**5.1. Time of action and reaction**

Time of action was determined by analysing the signal regarding pedals (brake and accelera‐ tor) and steering wheel. Every effort was put to identify the first action caused by the perception of a hazard. Approaching the obstacle an action on the brake was always detected; in the cases of scenarios with ADAS, sometimes the action on the accelerator was detected before the alarm was issued, mainly because the driver was prudently reducing the speed approaching an intersection with some traffic, so that the first action was considered braking; this happened 11 times out of 30.

Since the moment when the driver saw the pedestrian cannot be determined, it was not possible to obtain a reaction time following such event. In the cases of scenarios with ADAS, reaction time was here defined as the time interval between the alarm and the first successive action (on accelerator or brake); its mean value resulted to be 0.88 s.

An indication about the degree of emergency perceived by the driver can be the time difference between the first (releasing the accelerator) and the second action (pressing the brake pedal) *tA2* − *tA1*; the comparison between different scenarios, as well as the overall scenarios with ADAS, is shown in **Figure 5**. The *t*-test showed that there is no significant difference among the scenarios with ADAS (*P*-value > 0.5), whereas these last are significantly different from scenario A (no ADAS, *P*-value < 0.05).

**Figure 5.** Mean values of *tA2* − *tA1* ± SD.

#### **5.2. Time to collision**

Time to collision [15] is the time at the end of which a collision will occur if speed does not change; in this study it was evaluated at the moment of the first action (release of accelerator) and the second action (braking). In **Figures 6** and **7**, mean values of both parameters are shown together with their dispersion. Scenario A yielded a significantly lower value for *ttcA1* (1.46 s vs. 2.52 s, *P*-value < 0.0001) as well for *ttcA2* (1.34 s vs. 1.92 s, *P*-value < 0.01) in comparison to all the scenarios with ADAS.

**Figure 6.** Mean values of *ttcA1* ± SD.

**5.1. Time of action and reaction**

scenario A (no ADAS, *P*-value < 0.05).

**Figure 5.** Mean values of *tA2* − *tA1* ± SD.

**5.2. Time to collision**

11 times out of 30.

92 Autonomous Vehicle

Time of action was determined by analysing the signal regarding pedals (brake and accelera‐ tor) and steering wheel. Every effort was put to identify the first action caused by the perception of a hazard. Approaching the obstacle an action on the brake was always detected; in the cases of scenarios with ADAS, sometimes the action on the accelerator was detected before the alarm was issued, mainly because the driver was prudently reducing the speed approaching an intersection with some traffic, so that the first action was considered braking; this happened

Since the moment when the driver saw the pedestrian cannot be determined, it was not possible to obtain a reaction time following such event. In the cases of scenarios with ADAS, reaction time was here defined as the time interval between the alarm and the first successive action

An indication about the degree of emergency perceived by the driver can be the time difference between the first (releasing the accelerator) and the second action (pressing the brake pedal) *tA2* − *tA1*; the comparison between different scenarios, as well as the overall scenarios with ADAS, is shown in **Figure 5**. The *t*-test showed that there is no significant difference among the scenarios with ADAS (*P*-value > 0.5), whereas these last are significantly different from

Time to collision [15] is the time at the end of which a collision will occur if speed does not change; in this study it was evaluated at the moment of the first action (release of accelerator) and the second action (braking). In **Figures 6** and **7**, mean values of both parameters are shown

(on accelerator or brake); its mean value resulted to be 0.88 s.

**Figure 7.** Mean values of *ttcA2* ± SD.

Among the scenarios with ADAS, as expected, there were significantly higher values for scenario B as compared to scenario C as regards *ttcA1* (2.87 vs. 2.32 s, *P*-value = 0.06) and above all as regards *ttcA2* (2.13 vs. 1.46 s, *P*-value = 0.03). No significant difference was found between scenarios C and D.

#### **5.3. Braking**

Braking represented, for 27 drivers out of 29, the first active actions attempted by successful drivers (as said, only two avoided the obstacle by steering only) since releasing the accelerator pedal, in itself, has a little effect on speed reduction. Braking is one of the actions that showed differences throughout the experimentation. In **Figure 8**, for instance, some typical modes of actions on the brake pedal are shown; they are related to the emergency braking approaching the pedestrian during four tests, two successful and two failed.

**Figure 8.** Typical emergency brake pedal activation; tests 28 and 41 were successful, and tests 26 and 71 were failed.

The main difference lies in the different time that the driver used to reach the maximum pedal activation. In order to characterise such difference, the parameter *t*max – *tA2* is introduced, as the time difference between the beginning of the braking activation and the instant when the maximum action is reached. Such value could not be calculated for all the tests since in some failed tests the collision happened before the braking action reached a stabilised level. As shown in **Figure 9**, where *t*max – *tA2* is plotted as a function of time to collision, a clear trend is visible, indicating that when *ttc* decreases, the action on the brake tends to be faster.

**Figure 9.** Trend of *t*max − *tA2* as a function of time to collision.

all as regards *ttcA2* (2.13 vs. 1.46 s, *P*-value = 0.03). No significant difference was found between

Braking represented, for 27 drivers out of 29, the first active actions attempted by successful drivers (as said, only two avoided the obstacle by steering only) since releasing the accelerator pedal, in itself, has a little effect on speed reduction. Braking is one of the actions that showed differences throughout the experimentation. In **Figure 8**, for instance, some typical modes of actions on the brake pedal are shown; they are related to the emergency braking approaching

**Figure 8.** Typical emergency brake pedal activation; tests 28 and 41 were successful, and tests 26 and 71 were failed.

The main difference lies in the different time that the driver used to reach the maximum pedal activation. In order to characterise such difference, the parameter *t*max – *tA2* is introduced, as the time difference between the beginning of the braking activation and the instant when the maximum action is reached. Such value could not be calculated for all the tests since in some failed tests the collision happened before the braking action reached a stabilised level. As shown in **Figure 9**, where *t*max – *tA2* is plotted as a function of time to collision, a clear trend is

visible, indicating that when *ttc* decreases, the action on the brake tends to be faster.

the pedestrian during four tests, two successful and two failed.

scenarios C and D.

**5.3. Braking**

94 Autonomous Vehicle

As shown in **Figure 10**, *t*max – *tA2* is also influenced by the presence of ADAS; scenarios with ADAS yielded significantly higher values than scenarios without ADAS (in average 2.03 s vs. 1.09 s, *P*-value < 0.001). As a consequence, part of the advantage afforded by ADAS devices (seen above, for instance, in terms of time to collision) is wasted because of a slower action on the brake pedal; trials that ended with a collision (indicated by a cross) are in some cases characterised by relatively high *t*max – *tA2*, indicating that a different braking approach could sometimes help avoiding the collision.

**Figure 10.** Cumulated distribution for *t*max − *tA2* in tests with and without ADAS; the cross indicates a failed test.

#### **5.4. Actual degree of emergency**

The parameter *ADE* introduced above can provide indications on the degree of emergency (meant as the urgency to react) of a given hazardous situation, but also indicates if a manoeuvre based on braking only can be successful, since the deceleration that a vehicle can experience is limited by the friction available.

In **Figure 11**, the cumulated distributions of actual degree of emergency corresponding to the action on the brake are shown, for failed and successful trials. A statistically relevant difference was identified between the two samples (in average 8.78 m/s2 for the failed tests vs. 3.89 m/s2 for the successful tests, *P*-value < 0.001). It is evident that it is impossible to stop before the collision when having *ADE* values near or greater than the maximum possible deceleration. Actually, the maximum value of *ADEA2* that allowed a successful manoeuvre only acting on brakes was equal to 5.89 m/s2 ; the cases with higher ADE (tests 31 and 34, highlighted in **Figure 11**) were successful only because a steering manoeuvre was performed.

**Figure 11.** Cumulated distribution for *ADEA2* in failed and successful tests. The cross indicates trials in which the driver avoided the obstacle by steering instead of braking (tests 31 and 34).
