Adaptive Cruise Control (ACC) – Objective Metrics and Performance

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TROY, Mich. — This is the first of three articles by Bernat Ferrer, Manager Chassis and Active Safety at Applus IDIADA U.S.


The objectivization of the systems calibration and development processes has always been one of the most recurrent hot topics in the automotive industry. However, the reliability of an expert subjective assessment, based on a specific market or field, has nowadays still a huge weight during the decision-making and setup of many vehicle functions. The Adaptive Cruise Control (ACC) is one of these applications which usually counts on a subjective evaluation to validate its performance.

This study, based on the longitudinal braking actuation of the 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 test cases used to develop this methodology come from IDIADA’s internal resources and testing programs carried out in former years of dedication. Many different vehicles were used to get the outputs necessary for the comparison, including a wide range of segments (passenger cars) and brands, but always keeping the same criteria of evaluations and expertise.

Subjective assessment

The first part of this study considers the pure subjective evaluation of the ACC system performance (braking actuation only). The initial subjective evaluation is performed while the vehicle is being characterized objectively in a controlled environment (Proving grounds) allowing the tests drivers to experience a complete range of system reaction in a reduced amount of time. The evaluation is then completed by driving the vehicle in open roads under different scenarios and areas. This evaluation carried out by expert drivers try to determine the reaction of the final customer under safety and comfort points of view. An initial categorization can be set, according to what could it be expected from the customer perspective, with a range that moves from a total rejection of the system on the one side, to a satisfied or enthusiastic reaction on the opposite one:

  • Worst reaction: Rejection
  • Middle range: Complain < Slight critic < No reaction
  • Best reaction: satisfaction – enthusiastic

This initial classification can be still refined by adding more categories within each range, considering extra sensations or symptoms that the driver can receive due to the system actuation.

All in all, a ten-scale ordinance has been defined to give enough variety of evaluation and subjective perception:

Table 1: Subjective evaluation scale

Once this scaling structure has been generated, a second phase of organization is necessary to minimize the effect of generalization of the actuation evaluation. The driving maneuvers compress multiple scenarios; a typical mistake when trying to compare subjective evaluations with objective data appears because of the excessive generalization of the first one. Therefore, a restrictive strategy has been applied to the assessment process, looking for delimited phases of the braking actuation of the ACC system.

In this study, three different steps were selected in order to define the braking scenario. And below them, some additional concepts were designated to provide extra detail of its characterization:

Figure 1: Longitudinal deceleration example while braking
  1. Braking beginning: deceleration ramp-up. Start of braking actuation until the target or desired deceleration is reached
    a. Application progressivity: ramp-up slope, defining the promptness with which the deceleration increases.
    b. Absence of rebounds: ramp-up linearity, without the presence of slope changes and rebounds causing uncomfortable and inconsistent braking deceleration increase.
  2. Braking quality: deceleration steady-state. Behavior and stabilization while keeping a constant target deceleration.
    a. Absolute level: absolute level of deceleration reached during the snub (stabilized area), considering the adequation of the vehicle to reach a certain level and referenced to the front vehicle.
    b. Continuity: deceleration oscillation, constantness level during the supposed constant target level.
  3. Braking ending: deceleration ramp-down. Braking torque decrease until the complete braking release
    a. Stabilization: time necessary for releasing the braking torque and deceleration and get a constant state without longitudinal acceleration.
    b. Release progressivity: ramp-down slope, defining the promptness with which the deceleration decreases.
    c. Absence of rebounds:  ramp-down linearity, without the presence of slope changes and rebounds causing uncomfortable and inconsistent braking deceleration release.

All in all, the braking actuation of the vehicle is much more accurately characterized, by breaking down its phases and providing an individual rate for each of the abovementioned concepts. 


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.

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