Your physics makes no sense. Why is this modded informative? The ground is not a magical reference point!
If two cars travelling in opposite directions at 40 MPH slam into each other, that's exactly equivalent, in terms of energy dissipation and momentum transfer, to one car travelling at 80MPH slamming into a stationary vehicle. Each vehicle, in its own reference frame, sees another vehicle travelling at 80MPH.
Think about it: if two identical cars crash, and one is stationary, then for a moment (before they come to a stop due to friction against the pavement) they'll be moving together at half the speed of the moving car before the crash. One car goes from 80MPH to 40MPH (40MPH difference); the other goes from 0MPH to 40MPH (40MPH difference).
This is exactly equivalent to going from 40MPH to 0MPH (40MPH difference).
When you're working out simple kinematics like this you should be starting with momentum, which is linear with velocity. You can work out how much energy is released afterwards; you'll see that it works out:
(1/2) * (1500kg) * (36m/s) ^ 2 = 972 kJ - Amount of kinetic energy in the moving car at 80MPH
(1/2) * (1500kg) * (18m/s) ^ 2 * 2 = 486 kJ - Amount of kinetic energy left after the crash: 2 cars at 40MPH
972 kJ - 486 kJ = 486 kJ - Amount of kinetic energy dissipated in the crash
(1/2) * (1500kg) * (18m/s) ^ 2 * 2 = 486 kJ - Amount of kinetic energy in 2 cars at 40MPH
(1/2) * (1500kg) * (0m/s) ^ 2 * 2 = 0 kJ - Amount of kinetic energy left after the crash: in 2 cars at 0MPH
486 kJ - 0 kJ = 486 kJ - Amount of kinetic energy dissipated in the crash
(Yes, kinetic energy is 1/2 mv^2, not mv^2!)