Feb 25, 2011 ... 2005-08 models during 2006-09, trend based on 1/weight .... Toyota. Camry
Solara 2D. 95.6. 3,185. Toyota. Camry 4D 2WD. 94.2. 3,263.
The Relative Safety of Large and Small Passenger Vehicles www.iihs.org
NHTSA Mass-Size Safety Symposium Washington, DC ● February 25, 2011 Adrian Lund, Ph.D.
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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Motor vehicle crash deaths per billion miles traveled 1950-2009 80
60
40
20
11.3 per billion
0 1950
55
60
65
70
75
80
85
90
95
2000
05
www.iihs.org
Cumulative percentage of passenger vehicles By model year and curb weight (lbs) 120%
100%
80% 1983
60%
1988 1998
40%
2008 20%
0% 1,501-
2,001-
2,501-
3,001-
3,501-
4,001-
4,501-
5,001-
5,501-
6,001-
6,501www.iihs.org
Cumulative percentage of passenger vehicles By model year and shadow (sq ft) 120%
100%
80%
60%
40%
1983 1988 1998 2008
20%
0%
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Driver deaths per million vehicle registrations 1985-88 models during 1986-89, trend based on 1/weight 300 cars and minivans pickups
250
SUVs car trend 200
pickup trend SUV trend
150
100
50
0 1,500
2,000
2,500
3,000
3,500 4,000 4,500 curb weight (lbs)
5,000
5,500
6,000
6,500
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Driver deaths per million vehicle registrations 1995-98 models during 1996-99, trend based on 1/weight 300 cars and minivans pickups
250
SUVs car trend 200
pickup trend SUV trend
150
100
50
0 1,500
2,000
2,500
3,000
3,500 4,000 4,500 curb weight (lbs)
5,000
5,500
6,000
6,500
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Driver deaths per million vehicle registrations 2005-08 models during 2006-09, trend based on 1/weight 300 cars and minivans pickups
250
SUVs car trend 200
pickup trend SUV trend
150
100
50
0 1,500
2,000
2,500
3,000
3,500 4,000 4,500 curb weight (lbs)
5,000
5,500
6,000
6,500
www.iihs.org
Driver deaths per million vehicle registrations 2005-08 models during 2006-09, trend based on 1/shadow 300 cars and minivans pickups
250
SUVs car trend 200
pickup trend SUV trend
150
100
50
0 60
70
80
90 100 vehicle shadow (sq ft)
110
120
130
140
www.iihs.org
Driver deaths per million vehicle registrations per year, by model year and curb weight (lbs) 200
2005-08 model cars during 2006-09 1995-98 model cars during 1996-99
150
1985-88 model cars during 1986-89
100
50
0 2,000
2,500
3,000
3,500 curb weight (lbs)
4,000
4,500
5,000 www.iihs.org
Driver deaths per million vehicle registrations per year, by model year and shadow (sq ft) 200
2005-08 model cars during 2006-09 1995-98 model cars during 1996-99
150
1985-88 model cars during 1986-89
100
50
0 70
80
90 100 vehicle shadow (sq ft)
110
120 www.iihs.org
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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Curb weight vs. shadow, by make and series Model year 2008 cars and minivans (r²= 0.70) 8,400 curb weight (lbs) 7,400 6,400 5,400 4,400 3,400 Toyota Camry 4dr
2,400 1,400 40
60
80
100
120
140
160
shadow (sq ft) www.iihs.org
Relative collision claim frequencies By body style and size class, 2008-10 models
micro mini small midsize large very large
150
100 = all-passenger-vehicle result of 7.0
100
50
0
four-door cars
luxury cars
station minivans wagons
two-door cars
sports cars
SUVs
luxury SUVs
pickups
www.iihs.org
Curb weight vs. shadow, by make and series Model year 2008 cars and minivans (r²= 0.70) 8,400 curb weight (lbs) 7,400 6,400 5,400 4,400 3,400 Toyota Camry 4dr
2,400 1,400 40
60
80
100
120
140
160
shadow (sq ft) www.iihs.org
Variation in weight among 2008 model year cars of similar size (93-95 sq. ft) to Toyota Camry make
series
Nissan Ford Toyota Toyota Volkswagen Chrysler Ford Dodge Chevrolet Lincoln Volkswagen Chevrolet Toyota Lincoln Toyota Chrysler Dodge Volkswagen Volkswagen
Altima 4D Fusion 4D 2WD Camry Solara 2D Camry 4D 2WD Passat 4D 2WD Sebring 4D Fusion 4D 4WD Avenger 4D 2WD Malibu 4D (new) Zephyr/MKZ 4D Passat SW 2WD Malibu Hybrid 4D Camry Solara Conv MKZ 4D 4WD Camry Hybrid 4D Sebring 4D 4WD Avenger 4D 4WD Passat SW 4WD Passat 4D 4WD
Shadow in sq. ft. 93.2 95.4 95.6 94.2 93.7 94.2 95.4 95.2 93.6 95.5 93.6 93.6 95.6 95.5 94.2 94.2 95.2 93.7 93.7
Weight in lb. 3,112 3,181 3,185 3,263 3,305 3,310 3,325 3,355 3,415 3,469 3,512 3,537 3,615 3,672 3,680 3,699 3,738 3,819 3,829 www.iihs.org
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 www.iihs.org
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 www.iihs.org
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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