John Stott, in his book, The Cross of Christ, briefly mentions the role of the image of a fish in early Christianity:

Only the initiated would know, and nobody else could guess, that icthys ('fish') was an acronym for Iesus Christos Theou Huios Sotēr ('Jesus Christ, Son of God, Saviour'). But it did not remain the Christian sign, doubtless because the association between Jesus and a fish was purely acronymic (a fortuitous arrangement of letters) and had no visual significance.

Clearly Stott never had the privilege of driving around 21st century America.