Definition: Further forward in time; in advance; in the near future; in a forward direction; onward; forward.
Semantic variation: Further forward in space; in or toward the front.
These are two general signs in ASL sentences but it doesn't mean either of them can be used in every "ahead" found in English sentences (like every language in translation).
For example, the word "ahead" found in English, "The line of cars moved ahead slowly", does not apply to the ASL sign above. ASL has its own structure (grammar and vocabulary) for the same concept of the sentence.
In case you're wondering, use the dominant-handed horizontal "3" handshape (classifier) making "ahead" of the passive-handed horizontal "3" classifier, for a car making ahead of the other car.