# Jam Shock Speed

On a highway, there are 100 cars per kilometer and the cars are all moving at $$100 \text{ km/hr}$$. One car notices something and slams on its breaks, slowing down to $$50 \text{ km/hr}$$ and halving the distance between itself and the car behind it.

How quickly does the shock wave generated move relative to the ground (in$$\text{ km/hr}$$)?