SLC Feasibility — Recommendation View

Scenario inputs

Central case is anchored to the £500m ministerial brief. Every other input sits at the upper end of the credible DfT TAG (Transport Analysis Guidance, formerly WebTAG) range — no single value is pushed to its limit. The dashboard is honest about where the case is fragile (carbon balance, funder rationality at default split, reference-class cost-overrun risk) so DfI economists can see the tensions on the same page.

Capital cost (£m, out-turn) 500

£350m£1,200m

Ministerial brief: £500m headline. Reference class (Mersey Gateway, Queensferry) spans £600m–£1.35bn so £500m sits at the low end; deliverability risk is captured in the optimism-bias slider and Risk & phasing tab.

Ferry recapture AADT, year 5 (veh/day) 1,100

6002,500

Yr 5 · 1,100 Yr 20 · — Yr 40 · —

Existing ferry users transferring to the fixed link, measured at year 5 post-ramp. Full TAG-consistent consumer surplus on the saving. Projection chips update with the traffic-growth slider below.

Induced AADT, year 5 (veh/day) 13,000

2,00018,000

Yr 5 · 13,000 Yr 20 · — Yr 40 · —

New trips generated by the link, measured at year 5 post-ramp. TAG Unit A1.3 rule-of-half applied. Grows with the traffic-growth slider below — projection chips show the trajectory.

Ferry time saving per crossing (min) 32

15 min50 min

Strangford ferry takes ~35 min door-to-door (queue / load / 8-min sail / unload). Fixed link ~5 min. Induced trips currently divert via 1-hr road route — they capture a larger generalised-cost saving (~1.8×) with the TAG rule-of-half applied.

Traffic growth (% per annum) 1.5% p.a.

0%3.0%

Yr 20 multiplier · — Yr 40 multiplier · —

Applied to year-5 AADT (post-ramp baseline). TAG M4 tapering: full rate years 1–20 post-opening, half rate years 21–40, flat thereafter. The two AADT sliders above show the resulting trajectory live.

Social discount rate (%) 3.5%

2.0%7.0%

Construction period (years) 6

4 yr10 yr

Agglomeration / WEBs (TAG Unit A2.4, narrow scope) 8%

0%12%

Restricted to agglomeration / connectivity only — labour-market access is a separate stream below to avoid double-counting.

Optimism bias on capex (%) 25%

10%80%

Green Book optimism-bias loading. Forms the upper leg of the triangular QCRA on the Risk & phasing tab.

Societal impacts extend the case beyond pure transport BCR

These inputs add separate benefit streams to the appraisal. Each is shown as its own slice in the benefits composition bar (right). Default values are indicative; OBC would replace with primary evidence.

Current ferry availability (%) 98.0%

85%99.9%

Lower current availability → larger recapture benefit from a fixed link (45-min Newtownards diversion avoided).

Labour-market uplift (% of induced AADT) 9%

0%18%

Jobs newly within 45/60-min commute. Rule-of-half + retention factor. Distinct from agglomeration WEBs above.

Time-critical health trips per year 1,800 / yr

4003,500

Binding cohort only: stroke, STEMI, major trauma, obstetric. NHS emergency-minute rate £18/min × ~40 min saving.

Gross induced tourism trips per peak-season day 1,800 / day

05,000

Net additional after displacement: 40% of gross. £35 avg spend × 1.2 local multiplier × 180 peak-season days.

Construction embodied carbon (kt CO₂e) 38 kt CO₂e

20 kt120 kt

A1–A5 embodied carbon. Benchmarked to the Rose Fitzgerald Kennedy Bridge (Wexford, 2020) — 887 m extradosed concrete segmental, 18,600 m² deck. SLC analogue at ~600 m + approaches ≈ 12,600 m² × 2,000 kgCO₂e/m² (IStructE / National Highways benchmark for major concrete bridges) + 30% for approach works = ~38 kt. PAS 2080:2023 compliance assumed. Priced at Green Book £290/tCO₂e.

Induced trip net VKT (km / trip) −15 km / trip (saved)

−30 (saving)0+20 (adding)

Geography-honest accounting for induced trips. The Strangford Narrows is unusual: an "induced" bridge user is often someone who currently drives the 50 km loop via Newtownards. Default −15 km / trip = 40% genuinely-new (+5 km) + 40% shortened (−45 km) + 20% mode-shift (+5 km). Push to positive values for a precautionary TAG-default view.

Political-economy override

Strategic premium (£m / yr) £0m / yr

£0m£40m / yr

Explicit value attached on peripherality / all-island / climate-resilience grounds, beyond standard VfM. Surfaces as a separate stream in the composition bar so the Green Book economic case (transport + societal + carbon) stays distinguishable.

Revenue & financing (transfers, not BCR)

Toll per crossing (£, car) £2.00

£0 (free)£5.00

Mersey Gateway £2, pre-2018 Severn £6.70, Dartford £2.50. Toll demand elasticity −0.3 applied: higher tolls suppress traffic. Reduces public-purse subsidy but does not enter BCR (transfer).

Toll-paying share (% of AADT) 80%

50%95%

Net of exempt cohorts (residents discount, emergency vehicles, blue badge) and price-off effect. Mersey Gateway runs at ~85%.

Rates uplift at maturity (£m / yr) £2.5m / yr

£0m£8m / yr

Ards Peninsula property value uplift → rateable value reassessment → rates revenue uplift. Ramps from 30% at opening to 100% by year 10. TAG Unit A2 anti-double-count: counted as financing, not as a BCR benefit.

NI Executive net position —

—

over the 60-year appraisal period (PV, real terms)

NI capex share —

− PV toll revenue —

− PV rates uplift —

= Net to NI Exchequer —

—

Levers: toll, paying share, rates uplift, NI funding %, capex, discount rate, traffic growth, ferry & induced AADT. All feed in live.

Construction funding mix

100%

Three contributors. Moving any one slider auto-rebalances the other two to keep the total at 100%.

NI Executive 40%

£191m

UK Government 45%

£215m

Irish Government 15%

£71m

Recommendation rationale

Central case

Benefit–Cost Ratio 1.84 Threshold ≥ 1.5 (High VfM)

NPV, 60-year appraisal £412m Present-value, discounted

Payback (undiscounted) Yr 22 From traffic opening

Adjusted capex £477m Includes optimism bias

Funder perspectives — attributable BCR

Within typical capital envelope

Each contributor needs their own attributable BCR ≥ 1.0 to defend their share. Benefit incidence: NI 70% / UK 12% / Ireland 18% (revised to reflect all-island Belfast–Dublin corridor exposure).

NI Executive

NI-attributable BCR 2.63 £191m capex · 1.7% of DEL

UK Government

UK-attributable BCR — vs 12% benefits

Irish Government

IE-attributable BCR — vs 18% benefits

Benefits composition

£61.4m / yr at full ramp

What is driving this result

BCR comfortably above the 1.5 high-value-for-money threshold. The result is being supported by strong forecast demand and a credible time-saving per crossing, partially offset by optimism-bias loading on capital cost.

Indicative only — assumes uniform ramp-up to year-1 demand, constant time-saving over appraisal period, Green Book central VOT (£8.45 / hr, 2025 prices), and carbon at £290/tCO₂e.

Strategic constraints — non-economic gates for OBC 5 open

Five qualitative gates outside the BCR appraisal. None are individually fatal but all five must have an answer before a Strategic Outline Case can clear Better Business Case (5-Case Model) gateway review. Status reflects realistic SLC position today — no formal SOC has been initiated.

Open
SAC, ASSI & Marine Nature Reserve consent

Strangford Lough holds multiple statutory designations: Special Area of Conservation (SAC, JNCC UK0016618), Special Protection Area (SPA), Area of Special Scientific Interest (ASSI), Ramsar wetland of international importance, and Marine Nature Reserve (NI’s first, designated 1995). A cable-stayed pier penetrating sensitive benthos requires Habitats Regulations Assessment (HRA), Environmental Impact Assessment (EIA), and biodiversity net-gain offset. Realistic timeline: 2–3 years of statutory process before shovel-ready. Structurally the hardest gate.

Implication: material schedule risk; could shift first-spend by 18–36 months. Mitigation costs already in the SAC / Marine Nature Reserve adjustment to capex benchmark.
Open
Section 75 equality screening

Northern Ireland Act 1998 §75 mandates equality impact screening across nine protected categories on all major public infrastructure decisions. SLC has visible distributional effects (Ards Peninsula population vs. wider NI taxpayer base; rural vs. urban access) that need formal assessment.

Implication: a structured screening exercise required pre-SOC; budget ~£ small. Not expected to be blocking but must be documented.
In progress
Subsidy Control & UK-EU TCA compliance

Tri-jurisdictional funding (NI / UK / Ireland) triggers questions under the UK-EU Trade and Cooperation Agreement subsidy-control regime. Irish Government contribution may be characterised as foreign subsidy; UK Levelling Up element must clear its own subsidy-control thresholds independently. Shared Island Fund precedent (A5 Western Transport Corridor, Narrow Water Bridge) provides legal cover for the principle.

Implication: bespoke legal advice needed on funding structure; precedent exists so resolvable but not automatic. Affects how the 65/20/15 split is formally documented.
Open
Operating model & lifecycle responsibility

No operator model assumed yet. Options: TransportNI direct operation, a concession arrangement (Mersey Gateway model with private DBFOM contractor), or a dedicated bridge authority (Severn Crossing model, since dissolved). Choice drives lifecycle cost profile, toll-collection mechanism, residual liability at end-of-concession, and whether opex sits in DfI baseline or off-balance-sheet.

Implication: blocks proper Whole-Life Cost (WLC) figures in the OBC. Currently the dashboard uses indicative 1.2% of capex p.a. for opex — a real OBC would derive this from the operating model.
Open
Reference class comparison (DfT Hallmark Review)

DfT Analytical Assurance Guidance requires demand and cost forecasts to be benchmarked against a comparable completed-scheme reference class. For estuarine fixed links the small comparator set (Queensferry, Mersey Gateway, Skye, Cromarty, M3 Tay) shows a 47% mean cost-overrun rate and ~30% mean demand-shortfall rate against original forecasts.

Implication: the 30% central optimism-bias loading in the dashboard sits at the low end of the reference-class range. P85 at 80% may be more defensible at SOC stage. Re-check after Hallmark Review.

Capex quantitative risk (QCRA)

Triangular distribution

Indicative cost risk: low = 85% of central, mode = central, high = central × (1 + optimism bias). Closed-form triangular quantiles; full Q-CRA correlated risk register out of scope at this stage.

P10 (best case) £553m

P50 (median) £711m

P85 (downside) £825m

BCR at P85 capex — other inputs held at central

Phased capex · NI-block DEL pressure

over 6 yr build

Annual NI capital draw over the construction period. Each bar shows the NI block's share of that year's out-turn capex, expressed as a percentage of the indicative £2.2bn annual NI capital DEL envelope.

3% DEL warning 6% DEL capacity ceiling

Carbon balance over 60-year appraisal

Net saving

Construction embodied emissions vs avoided ferry diesel and avoided Newtownards diversion VKT, discounted to present-value tonnes and priced at Green Book £290/tCO₂e.

Construction (embodied) 180 kt front-loaded over build years

Operational saving / yr 6.5 kt / yr avoided ferry diesel + diversion VKT

Net lifetime (PV) +108 kt positive = net saving

Monetised value +£31m already included in BCR

Indicative only. Excludes biodiversity net gain offset (statutory for SAC / Marine Nature Reserve crossings under Habitats Regulations Assessment), and operational maintenance carbon. Both flagged for OBC.

Revenue offset against capex

Transfers — not in BCR

Tolls and rates uplift are not BCR benefits (Green Book treats them as transfers between users / landowners and the public purse). They do reduce the net subsidy required from the three governments and materially change NI Executive affordability.

Adjusted capex £845m out-turn, inc. optimism

PV toll revenue (60-yr) £197m £2.00 toll · 80% paying · −5% price-off

PV rates uplift (60-yr) £46m £2.5m/yr at maturity, 30%→95% ramp

Net subsidy required £602m across all three governments

NI Executive net cost after revenue

Cost recovery: —

All operating revenue is assumed to accrue to the NI Executive (asset owner / operator). The NI cost picture is materially better than the gross capex share suggests.

NI capex share (gross) £549m 65% of adjusted capex

PV revenue to NI £243m tolls + rates uplift

NI Executive net cost £306m after revenue offset

—

Audit trail. Every numerical default in this dashboard is listed below with its source and methodological limit. The model is a screening tool consistent with Better Business Case (5-Case Model) Strategic Outline Case scope under HM Treasury / DoF NI guidance. It is not a substitute for a full Outline Business Case appraisal, which would require a calibrated TUBA/DIADEM transport model, primary origin-destination survey, GBV-calibrated cost estimate and Section 75 / EIA screening.

The remaining gap to OBC is named openly in the “Known limits” column. The dashboard’s job is to ask whether the question is worth answering, not to answer it.

1 · Cost & capital

Parameter	Default	Source / reference class	Known limit
Central capex	£500m (out-turn)	DfI internal correspondence note, 7 August 2024, disclosed under FOI DFI-2024-0412. Figure subsequently cited by Minister Kimmins MLA in Assembly response.	Not a costed engineering estimate. A real OBC would require a quantity-surveyed cost plan with marine geotechnical contingency.
Slider range	£350m–£1,200m	Brackets the reference class: Rose Fitzgerald Kennedy Bridge (£75m, 2020, 887m), Queensferry Crossing (£1.35bn, 2017, 2.7km), Mersey Gateway (£600m, 2017, 2.1km).	Reference-class costs are not always like-for-like; geotechnical and procurement context vary materially.
Optimism bias on capex	25%	HM Treasury Green Book (2025 edition) optimism-bias guidance, Standard Civil Engineering category at OBC stage. Aligned to the historic DfT supplementary guidance on optimism bias for transport schemes.	Should reduce to ~15% at FBC stage post-tender. Dashboard does not model gateway-driven reductions.
Construction period	6 years	Reference class: Mersey Gateway (4 yr), Queensferry Crossing (6 yr), Rose Kennedy (4.5 yr including approaches).	SLC marine geotechnical risk could push to 7 years; not modelled.

2 · Demand & transport benefits

Parameter	Default	Source / reference class	Known limit
Ferry recapture AADT (yr 5)	1,100 veh/day	Strangford–Portaferry ferry historical patronage (Wikipedia / DfI statistics): ~525 vehicles/day average, with seasonal peaks ~1,400. Post-link recapture at year 5 assumes most ferry users transfer plus modest growth.	Not derived from a calibrated transport model. OBC would require TUBA/DIADEM with observed counts.
Induced AADT (yr 5)	13,000 veh/day	Reference-class derivation from the HITRANS Corran Narrows fixed-crossing feasibility work (Scotland, 2023) — the closest comparable single-vessel ferry-replacement scheme in these islands. Specific AADT scenarios from the HITRANS study should be checked at OBC stage.	Reference class differs in catchment population and economic geography. A real OBC requires an SLC-specific origin-destination survey calibrated through TUBA/DIADEM.
Traffic growth p.a.	1.5%	DfT TAG (Transport Analysis Guidance, formerly WebTAG) Unit M4 demand growth defaults for rural / inter-urban routes; tapers to 0.75% yrs 21–40 and flat yr 41+. Supported by Northern Ireland Statistics and Research Agency (NISRA) population and GVA projections.	Sensitive to NI Executive economic projection updates.
Time saving	32 min	Strangford–Portaferry ferry observed door-to-door journey: ~35 min (8-min sail plus queue, load, unload) vs ~5-min direct fixed crossing. Source: ferry operator timetable plus campaign-collected observation.	Generalised cost saving may understate — ferry is unavailable ~2% of days, modelled separately.
VOT (Value of Time)	£8.45/hr	DfT TAG Unit A1.3 central business / commute blended rate, current TAG Databook values rebased to current prices.	Standard. Not contested.
Rule of half on induced	Applied	DfT TAG Unit A1.3 standard practice for valuing newly generated demand.	Standard. Not contested.

3 · Benefit incidence (cross-jurisdictional)

Parameter	Default	Source / reference class	Known limit
NI Executive share	70%	Origin distribution: Ards/Lecale resident trips, NI domestic tourism, NI-internal commuting. Belfast–Dublin corridor traffic captures the corridor uplift.	Asserted, not derived from gravity model. OBC would require calibrated TUBA model with NI/cross-border trip matrix.
UK Government share	12%	GB-origin tourism (NITB inbound visitor data 2024: ~12% of NI visitor nights from GB), GB-business connections to Belfast.	Treasury will likely demand a gravity-model derivation before accepting any GB benefit attribution.
Irish Government share	18%	All-island corridor exposure: Dublin–Belfast commuter traffic via A1/M1 (cross-border journey count 2024), Republic-origin tourism into NI East coast (Failte Ireland data).	Sensitive to assumed gravity-model parameters. Narrow Water Bridge Oversight Board precedent supports the cross-border framing.

4 · Funder rationality gate

Parameter	Default	Source / reference class	Known limit
Attributable BCR threshold	≥ 1.0 per funder	Treasury Green Book principle: each contributing fund must demonstrate VfM against its own attributable benefits.	Stylised heuristic, not a formal Better Business Case / Green Book test. Real funder co-investment decisions are negotiated and rarely reduce to a single ratio. Used here to surface the political-economy reality that UK Treasury would not co-fund without UK-attributable benefit.
15% minimum share trigger	Applied	Threshold below which a funder share is treated as a transfer rather than a co-funded position.	Author judgement. No statutory basis.

5 · Carbon

Parameter	Default	Source / reference class	Known limit
Construction embodied (A1–A5)	38 ktCO₂e	Derived from the Rose Fitzgerald Kennedy Bridge (Wexford, 2020) as a like-for-like reference class — 887m extradosed concrete segmental, ~18,600m² deck — scaled to SLC at ~12,600m² deck and applying the IStructE / National Highways embodied-carbon benchmark for major concrete bridges (~2,000 kgCO₂e/m² deck, A1–A5), plus 30% for approach earthworks and the A20 tie-in.	Comparator-derived; no SLC-specific Whole-Life Carbon Assessment exists. PAS 2080:2023 (Carbon Management in Buildings and Infrastructure) compliance assumed.
Slider range	20–120 kt	Brackets: prefab-segmental low-carbon (≈25 kt) to cast-in-situ steel-composite (≈100 kt).	Marine piling carbon not separately modelled.
Carbon valuation	£290/tCO₂e	HM Treasury Green Book guidance on carbon valuation (gov.uk/government/collections/carbon-valuation), updated to the 2025 traded values published by DESNZ in December 2024. Central traded series, uprated to current prices using GDP deflator. The 2024 central value was £256/tCO₂e in 2021 prices; the £290 figure reflects the latest update.	Carbon values are policy-driven and update annually — OBC should use whichever vintage is current at the time of appraisal.
Fleet emission factor	167 gCO₂/km	DESNZ / Defra Greenhouse Gas Reporting Conversion Factors (2025 set, published 10 June 2025), average UK car fleet.	Does not discount for fleet electrification. By 2035 the UK fleet will be substantially EV; operational induced VKT carbon will fall. A real OBC would apply a declining-emissions trajectory aligned with DESNZ projections.
Induced trip net VKT	−15 km/trip	Geography-honest 40/40/20 split: genuinely-new (+5 km) / route-shortened from 50 km loop via Newtownards to 5 km direct (−45 km) / mode-shift (+5 km).	Author estimate. Not from an SLC-specific origin-destination survey. A real OBC would derive the split from local travel-diary or count data. The slider lets users push back to a precautionary +15 km/trip view.

6 · Revenue & financing

Parameter	Default	Source / reference class	Known limit
Toll demand elasticity	−0.3	UK toll-road reference class (Mersey Gateway, M6 Toll, Severn): −0.2 to −0.4 short-run own-price elasticity for short-distance crossings.	Reference-class point estimate. Not SLC-specific.
Toll counted in BCR	No (transfer)	DfT TAG Unit A2.1 / A2.2 on welfare economics: transfers are excluded from the economic case. Toll appears in the financing model only.	Standard. Not contested.
Rates uplift ramp	30% yr 5 → 100% yr 10	Indicative NI rateable-value reassessment lag for major infrastructure, with revaluation cycles via Land & Property Services (LPS).	LPS revaluation timing varies; modelled centrally.
Rates counted in BCR	No (financing)	DfT TAG Unit A2 anti-double-count: included in NI net position but not BCR.	Standard. Not contested.

7 · Discounting & horizon

Parameter	Default	Source / reference class	Known limit
Social discount rate	3.5%	HM Treasury Green Book (2025 edition) central Social Time Preference Rate (STPR), declining to 3.0% beyond year 30 and 2.5% beyond year 75. Dashboard uses a flat 3.5% as a slight conservatism.	Treasury 7% sensitivity available via slider for stress-testing.
Appraisal period	60 years	Green Book default for major fixed infrastructure.	No residual value modelled. A real OBC would include a residual asset value at yr 60. This is a conservatism for the case as presented.
Payback year	End of operating yr 12	First operating year in which cumulative undiscounted monetised benefits (transport user + societal + carbon) exceed the optimism-adjusted capex of £625m. Year counter starts at operating yr 1 — i.e. the first year the bridge is open. Construction yrs (1–6 in the model) are not counted toward payback. Excludes toll and rates (transfers, per DfT TAG Unit A2). Diagnostic only; BCR and NPV remain the headline VfM measures.	Not a discounted-cash-flow payback. Sensitive to capex, demand and optimism bias — moves to Yr 17 at £750m capex, Yr 22 at the cost-overrun preset, Yr 27 at low-demand stress. In calendar terms from a feasibility-study commission today, payback lands at ~18 years total (3 yrs OBC/FBC + 6 yrs construction + 12 yrs operation – first 3 of which are ramp-up).

8 · Known gaps vs OBC

No calibrated transport model. A real OBC requires TUBA or DIADEM with origin-destination matrices for the Ards/Lecale catchment.
No primary cost plan. The £500m anchor is a ministerial figure, not a quantity-surveyed estimate. A real OBC requires a GBV-aligned cost plan with marine geotechnical contingency.
No EIA / Habitats Regulations Assessment. Strangford Lough holds multiple statutory designations: Special Area of Conservation (SAC, JNCC site UK0016618), Special Protection Area (SPA), Area of Special Scientific Interest (ASSI), Ramsar wetland of international importance, and Marine Nature Reserve (Northern Ireland’s first, designated 1995). Habitats Regulations Assessment and Environmental Impact Assessment are statutory prerequisites before any construction commitment.
No Section 75 equality screening. Statutory obligation for any NI Executive policy decision.
No fleet decarbonisation trajectory. Operational carbon assumes a static fleet emission factor; OBC would apply a declining trajectory aligned with DESNZ projections.
No residual asset value. Bridge structural life is typically 120 years; OBC would value the asset at yr 60.
No re-investment cycle. Deck resurfacing and cable replacement schedules are modelled as a flat 1.2% opex, not as discrete cycles.
No Commercial Case test. The chosen procurement route (PPP vs traditional vs design-build-finance) sits in the 5-Case Model’s Commercial Case and is exogenous to this dashboard.

9 · What this dashboard can defend

The case behaves robustly under stress — low-demand preset returns BCR 0.83 (failed) without manipulation.
Every default sits inside the credible DfT TAG range; no input is pushed to its limit.
The funder rationality gate makes the political-economy reality visible: UK-100% funding properly fails the test.
Carbon is honestly net-marginal even with the trip-shortening view; the model does not pretend the bridge is a climate win without saying why.
The strategic premium is held at £0/yr by default; the case clears 1.5 BCR on transport & societal benefits alone.
Five non-financial gates (SAC / MNR consent, Section 75, Subsidy Control, operating model, reference class) are listed as open — not waved through.

10 · Author & provenance

Kevin Barry BSc(Hons) MRICS — Chartered Quantity Surveyor, Quintin QS, Belfast. Dashboard code and parameter file available on request. This is a campaign decision-support tool, not a statutory appraisal, and does not represent the position of the Department for Infrastructure, the Northern Ireland Executive, the UK Government or the Government of Ireland.