 5.9 Reaction Time at different task levels – compared between Scenarios 5.5.2.1 Secondary Task Performance in Car-Following
5.5.2.1.1 Task Level
In Car-Following, the reaction time increased with the task difficulty ( Chi2 (2) = 13.554, p = 0.001. The Post Hoc analysis suggested that drivers had a longer reaction time in task level 3 (M = 2.28, 95% CI [2.08, 2.47]) than that in task level 1 (M = 2.82, 95%CI [2.57, 3.07]). It was also observed that SDRT in level 1 was significantly higher than that in levels 2 and 3. No statistically significant difference across the task levels on ErP and ErD was found (p>0.05), similar with the findings in HASTE (Engström, Johansson & Östlund, 2005). The RT, SDRT, ErP and ErD across all three levels are illustrated in Table 5.8.
Table 5.8 Visual task performance in Car-Following compared between levels
Task Level RT (s) SDRT ErP ErD
Mean 2.28 1.08 12.91% 8.83 1 SD 0.99 0.70 12.06% 12.33 Mean 2.45 0.74 11.82% 9.86 2 SD 1.18 0.63 9.63% 13.08 Mean 2.82 0.91 11.64% 8.74 3 SD 1.29 0.82 11.32% 12.95
5.5.2.1.2 The Effects of Gender, Age, Experience and Mileage 5.5.2.1.2.1 Gender
On average, compared to their female counterparts, the male drivers reacted faster to the visual secondary tasks by 23.29% (RT for male: 2.34s; RT for female: 2.89s), with a 29.81% higher percentage of making mistakes (ErP for male: 13.42%; ErP for female: 9.42%) in
Car-Following (p < 0.05). This effect mainly existed in ErP in level 1, and in RT and ErP in level 3 (p < 0.05), although this was not significant. No gender effect was found for either SDRT or ErD. Table 5.9 demonstrates the RT and ErP compared between male and female drivers at different task levels.
Table 5.9 Visual task performance in Car-Following compared between gender
Task Level Gender RT SDRT ErP ErD
Male 2.21 1.09 14.84% 9.46 Female 2.41 1.04 8.86% 7.53 F 0.691 0.106 6.875 0.542 df 1, 45.996 1, 100 1, 79.763 1, 100 1 p-value 0.410 0.745 0.010** 0.463 Male 2.26 0.69 12.23% 10.87 Female 2.85 0.85 10.98% 7.78 F 3.792 1.244 0.376 1.241 df 1, 39.375 1, 49.714 1, 99 1, 99 2 p-value 0.59 0.270 0.541 0.268 Male 2.55 0.84 13.18% 9.04 Female 3.39 1.05 8.42% 8.13 F 6.944 1.526 5.731 0.109 df 1, 40.979 1, 100 1, 95.248 1, 100 3 p-value 0.012** 0.220 0.019** 0.742 Male 2.34 0.87 13.42% 9.78 Female 2.89 0.98 9.42% 7.81 F 10.114 1.380 1, 250.534 1.594 df 1, 126.280 1, 175.082 11.034 1, 303 Total p-value 0.002 0.261 0.001 0.869
Note: 1. the Diff in the table was calculated by:
(value from female – value from male) / value from male. 2. ** denotes a significant effect, where p<0.05.
5.5.2.1.2.2 Driving Experience, Age and Annual Mileage
Results showed that the driver’s age and experience were sensitive to almost all parameters of the visual task. The general trends can be described as following: the most experienced driving Group (#3) had the shortest reaction time, but made more mistakes (SNK and Tukey HSD, p < 0.05). This could be due to their higher skill and a more relaxed attitude, as suggested by their Post-Driving Interviews, in which they said they prioritised driving over the secondary tasks, and only performed them when they could. Experience Group #2 showed the best secondary task performance, with shorter reaction time, fewer mistakes and less error distance. The drivers with less than 10 years’ experience had slow reaction to the visual tasks (10.36%
slower than Experience Group #3), and they also made mistakes to a larger extent (22% larger than Experience group #3), see Table 5.10.
Table 5.10 Secondary task performance for different experience groups in Car-Following Experience Group RT SDRT ErP ErD
# 1 2.66 1.06 0.12 10.88
# 2 2.38 0.67 0.09 6.31
# 3 2.43 0.92 0.15 9.40
Age Group #3 had a much shorter RT than Age Group #2, a higher ErP and higher ErD. Age Group #2 showed a much longer RT, but lower ErP and ErD. Age Group #1 had a medium RT, lower ErP and higher ErD. The impact of Annual Mileage showed a similar trend, with the Group having the least mileage showing longer RT but lower ErP, while the Group with most mileage showed the fastest reaction, but higher ErP (SNK and Tukey HSD, p < 0.05).
5.5.2.1.3 Experience of, and Attitude towards, Using Manual IVISs
The group of drivers who had more experience in using manual IVISs had a lower ErP (ErP for more experienced IVIS group: 9.36%; ErP for less experience IVISs group: 12.75%). This suggests that previous experience of using manual IVISs improves the accuracy
(F (1, 303) = 7.273, p = 0.007). This group also showed a lower SDRT (F (1, 303) = 4.790, p = 0.029). Driver’s attitude towards manually using IVISs while driving showed that drivers who were more against using manual IVISs (Group #3) had short RT to the secondary tasks (F (2, 302) = 3.539, p = 0.030), with more mistakes (F (2, 302) = 6.879, p = 0.001) than the other two groups. The effects of experience and attitude on manual IVISs in Car-Following on visual task performance are listed in Table 5.11.
Table 5.11 STP by groups of experience and attitude on manual IVISs in Car-Following Expe_Visu ErP SDRT Atti_Visu RT SDRT ErP
Group #1 9.36% 0.87 Group #1 2.64 0.88 11.08% Group #2 12.75% 1.05 Group #2 2.81 1.02 8.39%
Group #3 2.33 1.04 14.65%
5.5.2.2 Visual Task Performance in Free-Driving 5.5.2.2.1 Task Level
In the Free-Driving Scenario, the task level has significant effects on RT (F (2, 294) = 4.781, p = 0.009) and SDRT (Chi2 (2) = 8.587, p = 0.014), but has no effect on ErD (F (2, 294) = 2.011, p = 0.136) or ErP (F (3, 294) = 0.648, p = 0.524). The Post Hoc analysis revealed that RT in task level 3 was longer than that in levels 1 and 2, and SDRT in level 1 was higher than that in
levels 2 and 3 (Tukey HSD Test, p < 0.05). The value of visual task performance for each level is shown in Table 5.12.
Table 5.12 Visual task performance for each level in Free-Driving Task Level RT(s) * SDRT* ErP ErD
Mean 2.64 1.25 17.17% 15.96 1 SD 1.07 0.79 14.45% 23.02 Mean 2.71 1.01 15.05% 12.82 2 SD 1.06 0.74 12.30% 16.52 Mean 3.10 1.00 15.60% 10.18 3 SD 1.20 0.74 13.85% 14.05 Note: * denotes that a significant effect exists, where p < 0.05.
5.5.2.2.2 The Effects of Gender, Age/Experience and Annual Mileage on STP in Free-Driving 5.5.2.2.2.1 Gender Effect of STP in Free-Driving
Both generally and in each task level, there was no significant gender effect on any of the measurements in the Free-Driving Scenario, but the results showed a similar trend with that in the Car-Following, i.e., female drivers had a longer RT, lower ErP, and less ErD.
Table 5.13 Visual task performance in Free-Driving compared between gender
Gender RT(s) SDRT ErP ErD
Mean 2.79 1.11 16.59% 14.27 Male SD 0.99 0.80 14.06% 20.54 Mean 2.87 1.03 14.52% 10.16 Female SD 1.39 0.69 12.29% 11.86 Diff 2.87% -7.21% -12.48% -28.80%
5.5.2.2.2.2 Experience/Age Effect in Free-Driving
In Free-Driving, the effect of driving experience STP showed similar findings with that in Car-Following. RT steadily decreased with the increased experience, especially for the
experience group #1, who had a much longer RT (F (2, 292) = 3.705, p = 0.026). Again higher ErP (F (2, 292) = 21.682, p < 0.001) was also found for the most experience group (Group #3). Experience group #2 demonstrated a relatively short RT, the lowest ErP to the lowest extent (ErD), still showing the best STP in all three groups. For details, refer to Table 5.14.
Again, the same as that in Car-Following, the age group # 1 had the shorter RT
(F (2, 292)=10.153, p < 0.001) and a lower ErP (F (2, 292) = 7.483, p = 0.001). The age group #2 showed a longer RT and a lower ErP. Age Group #3 reacted the quickest to the secondary
task, but with higher ErP. No difference was found for the age effects on SDRT and ErD (Kruskal Wallis Test p>0.05), see Table 5.14.
Table 5.14 RT, SDRT, ErP and ErD with different age and experience groups in Free-Driving Experience RT(s) SDRT ErP(%) ErD Age RT(s) SDRT ErP(%) ErD
#1 2.92 1.19 17.39 14.89 #1 2.66 0.99 14.28 11.30
#2 2.83 0.94 10.96 9.59 #2 3.17 1.17 14.54 13.20
#3 2.62 1.08 18.98 13.53 #3 2.61 1.09 19.02 14.43
In Free-Driving, Mileage Group #3 showed a shorter RT (F (2, 292) = 7.130, p = 0.001) and higher ErP (F (2, 293) = 4.523, p = 0.012). This is consistent with that found in Car-Following.
5.5.2.2.3 Effect of Experience and Attitude towards Manual IVISs on STP in Free-Driving Same with that in Car-Following, it was found that previous experience of manually adjusting IVISs improves the task accuracy (ANOVA Test, F (1, 294) = 14.719, p < 0.001). The group of drivers who had more experience in using manual IVISs had a lower ErP (ErP for more
experienced IVIS group: 12.07%; ErP for less experience IVISs group: 18.10%). Driver’s attitude towards manually using IVISs while driving showed that drivers who were more against using manual IVISs had short RT to the secondary tasks (Trans_RT F (2, 292) = 3.858,
p = 0.022) than the other two groups (RT for Group #1: 2.96s; Group #2: 2.99s; Group #3: 2.62s). The performance of visual tasks by different groups of experience and attitude on manual IVISs in Free-Driving is illustrated in Table 5.15.
Table 5.15 STP by different groups of experience and attitude on manual IVISs in Free-Driving Expe_Visu RT SDRT ErP(%) ErD Atti_Visu RT SDRT ErP(%) ErD
Group # 1 2.86 1.05 12.07 13.69 Group # 1 2.96 1.05 16.57 13.87 Group # 2 2.92 1.21 18 .10 17.11 Group # 2 3.00 1.13 12.24 17.00 Group # 3 2.62 1.27 16.31 16.20