From Florence to San Casciano Val di Pesa

The section from Florence to Tavarnelle of this itinerary follows the historic and scenic road which retraces the 13th-century Siena-Rome road, which also connected Florence to Poggibonsi and to the important Via Francigena, or Frank Road, that came down from Lucca.
The present-day Cassia road was indeed the main connection between Florence and Rome until the construction of the Autostrada del Sole motorway. This  accounts for the abundance of architectural and artistic sites (historic centers, villas, palazzi, churches, and convents) which were built along this road and throughout the surrounding hills.
The initial section of the Cassia road runs through the city of Florence and is called Via Senese. It then winds through hills which embrace the plain to the south - hills clad with olive trees and dotted with ancient secluded villages - and runs past monuments such as the 14th-century convent of San Gaggio (today a private residence), through the hamlet of Galluzzo, and the imposing monastery Certosa di Firenze.
Once past the Certosa, the Cassia road enters the Greve valley and continues towards the Bottai. At the Ponte degli Scopeti, the road branches off on the right towards San Casciano Val di Pesa. It passes through Sant’Andrea in Percussina - a cluster of rural houses amongst which is the 'Albergaccio', which is the house where Niccolò Machiavelli lived during his exile in Florence.
The main road, however, continues through the valley up to Ponte dei Falciani, then it too goes uphill towards San Casciano.

From San Casciano Val di Pesa to the Museum of Sacred Art in Tavarnelle Val di Pesa

Once past San Casciano, the Cassia road continues downhill towards the Pesa River, skirts the rural hamlets of Calzaiolo and Bargino, where there are a few industrial sites as well. Before reaching the bridge over the Pesa River, which falls within the Tavarnelle territory, the road branches off into a narrow road on the left which leads to Badia a Passignano (five-kilometers away) - a monastery built on a hill where the Pesa, Greve, and Elsa valleys meet. 
The Cassia road does not continue to the hamlet of Sambuca. The town developed around the Romanesque bridge over the Pesa River, which was the road's obligatory gateway to Tavarnelle. Today the Sambuca section of the road leads to  San Donato in Poggio, retracing another historic Roman road: the road between Sambuca and San Donato, which leads to Castellina in Chianti.
Back on the Cassia road, before getting to Sambuca the road branches  off towards Tavarnelle Val di Pesa, crosses the river at Ponte Nuovo, and winds through the hilly side on the left of the valley. At the enterance of the town, follow the directions to pieve di San Pietro in Bossolo, where the Museum of Sacred Art is situated.
Not far from Tavarnelle there are important religious monuments. The Carmelite convent of Santa Maria al Morrocco and the Marian sanctuary of Santa Maria delle Grazie and of the Madonna di Pietracupa are amongst the major ones.

After Tavarnelle the Cassia road continues towards  Barberino Val d’Elsa ('terra nuova', or 'new territory' of Florence in 13th century). A detour on the right leads to the hamlet of Petrognano, where there is a villa-farm and tower houses.  Petrognano lies near Semifonte - a city destroyed by Florence in 1202. Here there is an octagonal chapel built in 1597 by Santi di Tito as a model for the dome of Santa Maria del Fiore (Florence's cathedral).
On the opposite side, the road leads to the monumental complex of Sant’Appiano, which comprises an ancient parish church and an Antiquarium. The church is perhaps the oldest Romanesque church in the territory of Florence. The oldest parts of this church are in stone and date from around the 11th century. They consist of arcades supported by square pillars along the left aisle, and the apse, with its protruding arcaded decoration. Across from the church, bearing witness to the existance of an ancient baptestry, demolished in 1805, are four columns (two of which are not intact) whose capitals are sculpted with symbolic images: the tau, the cross, stars, etc.). 

From:

Museo d'arte sacra di Tavarnelle Val di Pesa. Guida alla visita del museo e alla scoperta del territorio, edited by Rosanna Caterina Proto Pisani (2005), Florence, published by Polistampa (Piccoli Grandi Musei).