Reflecting this route, Hallingdal and its neighboring valley of Valdres in Oppland to the north were originally populated by migrants from Vestlandet and spoke a western dialect.

In recognition of this, Cardinal Nicholas Breakespear, who was in Scandinavia as papal legate in 1153, included Hallingdal in the diocese of Stavanger.

Flå is bordered in the north by Sør-Aurdal, in the east by Ringerike, in the south by Krødsherad and Sigdal, in the west by Nore og Uvdal, and in the northwest by Nes.

Travelers from the south pass through the 65 m long tunnel Hallingporten on Norwegian National Road 7 (Riksvei 7) just located north of Gulsvik.

The Norefjell mountain range also includes parts of Flå as well as Nes, Ringerike and Sør-Aurdal municipalities.