For every addition of 100 people added the speed is halved so for the first 100 people added its speed is 1/2 * 500 km/hr = <<500*1/2=250>>250 km/hr.
For another addition of 100 people its speed is also halved, 1/2 * 250 km/hr = <<250*1/2=125>>125 km/hr. So for the total of 200 additional people, its speed is 125 km/hr
#### 125