I'm looking for a technical word to describe objects that are travelling in parallel pathways but in opposite directions from each other. For instance on a highway you have two cars , one in the southbound lane and the other in the northbound lane, you would say the cars are .............(fill in the blank).
ODO gives this definition of antiparallel:
Parallel but moving or oriented in opposite directions.
This fits your first two requirements (register and general definition) but would not fit your form and would not be suitable in most broadcast traffic commentaries.
For traffic in particular, we have oncoming cars
a car etc that is coming towards you
However, this is normally used from the vantage point of the street, or in close proximity to the traffic. It doesn't sound idiomatic to use it as a remote, conceptual description.