If one is travelling at u and the other at v (relative to the road) then
from the frame of reference of one the speed of the other is uv, where
u and v are now velocities.
By way of example, the car you are following while both are travelling
at 60 mph relative to the road is zero, the distance between the cars
remains constant. 60  60 = 0. The car approaching from the opposite
direction
will have velocity 60 mph relative to the road and the distance is now
changing. 60  (60) = 120.
If there is an angle, then the velocity is (vu).cos(theta).


Fumble Index  Original post & context: 6isvd.9173$A6.6830@fe2.news.blueyonder.co.uk 