Correct! As both cars have stopped, both their speeds are $$0 \, \mathrm{km/h}$$. So, the total speed is $$0 + 0 = 0 \, \mathrm{km/h}$$. This is obviously $$< 80 \, \mathrm{km/h}$$.
But, the total speed before and after a crash is not conserved. Before the crash, the total speed was $$80 \, \mathrm{km/h}$$. After the crash, it is $$0 \, \mathrm{km/h}$$. As the total before the crash is different from the total after the crash, we can say that the total speed isn't conserved.