'Bottom up' formula approach to calculating Transport S106 contributions - Public forum - Planning Advisory Service (PAS)

I am currently looking at updating our evidence base the out Planning Obligations SPD. As part of this process we are looking at the way in which we calculate a benchmark figure for transport contributions. We feel that it would be best to do this from a 'bottom up' formula approach...i.e one person = £x cost to transport infrastructure. Is there currently anyone working a system similar to this?