From Siena take the freeway (Fi-Si) towards Florence, go on until the San Donato in Poggio Exit.
From Pisa take the freeway (Fi-Pi-Li), few meters after the Lastra a Signa Exit you find, on your right, the entrance into the highway A1 towards Roma, take it and go on until Firenze Certosa Exit.
From Firenze Certosa take the freeway (Fi-Si) towards Siena and go on until San Donato in Poggio Exit.