System

New Tyne Crossing Transport Appraisal System, session 3 of decomposition. This session began decomposing the Economic Appraisal Engine subsystem but the session failed before requirements could be created. Only the architecture decision was committed.

Decomposition

The architecture decision (ARC-003) documents an eight-component decomposition separating each monetisation domain — user benefits, accidents, environmental, GHG, public accounts, wider impacts — into independent modules feeding a central Present Value and Discounting Engine. This topology mirrors the DfT TAG unit structure (A1 TEE/PA, A1.3 COBALT, A3 environment/GHG, A2.1 wider impacts) ensuring each module can be independently validated against its governing TAG unit.

The Present Value and Discounting Engine is centralised rather than distributed across modules because Green Book discounting rules (3.5%/3.0% split at year 30) and optimism bias uplifts must be applied consistently. The BCR and AST Generator is a separate consolidation module because AST format is prescribed by DfT.

Artefacts

Type	Count
Architecture decisions	1
Subsystem requirements	0
Interface requirements	0
Verification entries	0

Notes

Session crashed after committing the architecture decision. Subsystem requirements, interfaces, and verification entries were not created. Work continued in subsequent sessions.

Reconstructed from AIRGen data — original journal post lost to session failure.