Meaning of "Share the road. Don't be shellfish."
If a truck has a sign on its back saying "Share The Road. Don't Be Shellfish!"
What exactly does it mean? What does shellfish refer to?

Question

Answer

It sounds like they are making a joke by misspelling "selfish" as "shellfish". Does the truck belong to a shellfish company?

Answer

Is there a picture of a prawn or a lobster on the sign? Or perhaps it's a truck delivering seafood? 
Either way, this sign is a joke - a wordplay on the fact that the word 'shellfish' sounds a lot like 'selfish'.