sec-4 6 8 10
Frequency
0 2
Subject brakes before first system action
Subject brakes before or with second system action
Subject brakes after second system action
Figure 4.13: Distribution of driver reactions in the first situation.
tion, the pedestrian cannot be controlled dependent on the movement of the car. As a consequence, the TTC at the curb shows some variation between the subjects.
The following reactions in this situation are possible in general:
• The driver perceives the pedestrian and reacts before the system does. In this case, the system is deactivated.
• The driver gets the optical pre-warning and reacts.
• The driver gets the pre-warning and the acoustical warning and reacts.
• The last possible reaction of the system, an automatic braking, was not observed in the experiment.
Key findings include the time of driver reaction with respect to the system action, the TTC at the activation of the brake pedal, the time between the acoustical warning and the activation of the brake pedal, and the maximum deceleration. In addition the perception of the different warning signals by the drivers was investigated.
Fig. 4.13 shows the distribution of driver reactions in the first situation. 8 of 19 drivers braked after the two warnings. How critical the situation became during the test is indi-cated by the number of actual collisions. Considering the first situation only,
• 2 of 19 drivers had a collision using the system and
• 2 of 18 drivers had a collision not using the system.
4.4 Driver behavior in highly critical situations
With system Without system
TTC [s]
2.50
2.00
1.50
1.00
0.50
0.00
Figure 4.14: TTC to the pedestrian at activation of brake pedal by the drivers.
These numbers indicate that there is no difference in the result due to the experimental conditions. The impact speed cannot be evaluated due to low number of collisions. The distance to the pedestrians in all avoided collisions is comparable for both conditions (non-significant difference in the mean, t =−0.20). As these results seem surprising given the design of the experiment and functions of the system, possible reasons are explained and discussed in the following.
The situation was constructed based on the relevance of real-world accidents (see Sections 3.2 and 4.2). The subjects were visually distracted by a secondary loading task, which was calibrated using expert knowledge from previous experiments. The whole tim-ing of the situation was optimized in a pre-test with several experts and dummy subjects.
Fig. 4.13 shows that 11 of 19 subjects reacted before the acoustical warning. Fig. 4.14 gives the TTC to the pedestrian at the activation of the brake pedal. Regarding the first oc-currence of the situation, the driver reacted on average a bit earlier with the system (1.55 s versus 1.44 s), but this difference in the mean is non-significant (t = −0.88). Fig. 4.15 gives the duration in time from the onset of the acoustical warning (at an earliest TTC of 1.50 s) and the activation of the brake pedal. As the maximum is 0.075 s, it is obvious that the driver reaction isno reaction to the acoustical warning. It must be concluded that the driver observed the pedestrian earlier during the test and decided to brake. Whether the reaction was triggered by the optical warning cannot be assessed with certainty. Although theoretically possible (as the optical warning is given at an earliest TTC of 2.0 s and so takes place 0.5 s before the acoustical one) it seems unlikely that this explains the moment of reaction of the driver, since only 5 subjects reported that they had observed the optical warning.
3 4 5 6 7 8
Frequency
0 1 2
5 15 25 35 45 55 65 75 85 95
Time [ms]
Figure 4.15: Time between acoustical warning and activation of the brake pedal by the drivers.
Considering the reaction of the drivers itself, the following can be stated. All drivers applied the brakes in reaction to the imminent danger. Some drivers also made minor changes in their lateral position. The brake reaction itself is of interest, as the system includes a brake assist, which gives the driver an acceleration of -10.0m/s2in case a warning was issued and the brake pedal was activated. Fig. 4.16 shows the maximum deceleration for the first situation for both experimental conditions. Regardless of the condition, nearly all drivers were able to realize a deceleration beyond 9m/s2; without system the mean was 10.39m/s2 (SD 0.60m/s2) and with system 10.07m/s2 (SD 0.63m/s2).
Accident statistics reveal that drivers do not realize the maximum possible deceleration even in accidents (3.85m/s2, SD 3.33m/s2). The accident statistics thus indicate that a brake assist, as implemented in this system, has a great potential (Fig. 4.17).
A second effect is connected to this phenomenon. The duration from beginning of brake pedal activation to the maximum deceleration is also non-significantly different considering the means (t = −2.01), see Fig. 4.18. The experiment does not reveal the expected effect of the preventive system regarding maximum deceleration and time for building up the maximum deceleration. A difference between driving simulator and real vehicle explains these effects. Drivers tend to avoid the hard braking maneuvers in reality even in accident situations, as they try to avoid high decelerations. Due to technical limitations, the driving simulator scales the decelerations experienced by the subjects (and all other accelerations as well) to a lower level, whereas the kinematic deceleration, as implemented in the vehicle dynamics model of the simulator, is realistic. As a consequence, drivers tend to realize far higher decelerations in the simulator than they would in reality. Due to these circumstances, the results regarding deceleration obtained in this experiment cannot be used directly for the evaluation of the effects of the preventive pedestrian protection system or as input for a driver model as needed in simulations.
4.4 Driver behavior in highly critical situations
With system Without system
Acceleration [m/s²]
-6.00
-7.00
-8.00
-9.00
-10.00
-11.00
-12.00
Seite 1 Figure 4.16: Maximum deceleration in the first situation, stratified by experimental conditions.
Circles indicate near outliers and stars far outliers.
Driver reactions in accidents (GIDAS)
Acceleration [m/s²]
2.50
0.00
-2.50
-5.00
-7.50
-10.00
Figure 4.17: Mean deceleration in pedestrian accidents [10].
77
With system Without system
Time between activation of the brake pedal and maximum deceleration [s]
1.40 1.20 1.00 0.80 0.60 0.40 0.20
Seite 1
Figure 4.18: Time from beginning of brake pedal activation to maximum deceleration.
The basic idea behind this situation was to present a realistic pedestrian accident scenario (crossing from the right, no visibility obstruction, urban setting, daylight, ...) in the driving simulator. The drivers went through a long period of driving without any special event, were not informed about the research questions, and were visually distracted by a secondary task during the highly critical situation. The objective for this situation was to test whether an accident situation can be reproduced in a stable way in such an experimental environment. The difference to other studies is that not a critical situation but an accident situation should be created. As the results show, it was not possible to bring the subjects reliably and repeatably into the critical accident situation (TTC at braking should have been at least below 1.0 s TTC) as only a few accidents did happen under baseline conditions.
Several possible explanations and ideas for further studies have been developed. The subjects probably found a strategy to work on the secondary task and react properly to the traffic situations. A possible design change would be to further increase the level of distraction. The experimental situation itself, i.e., taking part in an experiment in the driving simulator, could additionally have influenced the subjects as well as the perception of the environment and the traffic situation in the simulator. Another technical possibility would be a visibility obstruction of the pedestrian. This deviates from the realistic scenario as described in Section 3.2, but could help bring the subject into the accident situation and thus be an experimental necessity.