Adaptive Cruise Control (ACC) – Objective Metrics and Performance (Post 3 of 3)

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TROY, Mich. — This is the final of three articles by Bernat Ferrer, Manager Chassis and Active Safety at Applus IDIADA U.S. on the topic of Adaptive Cruise Cotnrol.

Adaptive Cruise Control (Acc) – Objective Metrics and Performance (Post 1 Of 3)

Adaptive Cruise Control (Acc) – Objective Metrics and Performance (Post 2 Of 3)

This study, based on the longitudinal braking actuation of the Adaptive Cruise Control (ACC) function, aims to match up both objective and subjective characterizations of this system, in order to understand the potential of a tool that could reduce the development loops based on subjective assessments. 

The first part of this article introduced the problem, while the second part focused on the methodology —test scenarios, subjective and objective analyses. This third and last article presents the main results and conclusions.

  • Results

The main outputs obtained during this study come from the comparison and association of the parameters described in the previous methodology section. The aim is to evaluate the trends obtained by comparing each pair and to understand the validity of its objectivization. The most relevant and interesting combinations are presented in this results section.

Starting by the first phase, the braking beginning, the two metrics generated are compared to the subjective concepts described. The outputs show an interesting linearity between the two fields:

Figure 5: Subjective vs. objective – Braking progressivity

On the graph above, the application progressivity is plotted out, meaning the slope of the deceleration ramp while starting to brake (jerk). The graph shows how the driver perception improves when this jerk is higher, whereas it is worse when the ramp-up is very smooth. This can be translated into a safety feeling that the system is providing to the driver, who prefers a higher vehicle response and braking input.

On the other hand, the following graph is more focused on the comfort side:

Figure 6: Subjective vs. objective – Rebounds

It represents the linearity error of the system while increasing the deceleration. In this case, the positive feeling perception is higher while lower is this error, meaning that the driver will be more satisfied when the deceleration ramp applied by the system is more linear and without oscillations or rebounds.

The second phase, corresponding to the braking quality (stabilized state of the braking application) is also showing promising results. When it comes to the parameters relative to the target vehicle, the metrics defined evaluates how the vehicle under test is reacting to the front vehicle behavior:

Adaptive Cruise Control
Figure 7: Subjective vs. objective – Relative to target

The graph above is indicating a clear trend of the driver’s reaction to the deceleration difference between both cars, relative to the target; independently to the deceleration achieved by the front vehicle, when higher is this difference, lower is the subjective perception, as the vehicle is not accurately following or responding to the front maneuvers. As on the previous braking phase, this fact can be related to safety feeling reasons, as any incoherent behavior with respect to the front vehicle also implies a change on the distance and time to collision parameters; and also to comfort, as the systems is not being completely efficient and probably the duration of the braking and the deceleration oscillations can be more present with high values of this metrics.

Until now, the comparison of the factors have shown acceptably linear tendencies. However, there are also some of them that do not show this same behavior. Such is the case of the absolute deceleration target level when considering this same steady state phase of the braking:

Figure 8: Subjective vs. objective – Braking target level

For this specific case, the graph doesn’t provide any clear trend, which means that on the different scenarios tested, between the different levels of deceleration reached (2 m/s2 to 4 m/s2), they do not influence explicitly the driver’s subjective perception.

The last phase of the analysis considers the braking ending. When applying the same safety metrics approach (deceleration slope or jerk), the linear trend is also clear for this case:

Adaptive Cruise Control
Figure 9: Subjective vs. objective – Braking ending

However, in this occasion the cause seems to be more related to comfort reasons. From the safety point of view, the braking application is mainly based on the first two phases: deceleration rise, target deceleration obtention and time to collision or distance control. The last phase (braking ending) is nevertheless more focused on the braking release, looking for smooth and comfortable feelings. That is why the trend of the Figure 8 shows an opposite trend as in the first phase (Figure 5, with better driver perception for lower values of jerk): the user seems to be less disturbed with a softer torque release producing a slower deceleration ramp-down slope.

  • Conclusions

After the study completion, the results have globally shown interesting relationships between the driving subjective evaluations and the objective characterization in the same vehicles. All in all, the metrics defined and the values appearing in the stablished comparisons, represent appropriate methods of both characterizing and even calibrate the systems during some early or mid stages of its development.

To sum up, the main points that can be extracted from the overall study are as follows:

  • The division of the braking application in the three separate “steps” (beginning, steady-state and ending) seems to be accurate for establishing a good subjective-objective comparison base: reduced enough to subjectively evaluate many parameters of each phase, but also sufficiently varied to generate a certain amount of metrics.
  •  Within the three braking phases division, the one that provides a more robust relationship between subjective and objective concepts is the first one (braking beginning).
    • On the other hand, this also indicates that there are some differentiating factors in this phase that produce a change on the driver subjective reaction. The work and refinement of this braking ramp-up parameters can be the key to achieve good user feeling perception.
  • During the middle braking phase (steady-state), the reference with respect the vehicle in front is clearly the key aspect. The absolute values of deceleration or braking levels do not represent especially determining results, but the relative deceleration with respect to the target vehicle and actuating with similar braking response are clearly significative.
  • Both safety and comfort matters are present within the complete braking actuation, being the safety one especially important on the initial and middle phases. The last phase (braking release) is mainly related to pure comfort concepts.

On the other hand, this study also presents some limitations, basically founded on the short amount of sensors and signals used to build the possible correlations. However, this fact also provides. On the other hand, this study also presents some limitations, basically founded on the short amount of sensors and signals used to build the possible correlations. However, this fact also provides added value to the results obtained, as with quite few indicators the comparison shows potential and robust associations.

As next steps and potential study improvements, IDIADA is also working in some methodologies applied to the correlation between subjective and objective concepts. A higher involvement of signals and metrics, together with an advanced Artificial Intelligence software application, with the objective of a complete correlation of factors, will help to characterize, validate and calibrate the next generations of automated driving vehicle brake systems.


With more than 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.

Applus IDIADA is in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.

Tag: Adaptive Cruise Control

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