Small and large scale behaviour
These simulations look at composite objects - made up of balls linked together by bonds to form simple "vehicles" - and how they behave in a variety of collisions.
What happens in the simulations can be looked at on the small scale; how each ball (in each vehicle) responds to the forces on it - or on the large scale; how the vehicles behave and interact.
The computer program running the simulation works only on the small scale. It calculates the forces on each ball, and how its motion should change as a result. It doesn't know or work out how the vehicles behave, just the individual behaviours of each of the balls that they're make up of.
We, watching the simulation, can see both the small and large scale behaviour, but we're often only interested in the small scale (the balls) for the insights it can give us into the large scale (the vehicles).
Crumple or bounce
The vehicles in these crash simulations are made up of a 2-D rectanglar array of balls joined together by bonds. These bonds have some elasticity, but beyond a certain strain become permanently stretched or compressed. The outer bonds in vehicles with crumple zones are more easily permanently stretched or compressed than those in vehicles without crumple zones (which will tend more to spring back to their original length). Vehicles of the same size have the same mass.
Some of the simulations display graphs of the acceleration in the middle of the vehicle - the passenger compartment of a real car. This acceleration matches the forces on a seatbelted passenger - although sudden forces may be damped somewhat by any 'give' in the seat or seatbelt. In physics "acceleration" means "rate of change of velocity" - and so includes slowing down, speeding up or changing direction (although the acceleration displayed in the graph is just that in the horizontal dimension of your computer screen).
Crumple zones reduce the accelerating forces on passengers in two ways. First, the collapse (or crumple) of the crumple zone means that a smaller accelerating force is applied to the car over a longer period (than with a car without crumple zones which is subject to a larger force for a shorter period). Second, energy is absorbed by the crumple zone (rather than stored elastically) so there is little bounce back. For a car without crumple zones the collision (for a car hitting a large hard stationary object) doesn't just stop the car, it bounces it back in the direction it came from, increasing the severity of the collision.
Seatbelts work in conjuction with crumple zones by making the passenger slow down at close to the same rate as the passenger compartment - and also spreading the accelerating forces over parts of the body (shoulders, chest, hips) that can withstand them. Without a seatbelt a passenger would simple fly forward to hit part of the car that had already been slowed gradually by the crumple zone - and be stopped suddenly by a larger force against a smaller and more delicate area of the body. (see links for more discussion of seatbelts, airbags and crumple zones)
How the simulated vehicles interact
Interactions between vehicles occur through collisions between the balls that they're made up of.
Within each vehicle the balls feel only the forces of the bonds between them; the balls themselves are allowed to overlap.
Bonds are black with zero strain; turning blue (then cyan) as they are stretched, and red (then yellow) as they're compressed.
The blue arrows in the simulation indicate the velocity of each part of a vehicle. The red arrows show the forces on each part.
The simulated vehicles are somewhat more rubbery than the real thing, accentuating and making more visible the interactions that occur in a collision.
The simulations depart most from reality in that the vehicles aren't steered by wheels in a particular direction but slide along without friction regardless of orientation. The fact that vehicles are made up of a small number of discrete objects results in a certain lumpiness in the forces experienced throughout a collision.
Links
How Stuff Works: car safety
http://www.howstuffworks.com/category.htm?cat=Safe
seatbelts - http://www.howstuffworks.com/seatbelt.htm
airbags - http://www.howstuffworks.com/airbag.htm
Car safety measures
http://www.science.org.au/nova/057/057key.htm
Physics of car speed
http://www.science.org.au/nova/058/058key.htm
Physics of car safety design
http://suberic.net/~avon/mxphysics/max%20and%20andrew/theory.html
Hyper physics - an analytical approach:
inelastic collision examples
http://hyperphysics.phy-astr.gsu.edu/hbase/inecol.html#c3
Seatbelts
http://hyperphysics.phy-astr.gsu.edu/hbase/seatb2.html
http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html
large-small truck collision
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html
car crash - effect of crumplezones
http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html
vehicle stopping distance
http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
Cause of speedway driver deaths
http://www.tennessean.com/sii/00/07/08/safety08.shtml
Pedestrian safety
http://archive.cln.com/charlotte/newsstand/c041099/metro2.htm
Car mass and safety - and the SUV debate
Size and Safety: the different effects of a 100 pound weight reduction for cars and for light trucks
http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/808569.html
An alternative view - does higher vehicle mass significantly improve driver safety - or is good design more important?
http://www.ems.org/gas/safety_concerns.html
http://www.aceee.org/press/t021pr.htm
The Aggressivity of Light Trucks and Vans in Traffic Crashes:
http://www-nrd.nhtsa.dot.gov/departments/nrd-11/aggressivity/980908/980908.html
Insurance Institute for Highway Safety: Vehicle compatibility in crashes
http://www.hwysafety.org/srpdfs/sr3409.pdf
Impassioned rant against SUVs:
http://www.net-monster.com/blather_suvs_part1.html / part2 / quotes (including the following)
SUVLOVE "I want to drive a tank..."
http://www.wweek.com/html/leada041900.html
"We really hate it when the wreckage of Honda Accords gets all tangled up in our fenders"
http://www.post-gazette.com/columnists/20000606regcol.asp
Hyperphysics large-small truck collision
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html
Vehicle mass and crumple zones discussion
- read post from pete98m3, comment from mlangner and reply from pete98m3
http://www.pelicanbbs.com/posts/86222.htm
SUV dangers and precautions
http://www.iinc.org/cons_issues/auto/suvs.html
http://www.parent-teen.com/driving/nosuvsforteens.html
Friends of the Earth SUV site - safety issues
http://www.suv.org/safety.html
SUVs and the environment
Friends of the Earth SUV site - environmental issues
http://www.suv.org/environ.html
California Greenhouse Gas limits hit SUVs
http://www.planetark.org/dailynewsstory.cfm/newsid/15573/story.htm
http://www.cnn.com/US/9811/06/suv.regulations/
http://www.enn.com/news/wire-stories/2002/07/07032002/reu_47724.asp
http://www.enn.com/news/wire-stories/2002/04/04222002/reu_suvs_46993.asp
CSE/CEI conservative thinktank responses
http://www.cse.org/informed/issues_template.php/1054.htm
http://www.cei.org/gencon/019,02422.cfm
non-computer simulations...
Egg Crash Vehicles experiment - Smithtown Central School District
http://www.smithtown.k12.ny.us/msteched/eggcrashcars.htm