filum viae

from Bouvier's Law Dictionary, Revised 6th Ed (1856)
FILUM VIAE. The thread or middle of the road. 
     2. Where a law requires travellers meeting each other on, a road to 
drive their carriages to the right of the middle of the road, the parties 
are bound to keep ou their side of the worked part of the road, although the 
whole of the smooth or most travelled path may be upon one side of the filum 
viae. 7 Wend. 185; 5 Conn. 305. 
    

[email protected]