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TitleHuman Performance Evaluation of Light Vehicle Brake Assist Systems: Final Report
LanguageEnglish
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Page 1

DOT HS 811 251 April 2010

Human Performance Evaluation of
Light Vehicle Brake Assist Systems:
Final Report

Page 2

DISCLAIMER

This publication is distributed by the U.S. Department of Transportation, National Highway

The United States Government assumes no liability for its contents or use thereof. If trade names,
-

sential to the object of the publication and should not be construed as an endorsement. The United
States Government does not endorse products or manufacturers.

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CHAPTER 6. DISCUSSION


This discussion chapter is broken into two sections. The first section discusses the effect of BAS
on driver panic-braking performance, while the second section discusses panic braking in general
and the role that expectancy plays.

THE BRAKE ASSIST EFFECT

BAS has become a standard feature on many vehicle models since its introduction to the
marketplace by Mercedes-Benz in 1996. It was conceived from simulator research internal to
Mercedes-Benz in the early 1990’s that found that although many drivers apply the brakes
quickly during panic-braking maneuvers, many fail to apply braking forces capable of yielding a
vehicle’s maximum braking performance. The BAS safety feature addresses this human
physical limitation by supplementing drivers’ braking force upon detecting a panic-braking
maneuver.

This report presents the results of a comprehensive investigation of drivers’ panic-braking
performance with the BAS safety feature. Information was gathered by submitting surveys to
OEMs and Tier 1 suppliers that solicited information on the type of BAS they manufacture, its
operation, and the direction the technology was taking. This communication revealed that
electronic BAS would become commonplace as fundamental components become standard on
less expensive vehicle models. As such, two electronic BASs were selected for this
investigation: 1) a 2006 Mercedes-Benz R350 that had a vacuum-booster-based BAS, and 2) a
2007 Volvo S80 that had an ABS-pump-based BAS.

At the onset of this investigation, little was known about the brake pedal input necessary to
activate BAS, as well as the maximum reduction in stopping distance that BAS activation
produced. Both test vehicles were thus brought to the VRTC in East Liberty, Ohio to have their
BAS characterized. Using a mechanical brake controller, the brake pedal input necessary to
activate BAS was systematically identified. It was found that BAS activation with threshold
brake pedal input in the Mercedes-Benz R350 produced stopping distances 20.2 ft shorter than
stopping distances produced when BAS was disabled. In contrast, BAS activation with
maximum brake pedal input in the Mercedes-Benz R350 produced stopping distances 0.1 ft
shorter than stopping distances produced when BAS was disabled. Unfortunately, a BAS
activation threshold could not be systematically identified in the Volvo S80 because of
limitations with the brake controller. However, BAS activation generated by a professional
driver revealed that the activation threshold was higher than what the brake controller was
capable of producing. Overall, the characterization tests revealed that the benefits offered by
BAS are dependent on what drivers’ baseline brake pedal input is (i.e., drivers that apply brake
pedal input near the BAS activation threshold will benefit more than drivers that apply the
maximally allowed brake pedal input).

There was a concern that high BAS activation thresholds would prevent participants from
activating BAS in the human performance braking study. Preliminary testing at VTTI with
human subjects was thus performed. Here, it was found that drivers could activate BAS, but
only after they were shown how to press the brake pedal in the proper manner. This finding
encouraged the researchers to continue with the human braking performance evaluation, but to

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incorporate a component that measured drivers’ performance after they were shown how to
perform panic-braking maneuvers.

The human braking performance tests comprised various braking maneuvers to investigate BAS.
Sixty-four participants, balanced for age and gender, drove one of the two instrumented vehicles
down a closed-course test track at 45 mph. After a 25-minute familiarization period, drivers
were unknowingly presented with an inflatable barricade that spanned the entire road for which
some of them stopped the vehicle to avoid a collision. After drivers consented to continue the
experiment, a series of braking maneuvers were performed afterwards, including stopping at the
inflatable barricade again and performing numerous hard-braking maneuvers in response to an
auditory alarm. Drivers’ panic-braking performances were measured and the effect of BAS
activation on vehicle stopping distance was evaluated using more than one approach. Here, a
panic-braking maneuver was operationally defined as a braking maneuver in which ABS
activated and the vehicle came to a complete stop.

BAS was first evaluated by comparing the mean corrected stopping distance produced by BAS-
inactive panic-braking maneuvers to the mean corrected stopping distance produced by BAS-
active panic-braking maneuvers. The stops performed to the unexpected barricade were not
analyzed in this approach because no BAS activations were observed in this trial. The three
BAS-active panic-braking maneuvers performed in the Volvo S80 at the anticipated barricade,
however, were found to be on average 11.98 ft shorter than the three BAS-inactive panic-braking
maneuvers performed in the same vehicle. This difference was not found to be statistically
significant (p = 0.2752). This was likely because of the small sample size available. Stopping
distance comparisons for the anticipated barricade braking maneuver in the Mercedes-Benz
R350 were not made because BAS activations in it were not observed. When considering the
panic-braking maneuvers performed in the repeated braking session, the four BAS-active panic-
braking maneuvers performed in the Mercedes-Benz R350 were on average 4.61 ft shorter than
the 25 BAS-inactive panic-braking maneuvers performed in the Mercedes-Benz R350. This
difference was found to be statistically significant (p = 0.0079). The 17 BAS-active panic-
braking maneuvers performed in the Volvo S80 were on average 1.51 ft shorter than the 61
BAS-inactive panic-braking maneuvers performed in the Volvo S80. This difference was not
statistically significant (p = 0.4209). Here, BAS activation produced a larger reduction in
stopping distance for the trained Mercedes-Benz R350 drivers than it did for the trained Volvo
S80 drivers, while more trained Volvo S80 drivers were able to activate BAS than the trained
Mercedes-Benz R350 drivers. Overall, although not all findings were statistically significant,
because the mean stopping distance differences were all in the same direction, there appears to
be a trend that BAS activation reduces panic-braking stopping distance.

A potential criticism of the previous approach is that panic-braking performance varies across
drivers. To isolate the effect of BAS on driver panic-braking performance, drivers’ individual
differences should be controlled. The second analytical approach accomplished this by
considering only drivers that activated BAS in the repeated braking session and by comparing
the stopping distances they produced when BAS-activated to the stopping distances they
produced when BAS was disabled. Here, the mean BAS-active stopping distance produced in
the Mercedes-Benz R350 was 5.92 ft shorter than the mean BAS-disabled stopping distance.
This difference was not statistically significant (p = 0.5). The mean BAS-active stopping

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DOT HS 811 251
April 2010

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