While we've been beating this about (incorrectly on my part), no one has
stated just what is correct. Let me try again.
Considering the energy aspect, assuming that the collision shares energy
between the two cars equally (using a parked car as the fixed object), then
the head-on at v should be energetically equivalent to a collision at 1.414v
with a fixed car--both collisions will dissipate mv^2 of energy. Does this
imply equivalent forces on the cars? If so, then the 35 mph head on is
equivalent (ignoring the messy details of the collision) with hitting the
tree at 49.5 mph. Is this the point originally being made? Still seems
that the force question depends on the delta-t to stop, but then if we were
to assume incompressible objects and therefore identical delta-T's, then the
force in the fixed collision is still bigger than the head-on, but this
would take me back to one of the correct statements I made--"Now certainly a
collision with the parked car at speed v is less serious
than a head-on with both cars travelling at v." I've also started to worry
about the SR implications although I realize that the collisions imply
accelerations so one probably can't argue that the head on collision needs
to be indistinguishable whether we are in the earth frame or the frame of
one of the cars (which is perhaps the source of thinking that it is
equivalent to a 2v fixed collision). I've now confused myself enough--so
I'll go home.