Learn German: Der Radfahrer fährt an dem Schild vorbei. - The cyclist is riding past the sign.

an ... vorbei is a two-part preposition (often called a circumposition) that means "past" or "by". The first part, an, goes right before the noun, and the second part, vorbei, goes at the end of the phrase or clause.

Since the cyclist is moving, why doesn't an take the accusative case here?

That's a great catch! Usually, two-way prepositions like an take the accusative case when there is active movement towards a destination. However, the specific fixed combination an ... vorbei is an exception and always triggers the dative case. That is why we use the dative neuter dem Schild.

Can I say am Schild vorbei instead of an dem Schild vorbei?

Absolutely. In everyday spoken German, an dem is frequently contracted to am. Saying Der Radfahrer fährt am Schild vorbei is perfectly natural and very common.

Why is it dem Schild? Isn't Schild a masculine word?

German actually has two words spelled Schild, and they have different genders and meanings! das Schild (neuter, which becomes dem Schild in the dative case) means "sign" (like a street sign). der Schild (masculine) means "shield" (like the armor a knight carries).