Wegfahren, fortfahren, losfahren, abfahren — all these verbs mean to leave by car (e.g. drive away, drive off). But is there any difference between them or are they complete synonyms? In case of difference, please, provide an example.
As far as I know, there are other verbs in German to describe the leaving place action, such as abreisen and verreisen, but in the specific question I'm curious to understand the difference between all kind of *fahren verbs.
Wegfahren (drive off) is focussing on a short movement from a given location without concern for a final destination, although this movement might as well be only the beginning of a longer journey. The means of transport is usually a private one, a car, motorcycle, bicycle, etc.:
Fahr von dort weg, du stehst im Halteverbot.
Ich fahre noch kurz weg.
Wir wollten über Ostern wegfahren, doch leider bekam ich keinen Urlaub.
Als sie wegfuhr, wusste sie nicht, ob sie jemals zurückkehren würde.
The meaning of fortfahren (drive away) is close to that of "wegfahren" but implies a longer journey or absence. It is more often about driving away from home than from a temporary accomodation:
Ich fahre morgen fort, und ob wir uns wiedersehen, ist ungewiss.
But it can as well mean to continue (for this, see the answers to that question)
Losfahren simply means starting to move, to get under way, usually with a private means of transport:
Wann fahren wir endlich los?
Als sie losfuhren, brach ein schweres Unwetter herein.
It can also refer to a public vehicle, or any vehicle when the driver is not important in the sentence, but in this case the vehicle itself is the subject:
Als das Auto losfuhr, bemerkten sie, dass die Heckklappe noch offen war.
Der Bus fuhr sofort los, als alles Gepäck verstaut war.
Der Zug fuhr langsam los, als das Signal die Fahrt freigab.
...although for a train, "anfahren" (start) is more common.
Abfahren means taking off, usually on regular public service like train or bus.
Wann fährt dieser Zug endlich ab?
Abreisen (depart) usually signifies the end of a temporary stay at a certain location, departing by all proper means of transport for a longer journey.
