This is in reference to my comment that I took the wrong observer as my stationary frame, but that the speed of the observed motion would be the same for the other observer.

The main tenet behind Special Relativity is that two observers in different inertial reference frames measure exactly the some properties of motion. So if one observer measures that a truck is moving toward him at a given speed then the driver of the truck must also measure the observer moving toward him at the same speed.

This can be easily verified in this simple example by the equation used to add velocities:

When you switch the roles of v and u in the equation the "sum" of the velocities is still the same.

If you are wondering why this principle doesn't work for the "

Twin Paradox" the answer is that one twin is accelerating with respect to the other twin at four different times, so we can indeed specify which twin is in motion. Thus there will be differences in the properties of motion of the two twins. Specifically, since an acceleration is involved this problem is a matter for General Relativity. Curiously I have never seen an analysis of this problem done using GR.

-Dan