What's the best way to model tax credit?

[qzaus]

Vehicles are usually modelled by processes where the capacity is the number of vehicles, the input flow(s) represent the energy consumption, and the output flow(s) represent the amount of v-km / p-km / t-km (possibly several flows according to travel distance).

FLO_TAX and FLO_SUB can be used for defining a tax / subsidy in proportional to any of the flows of a vehicle process. However, if the credit is related to a tax on vehicle purchase (investment), you can use NCAP_ITAX / NCAP_ISUB for defining an investment tax / subsidy per vehicle. And if it is related to an annual tax on vehicle use, you can use NCAP_FTAX / NCAP_FSUB (annual fixed tax / subsidy per vehicle). I think it is up to the modeller to decide which is the best way to model any such tax credits, but instead of adjusting the vehicle investment costs, I think would rather use NCAP_ITAX / NCAP_ISUB.

If I want to impose tax credit on investment in vehicles, but I only have the aggregate vehicle miles traveled, not the number of vehicles, can I still use NCAP_FTAX / NCAP_FSUB or NCAP_ITAX / NCAP_ISUB?

In addition, since a lot of the tax-credit will be proportion-based, what will be a good way to impose a tax-credit like that? Thank you!