More damage is done and more life is endangered in a head-on collision of two cars, each traveling at 30 mph, than in a car crashing into a brick wall at 30 mph because:

Kinetic energy (KE) = 0.5mv^2

For two cars of equal mass moving towards each other,

KE = 2*0.5*m*30^2 = 900m J

For one car hitting a wall, KE = 0.5m*30^2 = 450m J (since the wall is stationary).

It can be seen that two cars hitting each other head-on, the energy to be dissipated is twice as that to be dissipated for a car hitting a wall. This resuts to more damage as such energy is taken up by the contents in the car.