Pedantic attack @ various people
Your example doesn't quite work as your initial statement is that the "2 people start running away from each other". Therefore the directional element of each velocity is expected to be the exact opposite of the other.
Suggesting that the one running moves with a negative velocity would put him on the same directional heading as the other, just with a greater magnitude. The faster runner will be running away from the slower but the two are not running away from each other.
The speed is just the magnitude of the velocity thus 20m/s is obviously faster then 10m/s given the fixed starting frame of reference.
"Celsius is not really a scientifically accepted scale"
Technically the Celsius scale is accepted as 1 degree C = 1K. The unit scientifically used for a change in temperature should be the Kelvin as it is the SI unit.