Drivers’ Individual Difference in the Baseline Driving

background information were required on how different drivers’ characters (e.g. gender, age/experience etc.) affect the driving performance in baseline driving, i.e., the normal driving without doing any additional secondary task. The One-Way ANOVA indicates that the driving performance varies enormously between the two driving scenarios – Car-Following and Free-Driving. Therefore the analysis was performed in two sets: Car-Following and Free-Driving.

6.4.1 Baseline Driving in Car-Following Scenario

6.4.1.1 Gender Difference

In baseline driving from Car-Following Scenario, some gender differences were found, mainly existing in speed and headway choices. Female drivers kept a slightly lower speed, a shorter

MNHW, a shorter MinTHW,and lower SDTHW (p< 0.05) in baseline driving, see Table 6.4. The “Diff (%)” column shows the difference between female and male, in percentage, which was calculated by the following formula:

mean(Female) mean(Male) Gender Diff mean(Male) − = (6.6)

Table 6.4 Gender difference in driving performance in baseline Car-Following

Gender MNSP* SDSP RR1.5 RR3 SE MNHW* SDTHW* MinHW* Mean 85.46 2.32 20.32 7.42 0.56 51.18 0.67 44.32 Male SD 5.96 1.94 10.83 7.98 0.12 22.03 0.37 20.74 Mean 82.44 1.76 21.58 6.69 0.56 42.66 0.52 37.29 Female SD 4.89 1.27 11.41 7.04 0.12 14.72 0.17 13.83 Diff (%) -3.53 -24.14 6.20 -9.84 0.00 -16.65 -22.39 -15.86 -3.53 Note: * indicates a significant difference existing between the three groups.

6.4.1.2 Experience and Age

No significant difference of speed choice was found between three experience groups. The results from ANOVA revealed that measurements of lane keeping, Reversal Rate, and RR3, SE, and Headway parameters (MNHW, MNTHW, ,MinHW, and MinTHW) by experience groups is statistically significant (p < 0.05). The Tukey Test for the Post Hoc analysis provided some more detailed information:

1. Experience Group #1 had a remarkably higher SDLP (indicating worse lane control), more per minute steering wheel adjustments (both RR1.5 and RR3), higher SE (indicating a less predictability in steering behaviour), and a slightly lower MinTHW, comparing with Experience Groups #2 and #3. It is noteworthy that they made more steering adjustments, but led to a worse lane control, also, slightly shorter headways. 2. Experience Group 2 kept a much longer distance and time headway. Drivers from this

group also kept the longest minimum distance and time head way.

3. Except SDHW and SDTHW, all headway measurements were sensitive to experience.

Table 6.5 illustrates the difference on driving performance between the three experience groups. It may be seen that the least experienced drivers showed less capability than other drivers, even in baseline driving.

Table 6.5 Effect of experience on driving performance in baseline Car-Following

#1 #2 #3

Experience Group Mean SD Mean SD Mean SD

Diff 1 & 3 MNSP (kph) 84.48 4.58 84.35 5.68 84.31 7.09 0% SDSP (kph) 2.33 1.76 1.86 1.34 2.04 1.95 14% SDLP (m)* 0.18 0.08 0.14 0.08 0.13 0.07 33% RR1.5 (times/min)* 25.13 12.42 17.95 8.54 17.3 8.74 45% RR3 (times/min)* 10.16 9.76 4.83 4.58 5.03 4.54 102% SE* 0.59 0.13 0.54 0.1 0.53 0.11 12% MNHW (m)* 42.32 17.38 61.25 19.81 47.46 20.06 -11% MNTHW (s)* 1.94 0.31 2.14 0.27 2 0.2 -4% MinHW (s)* 36.23 15.96 54.3 19.41 41.22 18.49 -12% MinTHW (s)* 1.09 0.29 1.37 0.17 1.26 0.25 -14% Note: * indicates a significant difference existing between the three groups.

* Diff 1 & 3 shows the difference between Group #1 and Group #3 in percentage calculated by: (Value for #Group1 - Value for #Group 3)/ Value for #Group1. The differences between age groups had a similar trend with that of the experience group, i.e., older drivers (Group #3) shows less minor steering wheel turns, while the younger drivers showed the highest steering entropy (p < 0.05); Group #2 kept a longer distance and time headway, but this effect was not significant. The effect of age is less noticeable than that of experience. This may suggest that for the current sample, the experience factor has a higher impact than that of age.

6.4.1.3 Annual Mileage

Drivers with the least annual mileage showed significantly different driving performance. The ANOVA and Tukey Post Hoc Tests (p < 0.05) suggested that less annual mileage (Annual Mileage Group #1) showed a remarkably higher lane deviation, more large steering wheel turns, but smaller headway, MinHW, MinTHW than that of Group #2 and Group #3 (p < 0.05). Similar to the effect of Experience, the group with the least annual mileage showed less

capability than other drivers, even in baseline driving. For more detail, see Table 6.6.

Table 6.6 Effect of annual mileage on driving performance in baseline Car-Following

# 1 # 2 # 3

Mileage Group Mean SD Mean SD Mean SD MNSP 84.14 5.58 84.64 4.43 84.38 6.89 SDSP 2.15 1.65 2.39 1.76 1.87 1.80 SDLP * 0.19 0.08 0.14 0.08 0.13 0.07 RR1.5 22.25 11.28 22.51 11.62 18.15 9.94 RR3 * 10.09 8.54 6.55 7.91 5.38 5.95 SE 0.57 0.11 0.57 0.14 0.55 0.11 MNHW * 37.08 19.90 55.66 17.33 50.88 18.72 MNTHW 1.84 0.25 2.16 0.27 2.01 0.24 MinHW * 31.50 17.72 47.60 16.12 45.32 18.52 MinTHW * 0.98 0.28 1.31 0.18 1.30 0.23

6.4.2 Baseline Driving in Free-Driving Scenario

As the Free-Driving Scenario was designed to create a condition where drivers could control their speed without interacting with leading vehicles, all data were collected when there was no leading vehicle which could impact the speed choice of the IV. Hence the headway data were not meaningful in this scenario. Only the MNSP, SDSP, SDLP, RR1.5, RR3, and SE data were analyzed and reported in the Free-Driving.

6.4.2.1 Gender Difference

For baseline driving in Free-Driving, female drivers showed a lower SDSP, lower RR1.5 (both p < 0.05) and marginally lower SE (p = 0.058), suggesting a more stable vehicle control, see Table 6.7.

Table 6.7 Gender difference in driving performance in baseline Free-Driving

Male Female df F p_value

Mean SD Mean SD MNSP 82.09 5.52 82.30 3.55 1,85 0.05 0.83 SDSP* 8.10 3.80 6.25 3.50 1,85 5.45 0.02 SDLP 0.18 0.08 0.18 0.08 1,85 0.03 0.86 RR1.5* 30.32 11.60 25.66 17.34 1,85 2.25 0.14 RR3 9.59 7.38 8.97 10.11 1,85 0.11 0.74 SE* 0.66 0.09 0.62 0.12 1,85 3.69 0.06

6.4.2.2 Experience/Age and Mileage

Steering behaviour was significantly different between three experience groups (RR1.5, RR3, p < 0.05). Post Hoc analysis (Turkey HSD Test) confirmed that the most experienced drivers had a lower number of steering wheel adjustments per minute (RR1.5 and RR3), and a better predictability of steering wheel adjustments (lower SE). For more details, see Table 6.8.

Table 6.8 Effect of experience on driving performance in baseline Free-Driving

# 1 # 2 # 3

Experience Group

Mean SD Mean SD Mean SD

Diff 1 & 3 MNSP 81.17 3.4 81.36 3.58 83.88 6.16 -3% SDSP 7.76 3.12 7.58 3.44 6.57 4.57 15% SDLP 0.19 0.1 0.18 0.07 0.18 0.08 5% RR1.5 * 33.1 17.55 29.42 9.16 22.09 11.37 33% RR3 * 12.68 10.75 8.5 4.65 6.09 6.61 52% SE * 0.66 0.12 0.67 0.09 0.61 0.08 8%

Note: * indicates a significant difference existing between the three groups.

* Diff 1 & 3 shows the difference between Group #1 and Group #3 in percentage calculated by: (Value for #Group1 - Value for #Group 3)/ Value for #Group1.

Similar effects on steering behaviour were found between the three age groups. In detail, Age Group #3 had a lower RR1.5, lower RR3 and a lower SE (ANOVA, p < 0.05 and Tukey Post Hoc Tests, p < 0.05).

The drivers’ annual mileage had significant effect on some measurements for driving performance in Free-Driving Scenario. SDSP, SDLP, RR1.5 and RR3 were found to be different between three mileage groups (p < 0.05). Post Hoc analysis confirmed that drivers with the least annual mileage (Annual Mileage Group #1) had a much higher lane deviation, while the most experienced group (Annual Mileage Group #3) showed lowest SDSP, and steering wheel turns (RR1.5 and RR3).

6.4.3 Summary for Driving Performance in Baseline Driving

In the Car-Following Scenario, female drivers kept a lower speed, shorter headway and minimum time headway, and a lower deviation in time headway. Drivers with less experience (driving experience less than 10 years) showed a remarkably larger deviation in lane position than the other two groups, more adjustments of steering behaviour, suggesting a worse lateral control by the less experienced drivers. This was also confirmed by higher steering entropy from this group, which indicates less predictability of steering behaviour. Drivers with “medium experience” (driving experience from 11 to 20 years) kept significantly longer headway (and minimum headway) and time headway (and minimum time headway). The effect of age showed a similar tendency, but was less significant. Results from mileage groups confirmed the effect of experience, where the least annual mileage group showed a remarkably higher lane deviation, more large steering (RR3) wheel adjustments (suggesting a more abrupt steering turns); and showed the relatively smaller headway, despite the poor steering performance.

In the Free-Driving Scenario, female drivers had a lower speed deviation, lower small wheel adjustments and marginally lower steering entropy, suggesting a more stable vehicle control. Experience, age, and annual mileage showed similar tendencies, i.e. the most experienced, higher age and higher annual mileage group made less steering wheel adjustments and lower steering entropy; while the younger drivers who have less experience showed a much higher steering wheel adjustments and less predictability in their steering. Especially, the lowest annual mileage group showed a significantly higher deviation in lane keeping.

It was therefore concluded that in both driving scenarios, female drivers showed a more stable vehicle control (i.e. lower headway deviation in Car-Following, and lower speed deviation in Free-Driving), compared to male drivers. And less experience/age groups showed higher

steering wheel adjustments, their steering wheel behaviour was less predictable, and they had higher lane deviation, compared to more experienced/higher age drivers.