Parking Brake Use Study

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Parking Brake Use Study
2013-01-0199
Published
04/08/2013
Steven Becker
Robson Forensic Inc.
Copyright © 2013 SAE International
doi:10.4271/2013-01-0199
ABSTRACT
In the United States most passenger vehicles have an
automatic transmission with a transmission shifter position
labeled Park. When the transmission shift selector is placed in
Park, a parking pawl (a pivotally mounted arm) engages a
parking gear on the output shaft to immobilize the drive shaft
and prevent the vehicle from moving. The driver also has the
option of engaging the parking brake with a lever, pedal, or
button to immobilize the vehicle. Many state driver's license
manuals and vehicle owner's manuals commonly suggest the
use of the parking brake every time the driver exits the
vehicle regardless of the transmission type. Testing is
performed by vehicle manufacturers on the automatic
transmission to insure the Park shift position pawl holds the
vehicle on steep slopes. This study was conducted to
investigate how often and why drivers use the parking brake.
Utilizing two survey techniques, the results and analysis
show that a majority of drivers with vehicles equipped with
automatic transmissions rarely if ever set their parking brake.
Driver's typically place the transmission in Park and rely on
the transmission to hold the vehicle after they park. The lack
of use of the parking brake and the requirement for its use
illustrates an industry problem not documented in literature.
INTRODUCTION
When parking drivers have multiple tasks: shifting to Park
with an automatic transmission or a low gear with a manual
transmission, turning the ignition Off, headlights Off,
unbuckling the seatbelt, etc… Similar items must be
remembered when starting the vehicle. At both parking and
starting the vehicle, the driver is often occupied with thoughts
of getting to the next destination and not the process of
driving. Many safety features have been designed to prevent
unintended actions: the multi-position key, Brake-to-shift
interlock, reminder warnings for seatbelts, automatic
headlights, and reminders for disengaging the parking brake
while driving. Mechanical vehicle systems are being replaced
with electronic systems, including the parking brake [1]. In
the case of the parking brake, drivers who do not apply the
parking brake when parking - rely on the transmission to hold
the vehicle after they park.
The hazard presented by unintended vehicle movement poses
a danger to pedestrians, the occupants as they exit, and other
objects near the vehicle. Unintended motion of a parked
vehicle creates an unexpected and nearly silent danger of
being knocked or run over to nearby pedestrians such as; a
child playing in his driveway, a former passenger exiting the
vehicle, or retrieving packages from the rear. This type of
collision is classified as a non-traffic crash, but statistics on
this type of crash do not distinguish whether the vehicle was
powered or parked [2 3 4]. With a manual transmission, this
could occur if the operator did not place the transmission in
gear, or if the weight of the vehicle on a slope overcomes the
static forces in the engine. The transmission gear engagement
could slip due to poor or worn engagement of the gear
synchronizers or shift system. A 4WD vehicle could move if
the transfercase slips into neutral. Defective design, improper
adjustment, and worn components can create a “False Park”
position with an automatic transmission, where the operator
shifts into Park and removes the key from the ignition, but
does not have the parking pawl completely engaged [5].
Other errors include the driver not placing the shift selector
into Park with an automatic transmission.
The parking brake hand lever mechanism is comprised of
several components. Figure 1 shows a typical parking brake
mechanism. Common failures occur at the clutch spring
anchor attachment, the cable slack adjustment, or at the wheel
in the brake apply mechanism, shoes or pads.
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7. Are you aware of how to apply the emergency brake/ebrake/parking brake on your primary vehicle?
8. When parking, how often do you set your emergency/
parking brake?
9. Why do you set your emergency/parking brake when
parking?
Figure 1. Parking brake lever components [6]
An online survey was created to determine how often and
why drivers use their parking brake. Additionally, 100
residentially parked vehicles were surveyed as to their
transmission type and parking brake use.
SURVEYS
The survey was a ten question online multiple choice survey
utilizing Surveymonkey.com online software. The number of
questions was kept short to ensure a high participation rate
and not delve from the survey's main goal. The main goal of
the survey was to determine what proportion of people use
their parking brake when parked. Secondary to that goal was
background questions included at the beginning and the end
of the survey to qualify and categorize the survey respondent.
These background questions included:
1. Do you drive an automobile/truck?
2. Which of the following vehicle brands do you currently
drive? (Check all that apply)
3. Which of the following vehicle brands do you primarily
drive? (The vehicle you drive the most)
4. About how many miles is your primary vehicle driven in a
typical year?
5. Are you male or female?
Part of the goal of the survey was to determine the survey
respondents' understanding of their parking brake and
transmission. The type of transmission was a critical
question, as a manual transmission does not offer a Park shift
position. The means to apply the parking brake may be
important, so a goal was to determine if a lever, pedal, or
button/switch affects how the often the person applies the
parking brake.
The goal questions included;
6.
Does your primary vehicle have an automatic
transmission?
The second survey was taken manually, by visual inspection
of vehicles that were parked in residential areas. This
included a mixture of urban street parked, self park lots, and
suburban driveways vehicles. As the inspection was visual,
only vehicles with a parking brake lever were sampled to
ensure correct identification of use. The transmission type
was identified by the shifter. Most driveways and parking
structures have some slope for water drainage. The street
parked vehicles sampled were on a mixture of mildly sloped
and nearly flat roads.
RESULTS
The surveys resulted in statistical data that could be analyzed
in several ways. Filtering with the background questions
sorted the data by key attributes. The online survey was
answered by 158 people of the about 265 people to whom it
was sent for a 60% response rate. All of the respondents were
drivers. The mix of vehicle brands was fairly consistent with
annual sales and market share trends with 27 vehicle brands
represented. The primary driven vehicle is 7 years old on
average and ranged from model year 1973 to 2012 new. The
sample is normally distributed for miles typically driven per
year with a midpoint of 10,000 to 20,000 miles per year.
About 78% of the primary vehicles had automatic
transmissions. About 55% of the vehicles had a parking brake
lever, about 39% had a pedal, and 4.4% had a button/switch,
3 respondents were not sure what activated their parking
brake. The vehicles equipped with a button/switch were
BMW, Jaguar, Porsche, new Chevrolets, and Volkswagens.
Drivers indicated why they apply the parking brake and could
list multiple reasons. Of course those who never apply the
brake listed no reason, but most (90%) cited safety and
preventing movement. 14% were just taught to apply the
parking brake.
71% of the respondents were male. US Department of
Transportation Federal Highway Administration licensed
driver study [7] indicated the US driving population of about
200 million drivers is about evenly divided male and female.
The survey results are therefore biased by gender.
Using the data from all respondents and filtering for
important characteristics, the percent of how often the
parking brake is applied is shown in Tables 1 and 2 for
various criteria:
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Table 1. All data various criteria
Table 3. All data grouped
An odd not note was that the respondents that were not sure
how to apply the parking brake, responded with “rarely” and
“only on a hill”. As transmission type was a significant factor
the data, it was filtered further for only responses from
automatic transmission vehicle drivers:
Table 4. Automatic transmission only data grouped
Table 2. Filtered data for automatic transmission only
The survey data can be further grouped into two types of
respondents: those who frequently apply the parking brake,
and those who infrequently or only on hills apply the parking
brake. The respondents who frequently apply the parking
brake were grouped as those who selected; always or
sometimes apply the parking brake. The respondents who
infrequently apply the parking brake were grouped as those
who selected; rarely, never, and only on hills. This group
determines what slope of the hill requires greater protection
by applying the parking brake only when they feel it is
needed. Grouped together Tables 1 and 2 can be simplified,
and the results show interesting trends presented in Tables 3
and 4 for the all data group and automatic transmission only
data, respectively; and filtered for only responses from
automatic transmission vehicle drivers.
From Table 3, the use of the parking brake shows 75%
infrequent use with automatics and only about 18% for
manual transmissions. The manual transmission owners
either notice the lack of a Park shift position or are more
aware of the greater potential hazard for unintentional
movement with a manual transmission vehicle. Of significant
note is the equality between the genders, with about 75% of
each infrequently applying the parking brake. When looking
at the parking brake application method, there is greater use
for the lever versus the pedal, and as expected, when the
operator is not sure how to apply the parking brake, it may be
because they do so infrequently. The vehicles equipped with
a button or switch to apply the parking brake were all manual
transmission and use was approximately evenly distributed,
but less than the use in other manual transmissions. The
parking brake application method is a factor that affects
driver usage.
For the visual parked vehicle study, parking brake application
could only be distinguished visually for vehicles with a
parking brake lever. The results were that 72% of all vehicles
observed did not have the parking brake engaged. Of the
vehicles with manual transmissions only 33% did not engage
the parking brake. While, of those vehicles with automatic
transmissions, 82% did not engage the parking brake. The
visual survey yielded results consistent with the online
survey.
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DISCUSSION
The hazard presented by a parked vehicle is unintended
vehicle movement. From the results, this hazard is currently
addressed by the transmission Park shift position for about
75% of the respondents. As shown in Figure 2, Park in a
transmission is accomplished by the parking pawl
(highlighted in blue) engagement of a parking gear (in red)
that immobilizes the driveshaft:
transmission shift position labeled “Park”, if parking is only
completed by the application of the parking brake, not the
transmission shifter. It may be too late to change the public
perception and the automatic shift position name of “Park”
after more than 60 years of production. Possible solutions
would be to automate the engagement and disengagement of
the parking brake when the driver is removing the key and
after starting the vehicle and moving the shifter into gear.
Automated application of the parking brake could be
accomplished with electronic parking brake systems.
Future work could include expanded geographic review of
parking brake use. Increased driver education on the
application of parking brake will be a difficult future task. As
fewer people learn to drive manual transmission vehicles in
the United States, there will be fewer people to train drivers
to apply the parking brake.
CONCLUSION
• The parking brake is infrequently applied in 75% of the
vehicles with automatic transmissions.
• About 82% of vehicles with manual transmissions are
parked with the parking brake applied.
Figure 2. Automatic Transmission Parking components
Ford C4 transmission.
• The visual survey yielded results consistent with the online
survey.
• The parking brake application method is a factor that affects
driver use.
Not disclosed by the survey results is the possible effect of
geography, as most of the respondents were from the
northeast United States. Also age was not considered as a
factor, and those less familiar with manual transmission
vehicles, could be less likely to have learned to apply the
parking brake
• While the purpose of this study could not address state
driver's manuals and vehicle operator manuals being right or
wrong in requiring that vehicles with automatic transmission
apply their parking brake, future work should be performed to
reduce the disparity in parking brake use versus the
requirement.
The sample size of vehicles with electronic parking brakes
was low. Also all of the electronic parking brake vehicles had
manual transmissions. An interesting future study would be
to address improving the operator and parking brake interface
to determine what factors could increase the use of the
parking brake, to address the feasibility of parking brakes that
are automatically applied and released by computer and
electronic controls.
REFERENCES
Discussion with industry professionals indicated that the lack
of use of parking brake is well known. A literature review did
not show prior published work on the topic of parking brake
use. Either state driver's manuals and vehicle operator
manuals are wrong in requiring that the more than 93% of all
vehicles sold, which are automatics, should have their
parking brake applied, or the message that applying the
parking brake prevents accidents is not getting to drivers, or
the parking brake is not needed on automatic vehicles. The
automobile industry may be confusing drivers by having a
1. http://www.bmw.com/com/en/insights/technology/
technology_guide/articles/
electromechanical_parking_brake.html accessed 9/7/2012
2. Chidester A., Isenberg, R. “Pedestrian Crash Data Study An Interim Evaluation” (PCDS) 1994, Isenberg, R. Chidester,
A. “Update on the Pedestrian Crash Data Study” 98-S6-O-05
National Highway Traffic Safety Administration
3. 2008 Pennsylvania Crash Facts & Statistics; Pennsylvania
Department of Transportation
4. http://www.walkinginfo.org/pedsafe/crashstats.cfm
accessed 10/19/12, U.S. Department of Transportation
5. Chevrolet Blazer Power Train Recall 94V093000 & Jeep
Grand Cherokee Power Train Recall 02V053000 & Nissan
Murano Power Train Recall 03V123000 & Ford Ranger
Power Train Recall 99V204000 & etc…
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6. Figure from http://www.jk-forum.com/stock-jk-tech-12/
oem-parking-brake-lever-study-135255/ accessed 9/17/2012
7. http://www.fhwa.dot.gov/ohim/onh00/bar7.htm accessed
9/17/2012
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