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 u-v, 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 (v-u).cos(theta).