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The Relative Safety of Large 

and Small Passenger Vehicles

NHTSA Mass-Size Safety Symposium

Washington, DC ● February 25, 2011

Adrian Lund, Ph.D.
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Overview

• What is the history of motor vehicle crash safety by size and 

mass? Historical trends

• Can down-weighting of vehicles occur without safety 

consequences if size is held constant? Physics of injury

• What does the future hold? 
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Historical trends
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0 

20 

40 

60 

80 

1950 55 60 65 70 75 80 85 90 95 2000 05

11.3 per billion

Motor vehicle crash deaths per billion miles traveled

1950-2009
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Cumulative percentage of passenger vehicles

By model year and curb weight (lbs)

0%

20%

40%

60%

80%

100%

120%

1,501- 2,001- 2,501- 3,001- 3,501- 4,001- 4,501- 5,001- 5,501- 6,001- 6,501-

1983

1988

1998

2008
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Cumulative percentage of passenger vehicles

By model year and shadow (sq ft)

0%

20%

40%

60%

80%

100%

120%

1983

1988

1998

2008
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0

50

100

150

200

250

300

1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500

cars and minivans

pickups

SUVs

car trend

pickup trend

SUV trend

Driver deaths per million vehicle registrations

1985-88 models during 1986-89, trend based on 1/weight

curb weight (lbs)
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0

50

100

150

200

250

300

1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500

cars and minivans

pickups

SUVs

car trend

pickup trend

SUV trend

Driver deaths per million vehicle registrations

1995-98 models during 1996-99, trend based on 1/weight

curb weight (lbs)
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0

50

100

150

200

250

300

1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500

cars and minivans

pickups

SUVs

car trend

pickup trend

SUV trend

Driver deaths per million vehicle registrations

2005-08 models during 2006-09, trend based on 1/weight

curb weight (lbs)
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0

50

100

150

200

250

300

60 70 80 90 100 110 120 130 140

cars and minivans

pickups

SUVs

car trend

pickup trend

SUV trend

Driver deaths per million vehicle registrations

2005-08 models during 2006-09, trend based on 1/shadow

vehicle shadow (sq ft)
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Driver deaths per million vehicle registrations 

per year, by model year and curb weight (lbs) 

0

50

100

150

200

2,000 2,500 3,000 3,500 4,000 4,500 5,000

2005-08 model cars during 2006-09

1995-98 model cars during 1996-99

1985-88 model cars during 1986-89

curb weight (lbs)
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0

50

100

150

200

70 80 90 100 110 120

2005-08 model cars during 2006-09

1995-98 model cars during 1996-99

1985-88 model cars during 1986-89

Driver deaths per million vehicle registrations 

per year, by model year and shadow (sq ft)

vehicle shadow (sq ft)
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Historical conclusions

• Passenger vehicles of all types and sizes provide their 

occupants with greater protection today than just a decade ago 

and much greater protection than two decades ago

• Occupants of the smallest and/or lightest vehicles still have 

death rates about twice as high as occupants of the largest 

and/or heaviest vehicles 
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Mass and Size –

Physics predicts independent 

effects on occupant injury risk
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What is the source of injury in motor vehicle crashes? 

“...in the highway safety area the problem is almost exclusively 

one of mechanical energy reaching people at rates that involve 

forces in excess of their injury thresholds.”

– William Haddon, Jr., MD, American Journal of Public Health, 1968

First head of what is now NHTSA (1967-68)

President of IIHS from 1969-1985
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What affects forces experienced by occupants in crashes?

• In a simple model of frontal crashes, forces act on the occupant to bring his or 

her pre-crash velocity to its post-crash velocity

• The longer the distance over which the occupant’s velocity change occurs, the 

lower the average force experienced by the occupant

– Change in occupant energy state ≈ Force * DistancepreV to postV

• The occupant’s stopping distance is a combination of

– The space between the occupants and stiff parts of the compartment in front of them

Given that drivers generally seat themselves similarly in different cars to optimize vehicle control, 

restraint system design and usage is the primary determinant of this factor’s relevance

– The effective crush distance of the car in front of the occupant compartment

Occupants of longer vehicles typically will have greater effective crush distance 

– The distance which the car’s momentum carries forward or is reversed by the 

momentum of the crash partner (car, tree, pole, wall) during the crash

Occupants of heavier vehicles typically will benefit from greater effective momentum
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Car size and weight are separate physical factors 

influencing the stopping distance of an occupant

How well can their effects be quantified in vehicle crash experience?

• Vehicle size and weight tend to vary together in actual cars

• Car size and weight can influence crash likelihood, including the likelihood of 

different types of crashes

– Rollover risk decreases with size and weight but injury risk given a rollover increases with 

size and weight

– Smaller vehicles are involved in more crashes (not fewer, as some have hypothesized)

• Many other vehicle characteristics that can affect crash likelihood and severity 

are confounded with size and weight

– Heavier cars for a given size often have larger engines, 4-wheel drive, or are convertibles
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1,400

2,400

3,400

4,400

5,400

6,400

7,400

8,400

40 60 80 100 120 140 160

curb weight (lbs)

shadow (sq ft)

Curb weight vs. shadow, by make and series

Model year 2008 cars and minivans (r2= 0.70)

Toyota Camry 4dr
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Relative collision claim frequencies

By body style and size class, 2008-10 models

0

50

100

150

100 = all-passenger-vehicle result of 7.0

four-door

cars 

luxury

cars

luxury pickups

SUVs

sports SUVs

cars

two-door

cars

station minivans 

wagons

micro

mini

small

midsize

large

very large
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1,400

2,400

3,400

4,400

5,400

6,400

7,400

8,400

40 60 80 100 120 140 160

curb weight (lbs)

shadow (sq ft)

Curb weight vs. shadow, by make and series

Model year 2008 cars and minivans (r2= 0.70)

Toyota Camry 4dr
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Variation in weight among 2008 model year cars 

of similar size (93-95 sq. ft) to Toyota Camry

make series Shadow

in sq. ft.

Weight

in lb.

Nissan Altima 4D 93.2 3,112

Ford Fusion 4D 2WD 95.4 3,181

Toyota Camry Solara 2D 95.6 3,185

Toyota Camry 4D 2WD 94.2 3,263

Volkswagen Passat 4D 2WD 93.7 3,305

Chrysler Sebring 4D 94.2 3,310

Ford Fusion 4D 4WD 95.4 3,325

Dodge Avenger 4D 2WD 95.2 3,355

Chevrolet Malibu 4D (new) 93.6 3,415

Lincoln Zephyr/MKZ 4D 95.5 3,469

Volkswagen Passat SW 2WD 93.6 3,512

Chevrolet Malibu Hybrid 4D 93.6 3,537

Toyota Camry Solara Conv 95.6 3,615

Lincoln MKZ 4D 4WD 95.5 3,672

Toyota Camry Hybrid 4D 94.2 3,680

Chrysler Sebring 4D 4WD 94.2 3,699

Dodge Avenger 4D 4WD 95.2 3,738

Volkswagen Passat SW 4WD 93.7 3,819

Volkswagen Passat 4D 4WD 93.7 3,829
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Conclusions about size, mass, and safety

• Mass and size have separate, inverse relationships with 

occupant injury risk in crashes predicted by physics

• Quantifying those separate effects in the real world of a car fleet, 

however, is complicated by 

– The high correlation between size and mass in real world vehicles, 

and

– the fact that, even after controlling for things like driver age and 

gender, there are important other confounding variables that covary

with size and weight and have their own separate effects on injury risk

• Failure to find the separate effects indicates a failure to 

adequately account for the confounds, not that physical laws 

have been repealed
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Size, mass, and

safety of future vehicles
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Predictions

• Vehicles will get lighter and smaller as fuel prices increase

One benefit of size-indexed CAFE is to keep larger, safer cars affordable 

longer for all income brackets

• The sky will not fall as the fleet downsizes

We will not see an increase in absolute injury risk because smaller cars will 

become increasingly protective of their occupants

• However, some people will die in motor vehicle crashes that 

would have survived without the downsizing

– Large cars still will have fatality rates half as large as smaller cars and the 

smaller (on average) fleet that results will not be as safe as it could be

• Those of us whose mission is highway safety will have to adapt 

our strategies to the reality created by consumer choices

We will be ok if we let data on what works – not wishful thinking – guide

our strategies
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Smart Fortwo frontal offset test
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Smart Fortwo into Mercedes C class
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Minitruck into Ford Ranger
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Dedicated to reducing deaths, injuries,

and property damage on the highway