There is no available measure of the capacity of terminal airspace to support VFR traffic whilst it is used by different volumes of IFR traffic. As a result airspace change sponsors have to make estimates about the anticipated levels of IFR traffic and the likely impact VFR traffic will have upon it and vice versa. Moreover there is no agreed benchmark to assess if the VFR access provided in terminal airspace is unreasonable.
To provide a benchmark, terminal airspace needs to be modelled under a range of IFR traffic conditions. This modelling methodology would be agreed by all users and become the standard basis for airspace design. It would be tested against a range of current scenarios as part of the benchmarking process. Measures would also be proposed to assess the operational efficiency of the airspace once it is in operation in order to verify the actual operation against the design assumptions.