If there are two vehicles on a road, and I want to differentiate them, can I say "the ahead vehicle", using ahead as an adjective?
No - "ahead" is an adverb and is always relative to something:
- One car is ahead of the other (compares the two positions)
- On the road ahead (compares to your relative position)
In your context, referring to a vehicle that is ahead of another, you could say:
- The car in front.
or, if the cars were competing in a race, you might say:
- The car in the lead.