The Relationship between Vehicle Weight Size and

Physics of Sustainable Energy:

The Relationship between Vehicle 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 SocietyBerkeley CA, March 1-2, 2008

by

Tom Wenzel Marc RossLawrence Berkeley National Laboratory

TPWenzel@lbl.govUniversity of MichiganMHRoss@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 gastechnologies 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

slide 2 of 33

—stronger roofs—vehicle-to-vehicle communication

Two views on vehicle weight and safetyTwo 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 relationshipsimplistic 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 andbenefit 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 iprotection

•Crashes with another vehicle or stationary object—reducing weight ratio would reduce deceleration of lighter

vehiclevehicle—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•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 aggressivitynecessary 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 probably more important even than width

slide 4 of 33

g p y p—electronic stability control promising technology to prevent

rollovers

Definition of riskDefinition of risk

•“Risk”: driver fatalities per year, per million vehicles registered as of Jan 2005Jan 2005—driver fatalities from NHTSA Fatality Analysis Reporting System

(FARS)• FARS includes many details on all US traffic fatalities

registered vehicles as denominator or measure of “exposure”—registered vehicles as denominator, or measure of exposure

•Because it is based on actual fatalities, our definition of risk incorporates:—vehicle designvehicle 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-vehicle crashes only risks to other driversbecause from two vehicle crashes 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 2000-04differences less than 10% not statistically significant—differences less than ~10% not statistically significant

•Risks by vehicle model—calculated using only 69 most popular vehicle models, to reduce

statistical uncertaintystatistical uncertainty—differences less than ~20% not statistically significant

slide 7 of 33

Risks by vehicle type

250

300Risk to drivers of other vehiclesRisk to drivers

39 38 68 90

147

199

150

200

isk

143 14233 30 46

39 66

68 90

94100

150Ri

35 4074 85 75

96143 142

8248

123 105 101 100

5223

40 40

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Min

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Mid

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Com

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slide 8 of 33

Impo

Sub C

Sub

Truc

k

Cro

Com 1/ 3/

Risk to drivers in rollovers and all other crashes

250

300Risk to drivers of other vehiclesRisk in rollover crashesRisk in all other crashes

200

sk

19 25

34 4449

43 47100

150Ris

27 3260 66 61 71

109 99

40 3174 61 45 53 37

8 8

14 19 1425

4317

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slide 9 of 33

Impo

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Sub

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Sub

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k-b

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1/2

3/4 1 F

Risks by vehicle type200200

1-ton pickups

150 3/4-ton pickups

100full-size vans 1/2-ton pickups

50

truck-based SUVs

large carscrossover SUVs

sports cars

compact pickups

minivans compact cars

00 20 40 60 80 100 120 140 160

midsize carsimport luxury cars

subcompact cars (high risk)

subcompact cars (low risk)

slide 10 of 33

0 20 40 60 80 100 120 140 160

Risk-to-drivers

Risks by vehicle model

100

110

120 Subcompact carsCompact carsMidsize carsLarge carsD k t

Ram 1500

80

90

100 gMinivansSUVsCompact pickup trucks1/2-ton pickup trucks

ExplorerTahoe

Dakota

Jimmy/Envoy

Chevy/GMC C/K 1500

F-150

Durango

50

60

70p p

Wrangler

XTerra

Chevy Suburban

BlazerTacoma

Ranger

S-10/Colorado

y y

Jetta, Accord, Camry, Caravan,

DevilleC ll M lib

Expedition

IntrepidGrand Am

Neon

FrontierGrand Cherokee, Cherokee/Liberty

Altima Taurus

GalantStratus

Venture

20

30

40

Town & CountryFocus4Runner

Corolla, Malibu, Grand Marquis

SC/SL/Ion

Sunfire, Escort

Grand AmCavalierAccent

Alero

CR-VRX300/330

OdysseySienna

Windstar/Freestar

M i

Century

Altima, Taurus, Impala

Sable

Venture

Beetle

0

10

20

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Avalon, Legacy

Elantra, Grand Prix, Civic, Sentra

Saturn LSLeSabre, Sebring

Passat

RX300/330 Maxima

slide 11 of 33

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Risk-to-drivers

Effect of vehicle design on riskg

•High risk to drivers of pickups and SUVs from their propensity to roll overover—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 1 15 28% chance of rolloveraverage SUV SSF is 1.15, 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 fatality rate is 5x higher when struck in side by SUV y g y

(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

MY02 Dodge Ram 150 pickup truck

Driver behavior and environment influence riskDriver behavior and environment influence risk

•Driver characteristics that affect riskage and sex—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 and driver’s record over last two yearsdriving in current crash, and 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)th i k Bl /T ilbl h l li htl d i (18%—the risky Blazer/Trailblazer has only slightly worse drivers (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 modelthan 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 (0.20 v. 0.38)g ( )

•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 ll h hi h i k d d i th l hJetta all have higher risk and worse drivers than lower 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 f i i kEffect of environment on risk

•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 respectively)420 people per sq 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

UrbanRural

Risk to drivers Risk to drivers of other vehicles

150

200Rural

100

150

50

0Car Minivan SUV Pickup Car Minivan SUV Pickup

slide 18 of 33

p p

Vehicle type

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 minivanspickups, SUVs and minivans

•We excluded models overly influenced by their drivers (young males or elderly)

slide 19 of 33

Weak relationship between car weight and risk…

180

200

140

160

80

100

120

R2 = 0.1740

60

80

0

20

2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400

slide 20 of 33

2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400

Inertial weight (curb weight + 300 lbs)

… unless one accounts for manufacturer

180

200

Big 3Korean

Big 3

R2 = 0.39140

160 Japanese/German

80

100

120

Saturn SC/SL

Ford Focus

Mitsubishi Galant

40

60

80

Saturn L/LS

Japanese/German

R2 = 0.540

20

2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400

slide 21 of 33

2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400

Inertial weight (curb weight + 300 lbs)

Strong relationship between car resale value and riskrisk

180

200

Big 3Korean

120

140

160 Japanese/German

R2 = 0.8280

100

120

all cars

40

60

0

20

$0 $5,000 $10,000 $15,000 $20,000 $25,000 $30,000

slide 22 of 33

Retail price of MY98 car in 2003

Debunking the “simple physics” argumentDebunking the simple physics argument

•“It’s simple physics; all else being equal, you are safer in a heavier vehicle than in a lighter vehicle”vehicle than in a lighter vehicle

•All else is never equal; vehicle design is important—how well stiff structures in two vehicles are aligned

presence of safety equipment (airbags new seatbelt technology—presence of safety equipment (airbags, new seatbelt technology, head rests)

—presence of interior padding

•We analyzed fatality ratio (fraction of car drivers who died, in y y (crashes with another car)—little relationship between fatalities in frontal crash with another

car and car weight

•I li ht t k h i it f t k i i t t th•In car-light truck crash, aggressivity of truck is more important than 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

80%

90%Subcompact cars (high-risk)Subcompact cars (low-risk)Compact cars

60%

70% Midsize carsLarge carsImport luxury carsSports cars

30%

40%

50%

10%

20%

30%

0%

10%

2600 2800 3000 3200 3400 3600 3800 4000 4200 4400

I ti l i ht ( b i ht 300 lb )

slide 24 of 33

Inertial weight (curb weight + 300 lbs)

I i b i j f i Intrusion becoming a major source of serious injuries in certain crashes (Patel et al., 2006)

•Three general causes of serious injury in vehicles: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%)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’ agreeEuropean researchers agree

•“The results from this project have overturned the original viewsThe 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 accidenteasiest 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 Europeeven 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 construction)

•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 SUVstruck based 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 t k b d SUVsome truck-based SUVs

slide 27 of 33

Crossover SUVs have lower risks than truck-based SUVs…

52160

180Risk to drivers of other vehiclesRisk in rollover crashesRisk in all other crashes52

64120

140Risk in all other crashes

56

46

57 71

44

60

80

100

Ris

k

37

6542

30 36

3435 19

11

29

20

40

60

3017

0All SUVs Compact

truck-basedSUVs

Midsize truck-based

SUVs

Large truck-based

SUVs

Compactcrossover

SUVs

Midsizecrossover

SUVs

slide 28 of 33

SUVs SUVs SUVs SUVs SUVs

… and about 17% higher fuel economy for same interior volume (MY05)( )

35

Crossover 2WD SUVs

25

30Crossover 2WD SUVsTruck-based 2WD SUVs

15

20

5

10

0

5

0 20 40 60 80 100 120 140 160 180 200 220

slide 29 of 33

Interior volume (cubic feet)

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 pickups impose on others is decliningp p p 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 pickups that have not yet adopted voluntary changesg p p y p y 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 SUVs and minivans (LDT1)although CA treats smaller 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 independently of fuel economy

slide 31 of 33

Safety can be regulated directly, independently of fuel economy

Both California and US standards continue “loophole” for light trucks

50CarLDT2

50.8

4037.2

33.531 5

CARB (Pavley)

20

3031.5

US CAFE (estimated by CARB)

10

20

02008 2010 2012 2014 2016 2018 2020

slide 32 of 33

Model year

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.aspxp g g p

•IIHS driver death rates—http://www.iihs.org/sr/pdfs/sr4204.pdf

•Public Citizen vehicle safetyPublic Citizen vehicle safety—http://www.citizen.org/autosafety/

•High and Mighty: SUVs: The World’s Most Dangerous Vehicles andHow They Got that Way, Keith Bradsher

slide 33 of 33

y y,