May 2005 DOT HS 809 846 May 2005 DOT HS 809 846 May
12 & 15 Passenger Vans Tire Pressure Study:
Preliminary Results
Kristin K. Thiriez, Eric Ferguson, Rajesh Subramanian*
Traffi c Safety Facts
Research Note
Traffi c Safety Facts
Research Note
Traffi c Safety Facts
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 20590
* Kristin K. Thiriez is a Special Studies Project Engineer employed by Calspan Corporation, a contractor working for the Crash Investigations Division of the National Center
for Statistics and Analysis, National Highway Traffi c Safety Administration. Eric Ferguson is a General Engineer in the Crash Investigations Division. Rajesh Subramanian is a
Programmer Analyst in the Mathematical Analysis Division of NCSA.
1 NHTSA Action Plan for 15-Passenger Van Safety, November 2004 Update, http://www.nhtsa.dot.gov/cars/problems/studies/15PassVans/15passvan.html
2 Research Notes and Reports from NCSA’s Tire Pressure Special Study (DOT HS 809 315, DOT HS 809 316, DOT HS 809 317, DOT HS 809 359, and DOT HS 809 366) can
be found at http://www-nrd.nhtsa.dot.gov/departments/nrd-30/ncsa/availinf.html The analysis conducted on TPSS data to evaluate Tire Pressure Monitoring Systems was
published at ESV 2003 (Paper Number 259) and can be found at http://www-nrd.nhtsa.dot.gov/departments/nrd-30/ncsa/ESV2003.html
Summary
The National Highway Traffi c Safety Administration’s (NHTSA)
National Center for Statistics and Analysis (NCSA) conducted
a 15-passenger van tire pressure study. The purpose of the
study was to determine the extent of underinfl ation and
observe the tire condition in 12- and 15-passenger van (and
vans with similar body styles) tires. The following topics are
covered in this research note: sampling, data collection
methodology, analysis and results.
Background
Crashes involving 12- and 15-passenger vans have raised
concern about the safety of these vehicles among the
public and NHTSA. In March 2003, Senator Olympia
Snowe introduced the Passenger Van Safety Act of 2003,
in the Senate. This bill, S.717, called on NHTSA to develop
a dynamic test to assess the rollover risk of 15-passenger
vans for a consumer information program; test 15-passenger
vans at different load levels as part of the rollover resistance
program of the NHTSA’s New Car Assessment Program
(NCAP); and test and evaluate the stability control and other
technological systems that may assist drivers in controlling
15-passenger vans under conditions that may cause vehicle
rollover. A similar bill was introduced by Representative Mark
Udall in the House, H.R. 1641.
The National Transportation Safety Board (NTSB) has also
issued a series of recommendations related to 15-passenger
van safety. NHTSA developed a 15-Passenger Van Action
Plan that addresses these recommendations. As part of its
15-Passenger Van Action Plan, the agency decided to study
the extent of underinfl ation in 12- and 15-passenger vans
through the infrastructure of NCSA’s National Automotive
Sampling System (NASS).
In support of rulemaking activities mandated by Section 13
of the Transportation Recall Enhancement, Accountability,
and Documentation Act, the NCSA conducted the Tire
Pressure Special Study (TPSS) in February 2001 and the
Tire Pressure Monitoring System Study (TPMSS) in 2003.
The TPSS was designed to assess to what extent passenger
vehicle operators are aware of the recommended tire
pressures for their vehicles, the frequency and the means
they use to measure their tire pressure, and whether and how
signifi cantly the actual measured tire pressure deviated from
the manufacturer’s recommended tire pressure. The TPMSS
was designed to compare the extent of underinfl ation of
vehicles equipped with TPMS to vehicles not equipped.
The current Van Tire Pressure Study (VTPS) was designed
to collect similar information to that collected in TPSS, but
for 12- and 15-passenger vans (and vans with similar body
styles).
Please refer to previous publications2 for results from both
of these studies and analyses of that data to estimate the
effectiveness of indirect and direct TPMS.
The TPSS results showed that 27% of passenger cars in the
United States had at least one signifi cantly underinfl ated tire (for
the purposes of this research note, “signifi cant underinfl ation”
is defi ned as 25% or more below placard). The data also
showed that less than 25% of the study participants were
aware of where to fi nd the recommended tire pressure for their
vehicles. NHTSA used this data in support of its rulemaking
for Federal Motor Vehicle Safety Standard (FMVSS) No. 138,
Tire Pressure Monitoring Systems.
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 205902
Sample Design
The VTPS was designed as a
convenience sample to capture a good
representation of multiple regions,
climates, and demographics. Data was
collected from 16 locations throughout
the United States. These locations were
distributed among urban, suburban,
and rural locations and were located
in the east, midwest, southwest, and
western United States. Estimates
of the number of registered vans in
specifi c targeted states were found
using RL Polk (a private company that
provides automotive data and market
information) registration data.
The VTPS sample was selected from
several different types of organizations.
The study targeted some locations with
only one vehicle and some locations
with a fl eet of vehicles. Manufacturers
of the vans included Ford, General
Motors, and Dodge.
Table 1 - Distribution of
Vehicle Organizations Sampled
in the VTPS
Organization Type Sample
Colleges and Universities 13%
Churches and Community
Groups 24%
Camps and Daycares 11%
Subtotal 48%
Vanpools 11%
Transportation and Limo
Services 13%
Subtotal 24%
Local Governments 7%
Military 1%
Subtotal 8%
Hotels 5%
Subtotal 5%
Other (individuals, etc.) 14%
Subtotal 14%
Total* 99%
*Total not 100% due to rounding.
Source: National Center for Statistics and Analysis,
National Highway Traffic Safety Administration
The agency chose to focus on
moderately to fully-loaded vehicles
and those that are used to transport
children and young adults. Church and
community groups, universities and
colleges, day care facilities, and camps
made up 48% of the sample, see Table
1 (vans were not loaded when tire
pressure measurements were taken).
The distribution of vehicles shown in
Table 1 is from a convenience sample
and does not represent the true
proportions of these vehicles in the
United States. Extents of underinfl ation
were analyzed and the results are shown
in this research note. Data collection
took place in spring and early summer
of 2004. [Note: One would expect to
fi nd more underinfl ation during the cold
winter months due to the temperature-
related drop in tire pressure.]
The total number of vehicle inspections
completed was 1,242, with 937 15-
passenger vans.
Data Collection Methodology
Field data collection was conducted
through the infrastructure of NCSA’s
National Automotive Sampling System
(NASS).
Researchers visited organizations on
weekdays during business hours. If a
researcher contacted the owner of a
vehicle by phone before an inspection,
the researcher explained the study
was related to van maintenance or
“safety systems.” In order to keep the
study from being biased, “tire” and
“tire pressure” were not mentioned in
the phone contacts with the potential
participants.
The pilot phase of the data collection
was conducted from April 26, 2004
through May 14, 2004. Full-scale data
collection was conducted from May
17, 2004 through June 25, 2004. The
vans inspected in the pilot phase were
included in the fi nal VTPS data. Vehicles
surveyed included 15-passenger vans,
12-passenger vans, 14-passenger
vans, and cargo vans with similar
Gross Vehicle Weight Ratings (GVWR)
and body styles.
Nine anecdotal interviews were
conducted to get an idea of the tire
maintenance, driver training, and
safety awareness of the managers
of a subsample of our vehicles. The
interviewees included vehicle fl eet
managers from 2 churches, 2 colleges,
a community organization, a hotel, a
daycare, a transportation company,
and a vanpool company.
The Vehicle Inspection Form, which
was completed for each observation,
contained vehicle profi le information
such as make, model, and model
year. It also contained information
documented from the vehicle’s placard
regarding recommended tire size,
recommended infl ation pressure, and
the Gross Axle Weight Ratings.
The Tire Inspection Form, which was
also completed for each tire (except
unmounted spare tires), contained tire
size and measurement information, and
a variable to document visible evidence
of tire aging and/or damage (loose
tread, sidewall rot, etc.).
At the conclusion of each observation,
the participant was given a “courtesy
card” which contained the infl ation
pressure measured on each tire, the
vehicle manufacturer’s recommended
cold tire pressure, and several tire
safety tips.
Special Equipment
Special equipment used for data
collection included a pyrometer to
measure tire sidewall temperature and
ambient air temperature, an air pressure
gauge, and a tread depth indicator to
measure tread depth.
Analysis
Good tire care improves vehicle
handling3 as well as fuel effi ciency
and tire life. Proper tire maintenance
can prevent such events as tread
separations and tire blowouts which
may cause loss of control of a vehicle,
when not handled properly, and result
in a rollover. Low tire pressure can
also increase stopping distances and
the chance of hydroplaning on wet
surfaces.
3 “Testing the Effects of TPMS Minimum Activation Pressure on the Handling and Rollover Resistance of a 15-Passenger Van”, Garrott and Forkenbrock, DOT HS 809 701,
June 2004
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 205903
Data were collected on over 1,200
vehicles during the VTPS. An analysis
was performed looking at the
percentage of the vans that had at least
one signifi cantly underinfl ated (25% or
more from placard pressure) tire.
The variables of interest in determining
the underinfl ation are the recommended
pressures for each vehicle and the
measured pressures for each vehicle.
These were compared to determine
the extent of misinfl ation (misinfl ation
is used to characterize the extent of
underinfl ation and/or overinfl ation) for
each tire and then for each vehicle as a
whole. The data were used to determine
both underinfl ation and misinfl ation
(overinfl ation was included because
indirect tire pressure monitoring
systems do not distinguish between
over and underinfl ation).
Results
For the purposes of this research
note, underinfl ation was determined by
comparing measured pressures to the
vehicle manufacturer’s recommended
pressures. It should be noted that the
vacated FMVSS No. 138, the regulation
that required TPMS, listed minimum
activation pressures (MAP) for the
warning lamp to illuminate. For Load
Range E rated tires, this MAP value was
46 psi (320 kPa). The VTPS found that
16% of the 15-passenger vans in the
study had recommended pressures
below this MAP value. Nonetheless,
underinfl ation was determined relative
to the recommended pressure.
VTPS found that 56% of all vans had
at least one tire underinfl ated by 25%
or more, see Table 2. That is more than
double the percentage of passenger
cars we found in the TPSS (TPSS
is a national estimate, while VTPS
is a convenience sample, so true
comparisons are not possible).
Table 2 - Percent of Vehicles
in the VTPS and TPSS with
at Least One Tire Improperly
Infl ated
Vehicle
Misinfl ated
by 25% or
More
Underinfl ated
by 25% or
More
15-
Passenger
Vans
74% 57%
Other Vans 68% 54%
All Vans 72% 56%
Light
Trucks
from TPSS
39% 29%
Passenger
Cars from
TPSS
39% 27%
Source: National Center for Statistics and Analysis,
National Highway Traffic Safety Administration
For all tables in this research note,
“Other Vans” include 12-passenger,
14-passenger, and cargo vans. This
study also compared the percentage of
vans that had all four tires signifi cantly
underinfl ated to light trucks and
passenger cars found in the TPSS.
Table 3 shows that 6% of vans (a rate
double that of passenger cars) had
all four tires underinfl ated by 25% or
more.
Average underinfl ation was calculated
by fi nding the worst underinfl ated tire
for each vehicle and then taking the
average of all vehicles. The average
underinfl ation of vans was 29% under
the recommended pressure; vehicles
with overinfl ated tires were included as
vehicles with zero underinfl ation for this
calculation. The average misinfl ation
(also using the worst misinfl ated tire
per vehicle) of vans was 35% from the
placard pressure.
Table 3 - Percent of Vehicles
in the VTPS and TPSS with All
Four Tires Improperly Infl ated
Vehicle
Misinfl ated
by 25% or
More
Underinfl ated
by 25% or
More
15-
Passenger
Vans
7% 4%
Other Vans 11% 9%
All Vans 8% 6%
Light
Trucks from
TPSS
7% 4%
Passenger
Cars from
TPSS
6% 3%
Source: National Center for Statistics and Analysis,
National Highway Traffic Safety Administration
Because it was learned from all three
of our tire studies that people use the
pressure indicated on the tire sidewall
as a guide when checking their tire
pressure, the study also analyzed
overinfl ation. It was found that 22% of
vans had at least one tire overinfl ated
by at least 25% of the recommended
pressure. This can be explained by the
large difference between the placard
recommended front pressure (typically
45 or 55 psi) and the maximum
pressure listed on the tire sidewall
(80 psi). Two out of 9 interviewees
said that they looked at the sidewall
to determine infl ation pressure. The
rear recommended pressures were
typically 80 psi, so overinfl ation due
to referencing the tire sidewall was not
seen here. See Table 4 for results.
Not only were vehicles overinfl ated
when compared to recommended
pressure, but many were infl ated past
the maximum pressure recommended
on the tire sidewall. Seven percent of
the vans had at least one tire infl ated
past the maximum pressure on the tire
sidewall. 31 vehicles had at least one tire
that was more than 5% over maximum
pressure; 14 vehicles were more than
10% over maximum pressure, and 4
vehicles were more than 20% over
the maximum pressure, which is more
than 16 psi over the maximum pressure
allowed.
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 205904
play a part in vehicle handling. In the TPSS, it was found
that underinfl ation was more prevalent in older vehicles. This
study produced a similar result.
The study found that 4% of vans had at least one tire with
visible evidence of aging and/or damage. The study also
examined the tires’ tread depth and found that 6% of the
15-passenger vans had at least one bald tire (2/32nds of an
inch or less). 18% of the 15-passenger vans had 4/32nds of
an inch or less of tread. See Table 5 for these results. Worn
tread has been linked with increasing underinfl ation2.
Table 4 - Percent of Vehicles in the VTPS with at
Least One Tire Overinfl ated
Vehicle Overinfl ated by 25% or More
Overinfl ated Past
Max Pressure
15-Passenger Vans 23% 6%
Other Vans 18% 8%
All Vans 22% 7%
Source: National Center for Statistics and Analysis, National Highway Traffic Safety
Administration
So far, the analysis has shown the percentage of vehicles
with at least one underinfl ated tire, one overinfl ated tire, or
one-misinfl ated tire. It is interesting to note that 6% of the
vans had both signifi cant underinfl ation and signifi cant
overinfl ation in the same vehicle.
Because vehicle handling characteristics differ depending
not only on infl ation pressures, but the location of misinfl ated
tires, the fi gure shows the distribution of misinfl ation by tire
location.
Figure 1 shows that underinfl ation is more of a problem with
tires on the rear axle. 50% of rear tires were underinfl ated
past the 25% threshold while between 8 and 9% of front tires
were underinfl ated past the 25% threshold. Keep in mind that
the typical recommended pressure for the rear tires is 80 psi,
while for the front they are between 45 and 55 psi.
In addition to tire pressure, the age and wear of a tire also
28%
30%
32%
34%
36%
38%
200
4
Source: National Highway Traffic Safety Administration, National Center for Statistics and Analysis.
A
ve
ra
ge
U
nd
er
in
fla
tio
n
22%
24%
26%
200
2
200
0
199
8
199
6
199
3
199
1
198
9
14%
12%
10%
8%
6%
4%
2%
0%
Underinflation Overinflation
Percent Misinflation
Right Front Tire Left Front Tire Right Rear Tire Left Rear Tire
-90
%
-80
%
-60
%
-40
%
-20
% 0% 20% 40% 60% 80% 100
%
P
er
ce
nt
o
f
Ve
hi
cl
es
Figure 1 - All Vans Percent Distribution of Misinfl ation
Figure 2 - Average Underinfl ation by Model YearFigure 2 - Average Underinfl ation by Model YearFigure 2 - A
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 205905
Table 5 - Percent of Vehicles in the VTPS with at Least One Tire
Below Certain Tread Depth
Vehicle
Tread Depth of 2/32nds
of an inch or less
(% of vehicles)
Tread Depth of 4/32nds
of an inch or less
(% of vehicles)
15-Passenger Vans 6% 18%
Other Vans 5% 17%
All Vans 5% 18%
Passenger Cars from TPSS 9% 34%
Source: National Center for Statistics and Analysis, National Highway Traffic Safety Administration
Conclusion
The data from this study show that a very high percentage of large vans have
signifi cantly underinfl ated tires, a much larger percentage than passenger cars.
This poor tire maintenance in combination with the conditions under which these
vehicles are driven (i.e. transporting groups of children, inexperienced drivers, etc.)
suggest a need for better van tire safety awareness. Six of the 9 interviewees were
familiar with the NHTSA advisories on 15-passenger vans. Research into potential
countermeasures to improve van safety might include improving driver training,
equipping vehicles with a tire gauge, requiring passengers to wear safety belts, and
removing the last row of seats.
Other possible countermeasures to poor tire pressure might include the integration
of tire pressure monitoring systems into 12- and 15-passenger vans. Regardless of
the presence of TPMS, public awareness of proper tire maintenance is key to the
improvement of tire condition. Each tire, including the spare, should be checked
monthly when cold and set to the recommended infl ation pressure as specifi ed on
the vehicle placard and in the owner’s manual.
For additional copies of this research
note, please call (202)934-8517 or fax
your request to (202)366-3189. For
questions regarding the data reported
in this research, contact Eric Ferguson
[202-366-9430] of the National Center
for Statistics and Analysis. This research
note and other general information on
highway traffi c safety may be accessed
by internet users at: http://www-nrd.
nhtsa.dot.gov/departments/nrd-30/
ncsa/.
NHTSA’s National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, D.C. 205906