The Relationship between Vehicle Weight
Weight, Size and
Safety
Physics of Sustainable Energy:
Using Energy Efficiently and Producing It Renewably
Sponsored by American Physical Society’s Forum on Physics and Society
Berkeley CA, March 1-2, 2008
by
Tom Wenzel
Lawrence Berkeley National Laboratory
TPWenzel@lbl.gov
Marc Ross
University of Michigan
MHRoss@umich.edu
slide 1 of 33
Improvements in both fuel economy and safety are
possible
• Fuel economy improvement is cost-effective (Greene 2007, EEA
2006)…
— technologies exist to raise fuel economy 50%, at current gas
prices ($3.00/gallon)
— includes some weight reduction in only heaviest pickups
— does not include new powertrains (hybrid, plug-in hybrid, HCCI,
fuel cells) or fuels (diesel, low-carbon fuels)
— more technologies become cost-effective as gas price increases
• …but weight reduction is easiest, and least-costly, step to increase
fuel economy
• Advanced materials (high-strength steel, advanced composites) may
allow large weight reductions, and fuel economy improvement,
without any sacrifice in safety
• Safety can be improved using new technologies, with little impact on
weight or fuel economy
— electronic stability control
— better seat belts
— stronger roofs
— vehicle-to-vehicle communication
slide 2 of 33
Two views on vehicle weight and safety
• Majority of National Academy of Sciences committee on the
Effectiveness and Impact of Corporate Average Fuel Economy
(CAFE) Standards (2002):
— “The downweighting and downsizing [of vehicles] that occurred
in the late 1970s and early 1980s, some of which was due to
CAFE standards, probably resulted in an additional 1,300 to
2,600 traffic fatalities in 1993.”
• Minority (two members) of same committee:
— “The conclusions of the majority of the committee … are overly
simplistic and at least partially incorrect … The relationship
between vehicle weight and safety are complex and not
measureable with any reasonable degree of certainty at
present…Reducing the weights of light-duty vehicles will neither
benefit nor harm all highway users; there will be winners and
losers.”
• Does reducing weight inherently increase fatalities, or not?
slide 3 of 33
Conclusion: “weight” and “size” provide only partial
protection
i
• Crashes with another vehicle or stationary object
— reducing weight ratio would reduce deceleration of lighter
vehicle
— crush space improves crashworthiness, but difficult to provide
when struck in side
— height and structure of “bullet” vehicle are more important
• Vehicles struck in side
— heavier and larger light trucks are much more aggressive to
other vehicles than lighter and smaller light trucks
— reducing frontal height and perhaps stiffness of light trucks is
necessary to reduce their aggressivity
— increasing side stiffness or crush space in cars could improve
their compatibility
• Rollover crashes
— weight has little effect on propensity to roll over
• width,
and to a lesser extent length, can reduce rollover
propensity
• height
g p
probably
y more important
p
even than width
— electronic stability control promising technology to prevent
rollovers
slide 4 of 33
Definition of risk
• “Risk”: driver fatalities per year, per million vehicles registered as of
Jan 2005
— driver fatalities from NHTSA Fatality Analysis Reporting System
(FARS)
• FARS includes many details on all US traffic fatalities
— registered vehicles as denominator
denominator, or measure of “exposure”
exposure
• Because it is based on actual fatalities, our definition of risk
incorporates:
— vehicle design
• crash avoidance (sometimes measured by consumer groups)
• crashworthiness (typically measured in artificial lab crash
tests)
— driver characteristics and behavior
— road environment and conditions
• Therefore, all risks are “as driven”; as a result, our risks don’t
correlate well with lab crash test results
slide 5 of 33
Two types of risk
• Risk to drivers of subject vehicle
— from all types of crashes (total, and separately for two-vehicle
crashes, one-vehicle crashes, rollovers, etc.)
• Risk imposed by subject vehicle on drivers of other vehicles (all
types and ages)
— often called vehicle “aggressivity” or “compatibility”
— because from two
two-vehicle
vehicle crashes only
only, risks to other drivers
tend to be lower than risks to drivers
• Combined risk is the sum of the two
slide 6 of 33
Two levels of analysis
• Risks by vehicle type
— four major car classes (plus luxury import and sports cars),
based on Consumer Guide
— pickups by size, SUVs, and minivans
— calculated for 133 popular vehicle models with relatively
consistent, strong sales over 2000-04
— differences less than ~10%
10% not statistically significant
• Risks by vehicle model
— calculated using only 69 most popular vehicle models, to reduce
statistical uncertainty
— differences less than ~20% not statistically significant
slide 7 of 33
0
23
40
74
85
40
123
82
105
101
48
Fullsize vans
200
1-ton pickups
142
3//4-ton pickups
96
68
1//2-ton pickups
75
38
Com
mpact pickups
143
Cro
ossover SUVs
39
Truck
k-based SUVs
39
Sports cars
150
Sub
bcompact cars
(high-risk)
40
Compact cars
C
33
30
Large cars
100
Sub
bcompact cars
(low-risk)
300
Midsize cars
35
Minivans
50
Impo
ort luxury cars
Riisk
Risks by vehicle type
Risk to drivers of other vehicles
Risk to drivers
250
199
147
90
66
46
94
100
52
slide 8 of 33
0
32
60
61
71
17
40
31
74
61
56
45
53
Fullsize vans
F
43
1-ton pickups
43
3/4-ton pickups
25
1/2-ton pickups
100
Comp
pact pickups
66
99
Crosssover SUVs
109
based SUVs
Truck-b
34
Sports cars
8
14
Subcompact cars
(high-risk)
8
19
Compact cars
14
Large cars
250
Subcompact cars
(low-risk)
300
Midsize cars
M
27
Minivans
50
Imporrt luxury cars
Rissk
Risk to drivers in rollovers and all other crashes
Risk to drivers of other vehicles
Risk in rollover crashes
Risk in all other crashes
200
150
44
49
47
15
37
slide 9 of 33
Risks by vehicle type
200
1-ton pickups
3/4-ton pickups
150
100
1/2-ton pickups
full-size vans
truck-based SUVs
50
minivans
compact pickups
large cars
crossover
SUVs
sports cars
compact cars
midsize cars
subcompact cars
(high risk)
subcompact cars
(low risk)
import luxury cars
0
0
20
40
60
80
100
120
140
160
Risk-to-drivers
slide 10 of 33
Risks by vehicle model
120
Ram 1500
110
100
D k t
Dakota
90
Durango
Tahoe
Chevy/GMC
C/K 1500
Explorer
F-150
80
Jimmy/Envoy
y
y
70
Expedition
Chevy Suburban
60
50
Jetta, Accord,
Camry, Caravan,
Deville
40
Windstar/Freestar
Odyssey
Sable
Beetle
M i
Maxima
Avalon,
Legacy
Passat
S-10/Colorado
Neon
Stratus
Corolla,
C
ll Malibu,
M lib
Grand Marquis
Grand Am
Accent
Cavalier
Alero
Sunfire, Escort
Century
RX300/330
10
Blazer
Tacoma
Focus
Town & Country
CR-V
20
Intrepid
Altima, Taurus
Altima
Taurus,
Impala
4Runner
Sienna
30
Wrangler
Galant
Venture
Ranger
Frontier
Grand Cherokee,
Cherokee/Liberty
XTerra
Subcompact cars
Compact cars
Midsize cars
Large
g cars
Minivans
SUVs
Compact pickup trucks
1/2-ton p
pickup
p trucks
Saturn LS
SC/SL/Ion
LeSabre,
Sebring
Elantra, Grand Prix,
Civic, Sentra
0
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160 170 180 190
Risk-to-drivers
slide 11 of 33
Effect of vehicle design
g on risk
• High risk to drivers of pickups and SUVs from their propensity to roll
over
— NHTSA’s static stability factor (SSF): tw/2h
tw = track width; h = height of center of gravity
— average car SSF is 1.40, 12% chance of rollover in a crash
— average SUV SSF is 11.15,
15 28% chance of rollover
• High risk to others from pickups and SUVs (and to a lesser extent
minivans) associated with chassis stiffness and height
— car driver fatalityy rate is 5x higher
g
when struck in side by
y SUV
(4x higher when struck by pickup) than when struck in side by
another car
— SUVs are built on pickup frames, whose rails often override car
bumpers and sills and puncture car bodies
• Rollover risk in SUVs, especially crossovers, and risk to others from
pickups are declining
slide 12 of 33
Stiff frame rails of pickups and truck-based SUVs
act as fork tines
MY02 Dodge Ram 150 pickup truck
slide 13 of 33
Driver behavior and environment influence risk
• Driver characteristics that affect risk
— age and sex
— alcohol/drug use, driving record
— seatbelt use
— education level/income
• Environmental variables that affect risk
— time of day (visibility)
— rural roads (poorly lit and designed, high speeds)
— weather ((road conditions))
slide 14 of 33
Effect of driver behavior on risk
• Two measures of driver behavior
— fraction of fatalities that are young males (<26 years old)
— bad driver rating based on: alcohol/drug involvement and risky
driving in current crash
crash, and driver’s
driver s record over last two years
(lower rating is better drivers)
• Truck-based SUV drivers are very much like car drivers
• Risky sports cars have highest fraction of young male fatalities
(39%), and worst drivers (0.77). Safe minivans have the best
drivers (4%, 0.21). Safe luxury import cars have slightly worse
drivers (21%, 0.57) than the average car (20%, 0.50).
• However, individual models do not necessarily fit these trends
— the much safer Civic (30%, 0.54) and Jetta (32%, 0.66) have
worse drivers than all other subcompacts (22%, 0.54)
— the
th risky
i k Blazer/Trailblazer
Bl
/T ilbl
h
has only
l slightly
li htl worse d
drivers
i
(18%
(18%,
0.50) than the average truck-based SUV (15%, 0.45)
slide 15 of 33
Effect of driver behavior on risk (cont.)
• Station wagons have lower risk, and for the most part better drivers,
than sedan/coupe versions of same model
— Escort/Tracer, Taurus/Sable, Saturn SC/SL/SW, Legacy,
Impreza, Volvo have lower risks
— all but Impreza and Volvo also have better drivers
• Crown Victoria Police Interceptor model has much higher risks (risk
in 144, risk by 366) than other Crown Vics (risk in 94, risk by 65)
— Police Interceptors have more young male drivers (15% v. 5%),
but lower bad driver rating
g ((0.20 v. 0.38))
• Some models with higher hp engines have higher risk, and worse
drivers, than lower hp engines of same model
— higher hp engines in Mustang, Camaro, Firebird, Grand Am, and
J tt allll have
Jetta
h
hi
higher
h risk
i k and
d worse d
drivers
i
th
than llower h
hp
engines in same model
— but for other models (Camry, Galant, Eclipse, Alero, Lincoln LS)
cars with higher hp engines do not have higher risk
slide 16 of 33
Eff
Effect
off environment
i
on risk
i k
• Rural roads (less well-lit, undivided, higher speeds, unenforced
speed limits, further from hospital) are less safe than suburban or
urban roads
— risks much higher in very rural areas
• Pickups are driven more on unsafe rural roads than other vehicle
types; the average pickup fatality occurs in much less dense areas
(250 people per sq mile) than average SUV or car fatality (340 and
420 people per sq mile
mile, respectively)
• Used California vehicle registrations by county to calculate risk in
urban vs. rural counties, by vehicle type
slide 17 of 33
Risks in California are higher in rural areas than in
urban areas, for all vehicle types
250
Risk to drivers
Risk to drivers of other vehicles
Urban
Rural
200
150
100
50
0
Car
Minivan
SUV
p
Pickup
Car
Minivan
SUV
Pickup
p
Vehicle type
slide 18 of 33
Is car weight the best predictor of risk?
• Quality of vehicle design appears a better predictor of risk than
weight
— manufacturer
— resale value (retail used car price from Kelley Blue Book)
• Analysis limited to cars; need truck weights by “model” to apply to
pickups SUVs and minivans
pickups,
• We excluded models overly influenced by their drivers (young males
or elderly)
slide 19 of 33
Weak relationship between car weight and risk…
200
180
160
140
120
100
80
60
40
R2 = 0.17
20
0
2200
2400
2600
2800
3000
3200
3400
3600
3800
4000
4200
4400
Inertial weight (curb weight + 300 lbs)
slide 20 of 33
… unless one accounts for manufacturer
200
Big 3
Korean
Japanese/German
180
Big 3
160
R2 = 0.39
140
120
Ford
Focus
Mitsubishi
Galant
100
Saturn
SC/SL
80
60
Saturn
L/LS
40
Japanese/German
20
0
2200
R2 = 0.54
2400
2600
2800
3000
3200
3400
3600
3800
4000
4200
4400
Inertial weight (curb weight + 300 lbs)
slide 21 of 33
Strong relationship between car resale value and
risk
200
Big 3
Korean
Japanese/German
180
160
140
120
100
all cars
R2 = 0.82
80
60
40
20
0
$0
$5,000
$10,000
$15,000
$20,000
$25,000
$30,000
Retail price of MY98 car in 2003
slide 22 of 33
Debunking the “simple
simple physics
physics” argument
• “It’s simple physics; all else being equal, you are safer in a heavier
vehicle than in a lighter vehicle”
vehicle
• All else is never equal; vehicle design is important
— how well stiff structures in two vehicles are aligned
— presence of safety equipment (airbags,
(airbags new seatbelt technology
technology,
head rests)
— presence of interior padding
• We analyzed
y
fatalityy ratio (fraction
(
of car drivers who died, in
crashes with another car)
— little relationship between fatalities in frontal crash with another
car and car weight
• In
I car-light
li ht truck
t k crash,
h aggressivity
i it off ttruck
k iis more iimportant
t t than
th
weight of car
— most serious injuries in car-light truck side impact crashes are
due to truck intrusion into car, not deceleration
slide 23 of 33
Weak relationship between fatalities in frontal crash
with another car and car weight
90%
Subcompact cars (high-risk)
Subcompact cars (low-risk)
Compact cars
Midsize cars
Large cars
Import luxury cars
Sports cars
80%
70%
60%
50%
40%
30%
20%
10%
0%
2600
2800
3000
3200
3400
3600
3800
4000
4200
4400
I ti l weight
Inertial
i ht ((curb
b weight
i ht + 300 lb
lbs))
slide 24 of 33
IIntrusion
i becoming
b
i a major
j source off serious
i
injuries in certain crashes (Patel et al., 2006)
• Three general causes of serious injury in vehicles:
— contact with interior surfaces
— contact with intruded surfaces of other vehicle
— restrained deceleration
• Intrusion injuries are the result of structural incompatibility between
vehicles, rather than weight differential
• Intrusion injuries are nearly twice as prevalent when cars have been
struck in the side (61%) as when struck in the front (35%)
— intrusion causes 35% of serious injuries in a car when struck in
the side by another car, but 58% when struck in the side by a
light truck
• Further research to define “intrusion” in frontal crashes
slide 25 of 33
European researchers
researchers’ agree
• “The
The results from this project have overturned the original views
about [car-to-car] compatibility, which thought that mass and the
mass ratio were the dominant factors.” (Edwards et al., 2001)
• “The scientific community now agrees that mass does not play a
direct role in [car-to-car] compatibility.” (Delannoy et al., 2003)
• “Moreover, if mass appears to be the main parameter linked to
aggressivity of cars [against other cars], it is because this is the
easiest and universal parameter that is collected in all accident
databases.” (Faerber, 2001)
• There are very few light truck-car crashes in Europe; compatibility
even more important in US than in Europe
slide 26 of 33
Are crossover SUVs a solution?
• Conventional SUVs built on pickup chassis, with high/stiff fronts
(body on frame construction)
(body-on-frame
• Manufacturers now making “crossover” SUVs built on car-like, unit
body chassis
• Crossover design lowers center of gravity (increases stability,
reduces rollovers) and lowers/softens front (reduces aggressivity)
— crossovers are safer, for both crossover drivers and others, than
truck-based
truck
based SUVs
SUVs…
— … and crossovers tend to have 17% higher fuel economy than
truck-based SUVs with the same interior volume
• However crossover SUVs tend to have lower towing capacity than
some truck-based
t kb
d SUV
SUVs
slide 27 of 33
Crossover SUVs have lower risks than truck-based
SUVs…
180
Risk to drivers of other vehicles
Risk in rollover crashes
Risk in all other crashes
160
52
140
Risk
120
100
64
57
71
56
80
60
44
46
34
40
20
35
19
30
36
Large
truck-based
SUVs
Compact
crossover
SUVs
29
65
42
37
11
17
0
All SUVs
Compact
truck-based
SUVs
Midsize
truck-based
SUVs
Midsize
crossover
SUVs
slide 28 of 33
… and about 17% higher fuel economy for same
interior volume (MY05)
(
)
35
Crossover 2WD SUVs
Truck-based 2WD SUVs
30
25
20
15
10
5
0
0
20
40
60
80
100
120
140
160
180
200
220
Interior volume (cubic feet)
slide 29 of 33
Light trucks are becoming safer
• Rollover risk in SUVs is declining
— due to increased numbers of safer crossovers, rather than
improvements to truck-based SUVs
— truck-based SUVs and pickups still have much higher (2x)
rollover risk than that of average car
• Increased use of Electronic Stability Control should dramatically
reduce rollovers in all vehicles
• Risk that ppickups
p impose
p
on others is declining
g
— some claim this is from voluntary changes made by
manufacturers to biggest pickups…
— …but risk to others is declining even for compact pickups and
larger
g p
pickups
p that have not yyet adopted
p
voluntary
y changes
g
— risk reduction may be due to changes made to cars to make
them more compatible with trucks
— even with improvement, the risk that pickups impose on others
still much higher (2x to 5x) than that of average car
slide 30 of 33
Going forward
• New standards
— California AB1372 (Pavley);
• regulates
tailpipe CO2 emissions, under Clean Air Act
exemption
— revised US CAFE standard
• regulates fuel economy (miles per gallon)
• Both regulations continue light truck “loophole”
— pickups and SUVs must meet a less stringent standard than
cars
— although CA treats smaller trucks
trucks, SUVs and minivans (LDT1)
as cars
• Better approach would be to require all vehicles to meet same
standard
— this would dramatically raise price of heaviest pickups and SUVs
— subsidize purchase of heavy pickups and SUVs for appropriate
commercial uses through tax incentives
• Safety can be regulated directly,
directly independently of fuel economy
slide 31 of 33
Both California and US standards continue
“loophole” for light trucks
50
50.8
Car
LDT2
CARB (Pavley)
40
37.2
33.5
31 5
31.5
30
US CAFE
(estimated by CARB)
20
10
0
2008
2010
2012
2014
2016
2018
2020
Model year
slide 32 of 33
Other resources
• LBNL reports
— http://eetd.lbl.gov/EA/teepa/pub.html#Vehicle
• NHTSA crash tests (NCAP)
— http://www.safecarguide.com/exp/usncap/usncap.htm
• NHTSA CAFE FAQ
— http://www.nhtsa.dot.gov/cars/rules/cafe/overview.htm
• IIHS crash tests
— http://www.iihs.org/ratings/default.aspx
p
g
g
p
• IIHS driver death rates
— http://www.iihs.org/sr/pdfs/sr4204.pdf
• Public Citizen vehicle safety
— http://www.citizen.org/autosafety/
• High and Mighty: SUVs: The World’s Most Dangerous Vehicles and
How Theyy Got that Way,
y, Keith Bradsher
slide 33 of 33