Unless you are highly concerned about deflection of the road, why any surcharge at all?   Some passes with a partly loaded truck will tell a lot.

Thanks for the reply. We have a temporary access road built on a reinforced slope of site won material. To design the reinforcement for the slope to British Standard BS8006-1, using internal stability analysis, the surcharge is required to design the thickness of backfill layers and length of geogrid reinforcement.

Surely the heavier the construction machinery using the road, the more reinforement is required on the slope?

For standard roadway design in the US we use a distributed load of 2ft of soil (approximately 240psf) for traffic load.  Do the conversion -  we predominantly use the imperial system.  How is the bearing soil? Is it very soft?  Note:  This is a temporary design, so use a lower factor of safely - don't be too conservative for temporary.