Option appraisal methodology

Extensive research has been undertaken to understand how the potential impacts of the options could be appraised, and this has informed the methodology used.

To provide a baseline comparison, the options have been assessed against a Do-Minimum scenario, which reflects the status quo in speed limits within Scotland.

Alignment with STAG Appraisal Criteria

STAG represents the first stage in the assessment of potential solutions to transport problems. As the options being considered within this report will have a significant impact upon the transport network, the appraisal of the options has been structured to align with NTS2 priorities and STAG Sub-criteria. Informed by the research undertaken, the process has appraised the options against the STAG Sub-Criteria outlined in Table 5-1 below.

STAG Criteria shown in have been included in the assessment. Other Sub-criteria have not been used in the appraisal due to these not being related to vehicle speeds on the road network.

Table 5-1 – Appraisal Criteria

NTS2 Priority	Associated STAG Criteria	Identified Appraisal Criteria (Aligned to STAG Sub-criteria) - Included	Identified Appraisal Criteria (Aligned to STAG Sub-criteria) - Not Included
Takes climate action	Environment	Air Quality Noise and Vibration	Biodiversity and Habitats Geology and Soils Land Use (including Agriculture and Forestry) Water, Drainage and Flooding Historic Environment Landscape
Takes climate action	Climate Change	Greenhouse Gas Emissions	Vulnerability to the Effects of Climate Change Potential to Adapt to the Effects of Climate Change
Improves our health and wellbeing	Health, Safety and Wellbeing	Accidents	Security Health Outcomes Access to Health and Wellbeing Infrastructure Visual Amenity
Helps deliver inclusive economic growth	Economy	Economic Efficiency of the Transport System (TEE)	Wider Economic Impacts (WEIs)
Reduces inequalities	Equality and Accessibility	Comparative Access by Geographic Location	Active Travel Network Coverage Public Transport Network Coverage Active Travel Network Coverage Comparative Access by People Group Affordability

Appraisal Methodology

The appraisal has identified six key criteria that will be considered. The methodology for each of these is outlined in this section of the report.

Transport Model for Scotland (TMfS)

The TMfS is a strategic transport model, that provides a broad representation of transport supply and estimates of transport demand and is used to help appraise the potential impacts of major investments or policy decisions. The current version is TMfS18 which has a base year of 2018. Key outputs of this model include journey routing, journey times and journey lengths of various types of road user that can be used to inform many parts of the appraisal. Amongst the key parameters the model uses road link types and traffic speeds.

The outputs from TMfS represent travel in Scotland on an average day in any given year. Representative modelling years outputted and used in the appraisal were for 2025, 2030 and 2045.

Future Scenarios

To allow the potential impacts of the options to be more clearly understood, different future scenarios have been modelled within TMfS. These have considered committed government policies and the level of compliance with proposed speed limits. Key areas of government policy identified includes:

• Achieving a 20% reduction in car kilometres by 2030 aims to reduce Scotland carbon footprint, improve public health and inequality by reducing the number and length of car journeys undertaken;
• Phasing out the need for new petrol and diesel cars and vans by 2030 aims to reduce Scotland Carbon footprint by facilitating the move to electric vehicles; and
• The commitment to achieving net zero carbon emissions by 2045 is a legally binding requirement for Scotland to have a neutral impact with respect to carbon emissions by 2045.

Within TMfS the impact of fulfilling these policies is represented by the application of rulesets (“with policy” and “without policy” future scenarios).

International evidence verifies that compliance with posted speed limits varies with respect to driver behaviours and the level of enforcement. These are represented within TMfS by varying free-flow speeds on different road link types. To demonstrate the significance of compliance, two future scenarios that have been considered within TMfS are:

• 100% Compliance – This future assumes that drivers will fully comply with posted speed limit alterations proposed under the option; and
• Realistic Compliance – This future assumes that drivers will reduce their actual speeds by an average of 2.5miles per hour (4kilometres per hour) for a reduction of 10miles per hour in posted speed limit (see Summary of Literature Review in Appendix B).

The four futures considered within this report are listed in Table 5-2 below.

Table 5-2 – Future Scenarios considered within the Detailed Appraisal

Future Scenario	Description
"With Policy"	This future considers the impacts with a 20 per cent reduction policy ambition on car kilometres'
"Without Policy"	This future considers no policy ambition on reduction of car kilometres
100% Compliance	This future considers the impacts of the travelling public fully complying with posted/ National Speed Limits.
Realistic Compliance	This future considers the impacts of the travelling public partially complying with posted/ National Speed Limits.

Environment – Noise and Vibration

A qualitative assessment of the potential noise impacts arising from the options based on the principles of the STAG Strategic Level of assessment. The assessment assumes that a change in speed will not lead to a change in vibration effects from the operation of the roads. 

The appraisal is based on a sample of transport corridors considered to be representative of road links, and associated nearby noise sensitive receptors, potentially affected by the changes. The assessment focused on one representative future year, 2045. The selection of the sample of transport corridors was based on proximity to noise-sensitive receptors and length of the corridors.

This work utilised road traffic data in the form of Annual Average Daily Traffic (AADT) and average speeds outputs from TMfS. The assessment was undertaken using a combined approach utilising spreadsheets and GIS platforms to determine the likely noise level changes in proximity to key road links. The road links used are listed below with further details provided in Appendix C:

• A90 single carriageway – As a representative rural single carriageway;
• A702 single carriageway– As a representative rural single carriageway;
• A96 dual carriageway– As a representative rural dual carriageway; and
• M8 motorway– A representative motorway.

Relevant aspects of the methodology in the technical memorandum Calculation of Road Traffic Noise (CRTN) and DMRB LA111 have been used to inform the appraisal. Basic Noise Levels (BNLs) and noise level changes have been determined due to speed variations in each of the option and future scenarios assessed. At this stage, a propagation noise model including topography and screening influence provided by intervening buildings has not been prepared. It is considered that the assessment of noise level changes at source provides a good indication of the likely changes potentially experienced by the nearest noise sensitive receptors. This level of detail is appropriate for Strategic Level of the STAG assessment. The study area for the assessment was set at 600m from the road links.

Noise level changes were assessed as having regard to the magnitude of noise change presented in DMRB LA111 for short term. All options and future scenarios were compared to a traffic dataset representing the traffic flows for the same year if the option does not go ahead.

Population density was included in the assessment as an early indication of the population likely to be affected by the change in noise levels. Population datasets published by the Scottish Government were used to assist in this assessment. The Small Area Population Estimates (SAPE) datasets published were used to derive population densities within the study area.

Environment – local air quality (airborne matter and Nitrous Oxide emissions)

A qualitative analysis of the potential air quality impacts associated with the Options and future scenarios have been carried out for the 100% compliance and the realistic compliance scenarios for 2025, 2030 and 2045.

Using output data from TMfS, annual emission rates (in kg per year) of NO_X, PM₁₀ and PM_2.5 for each road link have been obtained by using Department for Environment, Food & Rural Affairs’ (Defra) Emissions Factors Toolkit (EFT), published in December 2023.

For the prediction of future year emissions, the toolkit takes into account factors such as anticipated advances in vehicle technology and changes in fleet composition, such that emissions are assumed to reduce over time. The EFT allows for the calculation of emissions arising from road traffic for all years between 2018 and 2050. For the 2025 and 2030 modelled Options and future scenarios the respective data has been used but for 2045, 2030 emission factors have been used as this represents the limit of current available predictions. While the EFT provides emission rates for 2031-2050, these are provided for climate assessments and appraisals only. This approach also ensures a robust, conservative assessment of impacts beyond 2030.

Total emissions for each year associated with each Option and future scenarios for the whole of Scotland have been derived by summing together the annual emission rates for each road link. The resulting change in emissions of the Options and future scenarios has been compared to the Do Nothing scenario and used to determine the relative magnitude of change. The impact descriptor (minor, moderate, major) assigned has been based on the percentage change in emissions (1%, 5% and 10%). This approach has been adapted from Table 2.91 in the DMRB LA 105 air quality guidance.

Climate Change – Greenhouse Gas Emission

The GHG assessment is based on using the Department for Transport (DfT) Transport Analysis Guidance Databook and calculating emissions over the assumed 60-year appraisal period.

The calculations are based on the traffic forecasts for the Do Minimum (without policy) and Do Something (with policy). Information for the GHG assessment comes from TMfS outputs including Annual Average Daily Traffic (AADT), distance, speeds, and percentages of Heavy Goods Vehicles by transport model road link.

The future scenario baseline is considered over the assumed 60-year appraisal period. Non-traded CO_2e emissions (petrol and diesel vehicles) and CO_2e traded emissions (electric vehicles) have been calculated in accordance with STAG. The future baseline scenario involves no construction activities and therefore the construction baseline has been assumed to have zero emissions.

Health, Safety and Wellbeing – Accidents (All severity)

The Nilsson power law model (R. Elvik presented the Nilsson Power Law Model) was used to assess collision savings as it was expected that Cost and Benefits to Accidents – Light Touch (COBALT) would not be suitable due to the speed banding present within the accident rates not being sensitive enough to envisaged driver speeds. COBALT assigns accident rates based upon 10mph increments, while some of the options/future scenarios under consideration are expected to achieve a 2.5mph (4kph) reduction in traffic speeds.

This implies that if the modelled change in average speed is relatively slight, there may be no allocation to an alternative speed band within COBALT, resulting in no discernible impact. This issue will be enhanced in rural areas where alternative diversionary routes may not be available or attractive and consequently flow volumes also remain similar.

The impact of reduced speeds on the change in collisions was estimated using Nilsson (2019) power law model and disaggregated by road categories. This estimates the reduction in incidents using the following formula:

counter factual incidents = baseline incidents * ( (new speed) / (old speed) )^α

Where 𝛼 was selected as the exponent (best estimate), based on the road categories and type of incident/collision to calculate the counter factual incidents with the help of new and old speed. The difference between baseline and counter factual incidents will be a reduction in collision savings for a particular incident type.

These values were then monetised using the latest Transport Appraisal Guidance (TAG) standard values. The Transport Appraisal Guidance (TAG) has historical and reference information on all appraisal and modelling values.

Accident savings are usually based upon the most recently available data. However, this data in Scotland is generally accepted as having been distorted by the Covid 19 pandemic. To reflect this a sensitivity test involving two scenarios have been appraised. These are:

• Core – The Core scenario uses recorded collisions data from 2021 and 2022 as a baseline to identify potential savings; and
• Uplift – The Uplift scenario applies a 35% uplift to the 2022 collisions data. This represents an observed jump in fatal collisions between 2023 and early 2024.

Economy – Economic efficiency of the transport system

User benefits, which include time savings, fuel-related Vehicle Operating Costs (VOC), non-fuel Vehicle Operating Costs and operator and Government revenues typically form the main part of benefits attributable to nationwide schemes. The assessment reported here uses the DfT’s Transport User Benefit Appraisal (TUBA) software (November 2023 TAG update).

The matrix skims from the Do Minimum and Do Something option models were extracted and input into TUBA to calculate the difference in journey costs between future scenarios. The inputs used are:

• Demand – Traffic flow at an origin destination level;
• Journey Time – The time in hours for each origin destination pair within the model; and
• Distance – The length in km for each origin destination pair.

TUBA generates the following economic outputs:

• Time savings;
• Vehicle Operating Costs (VOC) savings;
• Greenhouse gases; and

Comparative Access by Geographic Location

Routes from Scotland ports to the English border with their associated journey times are significant to many industries. The impacts on journey times for HGVs which have perishable goods have been assessed to understand the likely impact. Journey times have been assessed from key locations (Aberdeen, Scrabster, Ullapool, Uig and Oban) to nearby the English border at Gretna Green to demonstrate the impact of the options.

Appraisal of the options (seven-point scale)

STAG uses a seven-point scale against which options are appraised. As the options would be applied across all of Scotland, the assessment proposes to appraise the impact of the options relative to this national scale, as demonstrated in Table 5-3 below.

Scale	Overview
Major benefit	These are benefits or positive impacts which, depending on the scale of benefit or severity of impact, the practitioner feels should be a principal consideration when assessing an option's eligibility for funding
Moderate benefit	The option is anticipated to have only a moderate benefit or positive impact. Moderate benefits and impacts are those which taken in isolation may not determine an option's eligibility for funding, but taken together do so
Minor benefit	The option is anticipated to have only a small benefit or positive impact. Small benefits or impacts are those which are worth noting, but the practitioner believes are not likely to contribute materially to determining whether an option is funded or otherwise
No benefit or impact	The option is anticipated to have no or negligible benefit or negative impact
Minor negative impact	The option is anticipated to have only a moderate cost or negative impact. Moderate costs/negative impacts are those which taken in isolation may not determine an option's eligibility for funding, but taken together could do so
Moderate negative impact	The option is anticipated to have only a moderate cost or negative impact. Moderate costs/negative impacts are those which taken in isolation may not determine an option's eligibility for funding, but taken together could do so
Major negative impact	These are costs or negative impacts which, depending on the scale of cost or severity of impact, the practitioner should take into consideration when assessing an option's eligibility for funding

It should be noted that an impact under one category is not equivalent to the same impact in another category and no weighting of criteria is considered. The purpose of the impact criteria is to allow a comparison across the options.