One of the 4 core parts of a self Driving Car Software stack is Planning. And RNDF or Route Network Definition File is the first thing PLANNING uses.
An RNDF is a tab-delimited ASCII file. It basically defines the Roadsegments and Zones.
The typical format of an RNDF file looks like the following.
RNDF_name filename (string)
num_segments number_of_segments (integer>0)
num_zones number_of_zones (integer≥ 0)