Empty running is driving without cargo, for example to the loading point or back after delivery. Empty kilometres cost the full amount but earn no revenue.
Impact: €800 of trip cost over 1,000 km, of which 200 km empty, means €1.00 per loaded kilometre instead of €0.80.
In costing: add empty kilometres to the route as a separate leg, or spread the costs over loaded kilometres only.
Reducing it: backhauls, groupage loads and freight exchanges.