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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 \(< 80 \, \mathrm{km/h}\).
Whenever a property doesn't change before and after an event, the property is said to be conserved. For example, the total number of chocolates is conserved even when you give it to your friends. (Obviously, if someone eats the chocolate, the number is not conserved!)

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.