Hola Allison
"Rápido" can be used both as an adjective and an adverb. When used as an adverb, it works like "rápidamente".
In this sentence though, it would sound a bit odd to say "Tú conduces demasiado rápidamente". It is only because it is accompanied by "demasiado", and rápido sounds better.
The same happens for example with lento/lentamente. They are interchangeable. For example: Ellos caminan lento (They walk slowly)= Ellos caminan lentamente. (They walk slowly).
Thanks for your question.
Inma : )
