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Vehicle Design and the
Physics of Traffic Safety
Professor Marc Ross
University of Michigan
Last Week’s Talk: Weight reduction
increases Fuel Economy
1. Smaller vehicles, as done in the late 1970s
2. Change basic structure, e.g. unibody (instead of today’s
body-on-frame SUVs - also called car-based & crossover)
3. Lighter materials
– ultra-high strength steels, aluminum
– composites, sandwich structures
4. Reduce Engine Size
– Smaller engine capable of switching to high power
• high speed engine, or
• turbocharge, or
• variable displacement, split engine
5. Smaller automatic transmission
– automatic "manual" or double clutch or CVT
6. Rationalization of Vehicle Parts
Weight, Displacement & Fuel Reduction
(late 1970s Oil Shortages & CAFÉ Legislation)
new cars, sales weighted
7.0
37% fuel intensity reduction, 1975-1981
or 60% fuel economy increase
Weight Displacement Fuel Intensity
(gal/100 mi)
6.0
24% weight reduction, 1976-1980
5.0
35% displacement reduction, 1976-1980
(liters)
4.0
(1000 lbs)
3.0
2.0
How was this accomplished?
1.0
Fuel Intensity - gal/100 mi
Inertial Weight - lbs
0.0
Engine Displacement - liters
1975 1980 1985 1990 1995 2000 2005
EPA: Light-Duty Automotive Technology & Fuel Economy Trends: 1975 Through 2004
Cars mass in 1970s now called SUVs
Cars: Percent Fraction of Sales by EPA Weight Class (lbs) …….
30
2001
25
1986
20
15
1976
10
5
0
2000 2500 3000 3500 4000 4500 5000 5500 6000
Source: Heavenrich & Hellman "fuel economy trends" http://www.epa.gov/otaq/fetrends.htm ………..
How does the mass effect
societal safety?
Note: it does not if the
vehicle is designed well
Demo 1: Car Track
Mass War
• It has been argued that it is much safer to
ride in a heavy vehicle.
• Auto manufacturers have responded by
offering very heavy SUVs (Hummer,
Excursion, Durango, etc.).
• Truth: Camry, has a risk for its own driver
similar to that for a midsize and large
SUVs; but the corresponding risk when a
Camry collides with another vehicle is
much lower.
Crash A: Frontal
Intact effects 1. At the same speed a
frontal crash is less
injurious.
2. If speed is not very
high, then there is no
intrusion in a frontal
crash. Restraints
High Energy Absorption can protect
occupants.
Crash B: Side-Impact 3. In a side-impact
crash, sedan
occupants are at risk
from hard-contact.
Intrusion effects 4. In both, the SUV
occupants are
slightly less at risk
except for rollover.
Demo 2: Egg Low Energy Absorption
Crash Test #5161 Video
Crash Test #5161 Video
Near-side Forces (G’s) vs. Time (ms) Far-side Forces (G’s) vs. Time (ms)
Δp = ΔF * Δt Δp = Δf * ΔT
• Pelvic forces (acceleration in G’s) much stronger than
center-of-mass acceleration of target vehicle
• Momentum conserved (Δp)
• Pelvis: Big Δforce & small Δtime
• Vehicle: Small Δforce & big Δtime
Demo 3
Force Transfer
1. Same impulse
2. Left-side (near-side):
serious injuries
3. Far side: injuries usually
less serious and fewer
4. Passenger experiences
~¼ the magnitude of the
fT Ft force the driver
experiences.
Risk: Deaths/million registered Vehicles
of given type 4 to 5 times
More Risk
2 to 3 times
More Risk
Risks: Driver Deaths/Year per Million Vehicles
175 (most-popular models 1997-2001)
Risk to drivers of other vehicle by named model
150 Subcompact cars
Compact cars
Midsize cars Ram 1500
Large cars
125
Minivans
in all 2-vehicle crashes
Dakota
Pickup trucks
F-150
SUVs
100 Sports cars Marquis, Malibu,
Chevy/GMC
C/K 1500
Taurus/Sable,
Grand Prix
Tahoe
Ranger
75
Chevy Suburban
Explorer Tacoma
Astro Van
Expedition Cherokee TownCar, S-10
Wrangler Blazer
Caravan, Windstar, Lumina
50 Altima Intrepid Mustang
4Runner
Town & Country G Cherokee
Grand Am Neons
Venture Deville Stratus Cavalier/
Sentra Escort/ Corvette
Sunfire
Camry 626 LeSabre, Corolla Tracer
25 CR-V
Avalon Legacy
Jetta Civic Saturn
Accord,
Maxima Century
0
0 25 50 75 100 125 150 175 200
Risk to drivers in vehicle by named model in all crashes
Risks: Driver Deaths/Year per Million Vehicles
175 (most-popular models 1997-2001)
• Corvette demonstrates both the role of driver
Risk to drivers of other vehicle by named model
Subcompact cars
150 behavior and vehicle design. Corvette is
Compact cars
Midsize cars
dangerous to its own driver. The driver is very
Ram 1500
Large cars aggressive as shown by violations record.
125 • In spite of aggressive driving, the Corvette is
Minivans
in all 2-vehicle crashes
Pickup trucks Dakota
for
relatively safe F-150drivers of other vehicles with
SUVs which it crashes. Chevy/GMC
100 Sports cars safest
• Jetta is one of the C/K 1500 cars in spite of
Marquis, Malibu,
Taurus/Sable,
Tahoe
aggressive drivers as shown by violations
Grand Prix
Ranger
75 record.
Chevy Suburban
Explorer Tacoma
Astro Van
Expedition Cherokee TownCar, S-10
Wrangler Blazer
Caravan, Windstar, Lumina
50 Altima Intrepid Mustang
4Runner
Town & Country G Cherokee
Grand Am Neons
Venture Deville Stratus Cavalier/
Sentra Escort/ Corvette
Sunfire
Camry 626 LeSabre, Corolla Tracer
25 CR-V
Avalon Legacy
Jetta Civic Saturn
Accord,
Maxima Century
0
0 25 50 75 100 125 150 175 200
Risk to drivers in vehicle by named model in all crashes
Risks: Driver Deaths/Year per Million Vehicles
175 (most-popular models 1997-2001)
• Here we are considering sub-compact cars.
Risk to drivers of other vehicle by named model
Subcompact cars
150 They have about the same mass.
Compact cars
Midsize cars
• The bottom of the market sub-compact cars
Ram 1500
Large cars have high risk for their own driver.
125
Minivans • The top of the market sub-compact cars have
in all 2-vehicle crashes
Dakota
Pickup trucks low risk for their own driver.
F-150
SUVs • Both kinds of sub-compact cars have low risk
100 Sports cars Marquis, Malibu,
Chevy/GMC
for other drivers. C/K 1500
Taurus/Sable,
Grand Prix
Tahoe
Ranger
75
Chevy Suburban
Explorer Tacoma
Astro Van
Expedition Cherokee TownCar, S-10
Wrangler Blazer
Caravan, Windstar, Lumina
50 Altima Intrepid Mustang
4Runner
Town & Country G Cherokee
Grand Am Neons
Venture Deville Stratus Cavalier/
Sentra Escort/ Corvette
Sunfire
Camry 626 LeSabre, Corolla Tracer
25 CR-V
Avalon Legacy
Jetta Civic Saturn
Accord,
Maxima Century
0
0 25 50 75 100 125 150 175 200
Risk to drivers in vehicle by named model in all crashes
Risks: Driver Deaths/Year per Million Vehicles
175 (most-popular models 1997-2001) Models: 2001 - 2005
Excursion SUV
Yukon
Bigger area, bigger societal risk
Risk to drivers of other vehicle by named model
100 Subcompact cars
150
Compact cars Outlander
Rodeo
90 Hummer/H2
Midsize cars Ram 1500
Large cars Explorer Compact SUVs
125 Tahoe
80 Minivans Midsize SUVs
in all 2-vehicle crashes
Durango Dakota Large SUVs
Pickup
M/ML Class trucks F-150
70 SUVs Navigator Chevy Suburban Compact CSUVs
100 Sports cars
Chevy/GMC
Marquis, Malibu, Blazer/Trailblazer Midsize CSUVs
Cherokee/Liberty Jimmy/Envoy C/K 1500
60 Taurus/Sable,
Tribute Grand Prix Vitara/Grand Vitara
Tahoe GMC Suburban/Yukon XL
4Runner Expedition Ranger
50
75 Aztek
Chevy Suburban
Murano Rendezvous
Pilot Explorer Tacoma
Astro Van Sorento Wrangler
40 Sequoia, Expedition Cherokee TownCar, S-10
RX300/33 Element Wrangler Blazer
Caravan, Windstar, Lumina
Pathfinder, Grand Cherokee, Escalade
50 Altima Intrepid Mustang
4Runner
30 X5
Town & Country G Cherokee
Xterra Grand Am Neons
Highlander Mountaineer, Vue
Stratus Tracker
Venture Deville Cavalier/
Escort/ Corvette
20 MDX Sentra Sunfire
25 CR-V
CR-V
Camry Jetta
626 LeSabre, Corolla Tracer
Escape, Forester, Discovery
Avalon Legacy
Jetta Civic Saturn
10 Santa Fe, RAV4
Accord,
Maxima Century
0
0 10 20
25 30 40
50 50 60
75 80
70 100 90 125 110 150 130 175
100 120 160
140 150 200
Risk to drivers in vehicle by named model in all crashes
Risks: Driver Deaths/Year per Million Vehicles
175 (most-popular models 1997-2001)
- Is weight the key factor?
- What about crash test rating?
Risk to drivers of other vehicle by named model
150 Subcompact cars
Compact cars
Midsize cars Ram 1500
Large cars
125
Minivans
in all 2-vehicle crashes
Dakota
Pickup trucks
F-150
SUVs
100 Sports cars Marquis, Malibu,
Chevy/GMC
C/K 1500
Taurus/Sable,
Tahoe Grand Prix
Ranger
75
Chevy Suburban
Explorer Tacoma
Astro Van
Expedition Cherokee TownCar, S-10
Wrangler Blazer
Caravan, Windstar, Lumina
50 Altima Intrepid Mustang
4Runner
Town & Country G Cherokee
Grand Am Neons
Venture Deville Stratus Cavalier/
Sentra Escort/ Corvette
Sunfire
Camry 626 LeSabre, Corolla Tracer
25 CR-V Avalon Legacy
Jetta Civic Saturn
Accord,
Maxima Century
0
0 25 50 75 100 125 150 175 200
Risk to drivers in vehicle by named model in all crashes
Consumer Report Safety Rating The Curb Weight by Car Model
by Car Model as an Indicator of Risk as an Indicator of Risk
200
Risk to drivers
38 cars 29 cars
180
2
R = 0.40 R2 = 0.48
Neon
160
140
120
100
80
Regal
60
Risk to drivers
40
20
0
0
0.0 2000.5 400 1.0 600 1.5800 1000
2.0 1200
2.5 1400 3.0 1600 1800
3.5 2000
4.0
two tons - Curb Weight (in pounds) CRSAmax - CRSA
The Inverse of Price of a 5-Year Old Car Model
is the Best Indicator of Risk
200
38 cars
180
R 2 = 0.82
160
140
Risk to drivers
120
100
80
60
40
20
0
0 High Price
25 50 75 100 125 150 175 Low Price 225
200
1/P, where P is the retail price of MY98 cars in 2003
Historical Safety Record
Vehicle Design and the
Physics of Traffic Safety
Professor Marc Ross
University of Michigan
Email: mhross@umich.edu
1. Strong passenger compartment to inhibit
intrusion
2. Avoid large differences in vehicle structure
(height, rigidity, mass)
3. Convert kinetic energy into crush energy
(structural interaction)
