Background

Environmental assessment of transport impacts is a requirement for major infrastructure projects such as HS2. The scale, complexity, and delivery timescales for the multi-phase HS2 programme presents unique challenges for transport, different to that of other major projects. Our approach to transport assessment has been designed to robustly meet the requirements of the Environmental Impact Assessment process ^[1]. The methodology was developed in consultation with a wide range of stakeholders and with consideration of relevant local and national policies and guidance. Details on stakeholder engagement and the relevant policy and guidance considered are set out in Part 1 of the Transport Assessment^[2] published for each phase of the HS2 scheme. HS2 Transport Planning inputs relate to highways, station design, environmental assessment and not the engineering design and alignment for the operational railway.

The transport assessment work undertaken in relation to the HS2 scheme covers three phases of the project: Phase One – London to the West Midlands, Phase 2a – West Midlands to Crewe, and Phase 2b – Crewe to Manchester. To secure the powers to deliver HS2, the parliamentary process for a hybrid bill has been pursued for each phase of the project, this is explained at HS2 railway, UK – the parliamentary process to Royal Assent (2021)^[3]. Bills for phase one and 2a of the HS2 scheme received Royal Assent in 2017 and 2021 respectively, and the government deposited the Bill for phase 2b in January 2022.

This paper aims to share our learning legacy from over a decade of transport planning work covering all phases of the HS2 project. It focuses on three themes:

1. The role of transport assessment through the major projects lifecycle
2. The approach to assessing and reporting transport impacts
3. Developing and applying guidance to identify environmental effects

Role of transport assessment through the major projects lifecycle

The phased HS2 programme sees development and delivery over multiple decades. Transport planning and assessment is integral to scheme design, environmental assessment and delivery. The approach sees assessments for each phase that also account for impacts of simultaneous construction across different phases, and the wider impacts of the full end-state railway in operation.

Early design stage

Transport planning is integral to the design process. Early design development includes options assessment informed by proportionate transport planning analysis, and covers multiple aspects of the HS2 scheme including route development, station planning and highway design. The design development process sees consideration of all related environmental topic areas, as well as cost, deliverability and stakeholder impacts.

Hybrid Bill stage

As the scheme is further developed, an Environmental Statement (ES)^[4] and associated Transport Assessment (TA) is produced for each phase of HS2. The TA reports transport impacts that arise during construction and operation of the scheme. These impacts are then used to identify effects relating to traffic and transport as well as other topic areas that are reported in the ES. Identified environmental effects may be mitigated through design of the scheme or additional mitigation such as a junction improvement. The ES reports any residual environmental effects during construction and operation of the HS2 scheme.

The proposed scheme design, construction methodology and programme are further developed during passage of each hybrid Bill through parliament. This can be to respond to feedback from stakeholders, petitioners or the public, to drive efficiencies or to reduce adverse environmental impacts. Transport planning analysis is undertaken to both assess the impact of changes, and inform changes to the scheme. A Supplementary Environmental Statement (SES)^[5] provides environmental information on changes that do not require additional Bill powers, reporting any new or changed environmental effects using information from an updated TA. Changes that require additional Bill powers are promoted by way of an Additional Provision (AP) underpinned by an Environmental Statement 4.

Design, Delivery and Environmental Minimum Requirements

Should the hybrid Bill pass through parliament and be granted Royal Assent (securing the powers to construct the scheme), the scheme will progress through further design stages, before moving to delivery, in construction and operation. The TA and associated analysis, data and models are important throughout this process.

During design development (for example station designs and highway schemes), the transport planning work that has informed earlier design will be revisited and if necessary updated. Elements of this work will also be translated into technical standards and guidance for issue to design teams.

Throughout the design and delivery stages, the final TA as at Royal Assent is important in providing a record of the baseline assessment of the scheme. As the design and delivery of the scheme is progressed, any changes to the assumptions underpinning the TA are considered to understand whether these would lead to a material change in the assessment of transport impacts against the baseline assumptions. Of particular importance is ensuring that any change in transport impact would not lead to a new or different environmental effect.

Environmental Minimum Requirements (EMRs)^[6] are established through the passage of each hybrid Bill and apply to the design, construction and operation of HS2. The EMR general principles require that, save for specific exceptional circumstances, the environmental impacts reported in the ES will not be exceeded.

A key learning has been the value in establishing effective records of data, models and documentation developed for the TA at the earliest possible point, to ensure efficient retrieval of information and transfer of knowledge when required in latter stages of the scheme, for example when assessing compliance against the EMR general principles in delivery.

Multi-phase project

The scale and complexity of the HS2 scheme has required a phased programme for development and delivery. Design development, transport and environmental assessments are undertaken for each phase. The design and assessments for each phase also account for other phases that are already consented to understand any combined impacts from simultaneous construction across different project phases, and the wider impacts of the full end-state railway in operation.

Approach

The approach to assessing and reporting transport impacts

The Transport Assessment for each phase assesses the impact of the proposed scheme on a range of travel modes, during both construction and operation. The assessment covers local, regional and national impacts using a range of assessment techniques. Impacts are reported in the Transport Assessment, and various materials are produced for the public and select committee to make this technical information accessible. The approach addresses the unique challenges relating to the scale and complexity of the HS2 programme, drawing on guidance developed to ensure consistency and robustness of assessment across multiple phases of the project.

Construction and operational assessment scenarios

To assess the construction impacts of the proposed scheme a future baseline year is established against which a range of construction scenarios are assessed. To ensure a robust assessment, the construction scenarios capture the period likely to have the greatest construction impact. A range of construction activities are included in the assessment year to reflect that construction activities could occur in combination and could have greater combined impacts.

Two assessment years are considered for operation, the first capturing the opening year of the scheme and the second capturing changes after the railway has been operational for several years.

For each assessment year the future demand and transport network without the proposed scheme is forecast and compared to future demand with the proposed scheme. Other phases of HS2 are included in the future demand forecasts, for example demand changes resulting from Phase One of HS2 are accounted for in the baseline of the Phase 2a assessment.

Transport modes and geographic scope

The TA assesses the impacts of the proposed scheme on groups that may be impacted: pedestrians, cyclists, equestrians, people with mobility impairments, and users of highways, public transport and waterways.

There is no simple geographic limit, as transport impacts can occur across a broad area. The TA therefore covers a wide geographic area from the HS2 route alignment to capture all likely impacts. The assessment area is split into Community Areas (CAs), in line with the wider ES. Figure 1 illustrates how the area around the Phase One scheme was divided into 26 CAs. The TA for each phase is a single assessment with the reporting structured by CA for ease of navigation.

The size of the scheme considered for each phase (as illustrated for Phase One in Figure 1), requires a TA to be undertaken at various levels of scale and detail.

At the national level, the impacts of the scheme must be assessed and this will typically require an analysis that considers the interactions between different modes of travel, for example an improvement in journey times on HS2 will attract more people to travel by rail and in turn reduce demand for air and car travel. Impacts are considered both ‘route-wide’ and ‘off-route’. Route-wide impacts include changes that may occur over a wide area from the actual scheme, for example due to changes in rail journey times. Off-route impacts include changes that occur in areas which are more remote from the scheme but are impacted indirectly, for example changes in passenger footfall at stations due to wider train timetable changes, works required at stations remote from the scheme and new depots.

At a regional scale, more detailed assessments are undertaken to understand the impacts of the scheme on the regional transport infrastructure. For example this will include analysis of changes in highway travel patterns during HS2 construction, to identify areas of likely congestion and potential mitigation.

At a local level, the impacts on local transport networks are assessed. This may include impacts on specific junctions for vehicle users and vulnerable road users, public transport networks, public rights of way, parking and loading, accidents and safety.

Transport modelling approach

Transport modelling is integral to the identification of transport impacts in the TA. The traffic modelling, combined with other information, specifically informs the following areas of assessment:

• Public transport delay, specifically changes to bus journey times;
• Traffic delays to vehicle occupants, due to road diversions or traffic congestion;
• Traffic related severance for non-motorised users, informed by changes in all vehicle traffic and/or Heavy Goods Vehicle traffic; and
• Accidents and safety, informed by changes to Annual Average Daily Traffic (AADT).

Transport models have been used to establish traffic forecasts for construction and operation. The baseline represents conditions in the base year of the model and is used to inform future forecasts. Future baseline demand forecasts are established to separate the impact of the HS2 scheme from other factors that are anticipated to change future demand levels. The model scenarios are summarised in Table 1.

Assessment scenario	Future baseline	Future baseline + HS2 scheme
Construction assessment year	Future baseline 1	Future baseline + HS2 construction scenarios
Operational assessment year	Future baseline 2	Future baseline + HS2 operational scenario 1
Further operational assessment year	Future baseline 3	Future baseline + HS2 operational scenario 2

The assessment covers traffic conditions during morning and evening peak hours which typically represent the busiest network conditions and therefore represent a robust assessment scenario. The modelling approach follows Transport Analysis Guidance^[8] published by the Department for Transport.

Use of strategic models has been determined considering the need to assess diversionary traffic impacts and the robustness of available models. Where appropriate, strategic models are used to forecast traffic levels on key routes across large geographic areas. These models represent traffic demand and assignment on roads and junctions across the model area. Interactions are represented in the model, including diversionary effects whereby users change their route as a response to a network change, congestion impacting travel time or other factors. Local models, using outputs from strategic models, are used to understand traffic impacts on a specific network element in more detail, for example how a specific junction operates. Local models assess a network element in isolation and don’t represent wider interaction and diversion, and so this combined approach of running strategic and local models is used. Additionally, microsimulation traffic models are used for some complex junctions where this is needed.

Through engagement with stakeholders, the suitability of existing strategic and local traffic models is assessed and models selected that are appropriate for assessment of the proposed scheme. Updates are applied so that models include any changes to the road network since the model was originally developed, and the models contain up-to-date demand data from surveys and other sources. Model development is undertaken for specific areas of each model as required, for example where greater road network detail is needed to understand impacts in specific locations that are likely to be impacted by the proposed scheme. Where existing suitable models are not available and these are required, HS2 has commissioned new models.

HS2 traffic models account for growth using Department for Transport National Trip End Model travel demand forecasts, Local Planning Authority Local Plans and information on committed development.

HS2 construction vehicles and trips by HS2 construction workers are calculated by specialist HS2 Construction and Logistics teams drawing on the programme of construction activities. HS2 construction demand is then used as an input to the traffic models.

Outputs from public transport and multi-modal models are utilised for London and Manchester as an input to the calculation of traffic demand forecasts.

The introduction of HS2 infrastructure and services, as well as consequent changes to conventional rail services, will change user travel patterns and rail demand. The PLANET Framework Model^[9] is used to forecast rail user demand on train services and at stations served by HS2 and stations outside of the HS2 network.

Forecast rail user demand profiles are used to inform the design of HS2 stations in line with HS2 design and space standards. This is supported by pedestrian microsimulation modelling to test that the movement of people within the station is safe and efficient. Changes to rail demand at stations outside of the HS2 network due to the proposed scheme are also assessed and design changes progressed in consultation with partners if required.

Road network changes

A number of changes to the local and strategic road network form part of the proposed HS2 scheme. These are required to facilitate the scheme alignment and footprint, and to accommodate changes in travel patterns during construction, and latterly with the introduction of HS2 services.

During the construction phase, construction vehicles, worker trips and diversion of existing traffic places new pressures on the road network. A phased work programme, traffic management and road network changes form part of the proposed HS2 scheme to mitigate impacts during construction. Some road network changes are temporary for the construction period and others are permanent as they are also required or beneficial during operation of HS2.

Figure 2 illustrates the scale and complexity of some of the road network changes that the scheme requires, which can be considered significant projects in their own right. The modelling processes outlined above must capture all of these changes and consider their cumulative impact on the transport network, alongside the impact of the HS2 railway itself.

Transport planning in the hybrid bill parliamentary process

To secure the powers to deliver HS2, the parliamentary process for a hybrid Bill has been pursued for each phase of the project, with the respective ES and TA forming part of the evidence base. A select committee is appointed to oversee petitioning against each bill, whereby individuals and bodies directly and specially affected are provided the opportunity to petition and seek amendments to provisions within each bill.

A range of materials are produced to support the accessibility of transport planning information to the select committee, petitioners and the public. This includes materials to address specific points raised by petitioners or the select committee and a set of traffic and transport standard exhibits as listed in Table 2. Additionally, transport planning specialists support direct engagement with individuals and stakeholders with the aim of resolving issues prior to select committee hearings.

Standard exhibit type	Description
Overview of construction traffic routes map	Provides an overview of all roads that will be used by HS2 construction traffic
Construction route maps	Shows HS2 construction traffic routes and for these routes, peak daily two-way construction HGV traffic flows and the duration of use
Traffic flow maps	Presents background and HS2 traffic flows at key locations on the road network on a typical weekday in the peak month during construction
Construction traffic histograms	Shows the average weekday volume of construction traffic for a specific section of road during each month of the construction period

A key learning derived from developing three TAs to be delivered alongside a hybrid Bill is the need to find balance in delivering technical work that on the one hand is comprehensive, robust and able to stand up to scrutiny many years later, and yet can be readily understood by non-specialists, and can also be efficiently adapted to consider scheme changes. It is imperative at the outset of a TA of similar scale to consider this balance over the full lifecycle of the assessment.

Developing and applying guidance to identify environmental effects

HS2 has developed guidance on how transport-related environmental effects are determined for the proposed scheme. This has provided a clear, robust, and consistent methodology that has been further developed with each phase of HS2.

Guidance development

The TA delivers a comprehensive view on the traffic and transport impacts that arise due to the construction and operation of the HS2 scheme. For the environmental effects of these impacts to be established and reported in the Environmental Statement, it is necessary to define a set of relevant criteria.

A Scope and Methodology Report (SMR) for each phase outlines the overall approach to the Environmental Impact Assessment. This includes detailed guidance on how traffic and transport effects are determined for the proposed scheme. The SMR and detailed guidance provides a clear and consistent methodology that has been further developed with each phase of HS2. The latest SMR and guidance was published for Phase 2b in October 2018^[10] and July 2023^[11] respectively. These documents have evolved from those produced for earlier phases of the HS2 scheme.

HS2 guidance aligns with requirements of the wider EIA process and when developed for each phase of the scheme has drawn on relevant guidance at the time. Further detail on this guidance is provided in the Traffic and Transport section of the SMR for each phase.

Applying guidance to identify significant effects

The HS2 Scope and Methodology Report (SMR) for each phase contains the criteria used to determine likely significant effects. The detailed guidance covers how to categorise the level of significance attributed to the identified transport effects of the proposed scheme – minor, moderate or major.

The process of identifying likely transport significant effects involves application of the SMR and HS2 guidance using data and analysis from the TA. The need for mitigation is then considered following the identification of effects. Mitigation of minor adverse effects is usually considered at the detailed design stage. Mitigation of moderate and major adverse effects is considered where proportionate mitigation is possible. Any residual effects are reported in the ES and beneficial effects are also identified and reported using the same process. TA outputs also inform other environmental assessment topic areas including air quality, noise, community, health and equality.

Transport impacts of HS2 are assessed for construction and operation across the following areas:

• Public transport delay;
• Disruption at stations/interchanges;
• Traffic delays to vehicle occupants;
• Vulnerable road users (traffic related severance, non-traffic related severance, amenity and ambience);
• Accidents and safety;
• Parking and loading; and
• Waterways.

A distinction has been made between impacts and effects. Impacts are defined as the predicted change attributable to the proposed scheme, for example changes in traffic levels or delays, and the duration for construction impacts. Effects are the consequence of impacts; most commonly for transport this is the number of people affected.

A two-stage process is used to identify likely significant effects. The first stage sees application of the SMR criteria to determine whether the effect identified is significant. The second stage sees application of the guidance to categorise the significance of the effect as minor, moderate or major. The significance of a transport effect is a combination of the magnitude of the impact, and the sensitivity and/or value of the receptor affected.

Example of significant effect criteria

Traffic delays to vehicle occupants due to congestion or diversion is one area of assessment. Figure 3 shows the table used for assessment of traffic delays to vehicle occupants due to traffic diversions during construction. The criteria to determine the magnitude of impact comprises the change in travel distance and level of road use (number of vehicles per day), combined with the duration of impact. The criteria used to determine significance relates to the number of travellers diverted.

Many key learning opportunities have been afforded to the overall project by the phased delivery of the scheme. In relation to understanding traffic and transport environmental effects, the application of the SMR guidance during detailed design and delivery of Phase One has identified areas where the original guidance could be improved (for example to make interpretation clearer). It has therefore been possible to capture these improvements in updates to the guidance in subsequent phases of the scheme.

Conclusion

This learning legacy covers the role of transport assessment through the major projects lifecycle, the approach to transport assessment and how HS2 identifies transport-related environmental effects.

The scale, complexity, and delivery timescales for the multi-phase HS2 programme has presented unique challenges for transport that are different to other major projects. This report identifies a number of transport assessment related learning outcomes derived from addressing these challenges.

HS2 and its Professional Services Contractors have successfully delivered robust transport and environmental assessments for the HS2 route from London to Manchester. The Bill for Phase one received Royal Assent in 2017, and construction is well underway. The Bill for Phase 2a received Royal Assent in 2021 and the government deposited the Bill for phase 2b in January 2022.

By sharing this learning legacy it will allow other major projects to benefit from the experience developed at HS2, and will inform the approach to future transport and environmental assessments. If you want to understand more about transport planning at HS2 please do contact the team.

Acknowledgements

The authors wish to acknowledge HS2 colleagues and our Professional Services Contractors involved in the transport and environmental assessment process.

References

1. Department for Levelling Up, Housing and Communities. Environmental Impact Assessment Guidance. 2020. [Accessed 4 June 2024].
2. High Speed Two Limited. HS2 Phase One environmental statement volume 5: traffic and transport.  HS2 Phase One environmental statement volume 5: traffic and transport. 2013. [Accessed 4 June 2024].
3. Lagerweij L, Knight S. HS2 railway, UK – the parliamentary process to Royal Assent. Proceedings of the Institution of Civil Engineers – Transport. 2021. [Accessed 4 June 2024].
4. High Speed Two Limited. HS2 Phase One environmental statement. HS2 Phase One environmental statement. 2013. [Accessed 4 June 2024].
5. High Speed Two Limited. Supplementary Environmental Statement. 2017. [Accessed 4 June 2024].
6. High Speed Two Limited. Environmental minimum requirements for HS2 Phase One.  2017. [Accessed 22 February 2024]. 
7. High Speed Two Limited. London-West Midlands Environmental Statement Non-technical summary.  2013. [Accessed 22 February 2024].
8. Department for Transport. Transport Analysis Guidance . 2024. [Accessed 4 June 2024].
9. High Speed Two Limited.    HS2 Planet Framework Model version 10a: model description. 2023. [Accessed 4 June 2024].
10. High Speed Two Limited. HS2 Phase 2b Environmental Impact Assessment Scope and Methodology Report. 2018. [Accessed 22 February 2024].  
11. High Speed Two Limited. HS2 Phase 2b SES2 and AP2 ES Volume 5: Traffic and transport – Technical note – Updated guidance on significance criteria for traffic and transport (CT-001-00004). 2023. [Accessed 22 February 2024].