I think it is difficult to decide if I would rather be going 30 mph and
collide with a concrete wall or with an identical car going 30 mph in
the opposite direction. I might lean toward the concrete wall.
In favor of the concrete wall, assuming I hit it perpendicularly, the
primary sequence is the car begins to stop, the air bag deploys, I hit
the air bag and seat belt. Pretty much all motion is straight ahead
until coming to rest. This is the ideal situation for the seat belt,
shoulder harness, and especially the air bag, to protect me.
In contrast, the collision between cars, even if identical and even if
head on, can lead to spinning or other non-straight-ahead motions before
coming to rest. Any slight assymetry in the collision can result in
this. The bumper of one car can even ride up over the other, depending
on whether there is perfect symmetry and also on how things begin to
crumple. It is more difficult to protect the occupants from hitting
their heads on the door frame, etc. in these situations. Side cushion
air bags can help, if you have them.
Of course, in a real situation I might be able to avoid the other car,
which is not really part of this question. But note that if I try to
avoid the other car and do not succeed, my actions can change the
head-on collision to a partially head-on collision. Whether that is
worse or better depends on where you are sitting compared to where the
car contact is, an it depends on what happens next (such as a rolling
the car, going into the ditch, going off the road and hitting a tree,
ending up in other lane and getting hit by another car, etc.).
The point is, there are a lot more variables when hitting another car
versus hitting a concrete wall. It's hard to protect the occupants when
so many more bad things are possible.
Of course, when this question is asked in a physics forum, the
assumption is supposed to be that the motion is straight ahead in both
cases until the cars stop. In that case, I guess I don't care.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu