Two break-even thresholds — and why both are honest
The appraisal can be done two ways, depending on what benefits are counted.
TEE-only break-even
285 vpd
Counts transport user benefits only: journey time saving, vehicle operating costs, reliability. The narrow, most conservative test. Against £625m adjusted capex (£500m ministerial ×1.25 HM Treasury optimism bias).
Full Green Book break-even
5,000 vpd
Adds societal benefits: health gains, tourism revenue, labour-market access, carbon. The standard HM Treasury Green Book / Better Business Case test adopted across NI public investment appraisal since 2021.
| Scenario |
AADT |
TEE result |
Green Book result |
| TEE break-even |
285 |
BCR = 1.0 ✓ |
BCR < 1.0 ✗ |
| Today’s ferry |
650 |
BCR > 1.0 ✓ |
BCR < 1.0 ✗ |
| Low-demand stress |
1,200 |
BCR > 1.0 ✓ |
BCR 0.83 ✗ |
| Full BCR break-even |
5,000 |
BCR >> 1.0 ✓ |
BCR = 1.0 ✓ |
| Central (Yr 5) |
14,100 |
BCR 2.41 ✓✓ |
BCR 2.41 ✓✓ |
| High (Corran-class) |
16,000 |
BCR ~3.0 ✓✓✓ |
BCR ~3.0 ✓✓✓ |
How the 285 vpd figure is derived: Setting £625m adjusted capex equal to the present value of transport user benefits alone: 60-year appraisal at 3.5% Green Book STPR (annuity factor ~24.9), £8/return-trip TAG-priced benefit (30-minute time saving × £8.45/hr blended VOT + vehicle operating costs + reliability), giving £25m/yr required — equivalent to 275–285 vpd at typical paying share.
How the 5,000 vpd figure is derived: From the Quintin QS feasibility dashboard’s full appraisal engine, which adds societal benefit streams (tourism £95–110M, labour-market access, health gains) and carbon (DESNZ Dec 2024 values) to the transport user benefits. Break-even located by bisection at fixed central inputs. Both calculations use the same £625m adjusted capex anchor.
Capex anchor: £500m stated by Minister Kimmins, Assembly 11 May 2026. Adjusted by HM Treasury standard optimism bias of 1.25× =
£625m. DfI source document (FOI DFI-2024-0412, 7 Aug 2024) described its own estimate as a “very rough cost estimate.” The Quintin QS evidence-based component build-up gives £321m (pre-OB), £350m (reduced OB). Full methods:
feasibility dashboard →