Diverging diamond interchanges (DDIs)
The project aims to improve junction safety and performance by assessing the use of Diverging diamond interchanges (DDIs) on the Strategic Road Network, evaluating their benefits, risks and implications for UK design standards.
There are no DDIs on the Strategic Road Network and no guidance or requirements for them particularly within the DMRB standard CD 122. International research shows that DDIs can improve traffic flow efficiency and safety, and there is growing interest in them in the UK due to these demonstrated benefits. An example in Flanders shows that a DDI can be installed on an existing site at relatively low cost with a significant reduction in collisions and potential to reduce or limit carbon through retrofitting.
A DDI is an innovative grade separated junction in which traffic on the minor road temporarily crosses to the opposite side, allowing right-turning vehicles to proceed without conflict from opposing traffic.
The project will aim to measure the safety performance of DDIs to clearly understand how they are safer than existing junction types. It will examine potential risks and hazards such as wrong way driving. It will review international standards and determine relevance.
It will also review the Design Manual for Roads and Bridges (DMRB) standards CD 122 ‘Geometric design of grade separated junctions’ and CD 123 ‘Geometric design of at-grade priority and signal-controlled junctions’ to understand how requirements and advice would need to change to encompass DDIs.
Objectives
Establish whether DDIs can deliver improved safety on the SRN compared with existing junction types.
Identify and examine key risks, including wrong way driving and other potential hazards.
Review international standards and determine relevance to the UK.
Review DMRB standards CD 122 and CD 123 to understand how requirements and advice would need to change to encompass DDIs.
Build the evidence case that will inform future feasibility activity and support the development of any required guidance.
Success criteria
A clear assessment of DDI safety performance, including comparison with existing junction types and insight into risks such as wrong way driving.
An understanding of relevant international standards, identifying which elements apply.
Defined changes required to DMRB CD 122 and CD 123 to encompass DDIs.
An evidence base that informs future feasibility activity and supports decisions on whether a trial location should be identified.
How the project supports National Highways KPIs
Network availability improves through smoother flow, reduced disruption from fewer collisions and more consistent speeds, with further benefits when DDIs are implemented in a series.
Safety (KSIs) improves by removing right turn against traffic conflicts and addressing wrong way risk through design and operations
Efficiency improves when designing a DDI by reducing the need for multiple departures and lowering the time required from consultants and SES specialists.
Carbon emissions reduce where retrofitting DDIs avoids large scale reconstruction and excess land take.
Road user satisfaction increases through safer, smoother journeys with fewer serious collisions and disruptions.
The approach
The project progressed through a scoping phase to assemble the evidence needed to understand how a DDI could apply to the Strategic Road Network.
The work began with a review of international research on safety and operations, drawing on examples from Europe and the USA and noting that Ireland was also exploring a DDI for its network. The team examined existing UK standards by reviewing DMRB CD 122 and CD 123 to understand how requirements and advice would need to change to encompass this junction type.
The scoping work also assessed key risks and hazards, with a particular focus on wrong way driving and the implications of potential signal failure.
Engagement activities informed the developing evidence base, building on previous National Highways and Imperial College work on road space allocation and incorporating sector insights from Safety, Engineering and Standards (SES) specialists and the Conference of European Directors of Roads (CEDR) peers. This included learning from a technical visit to the Flanders DDI, which had been retrofitted to an existing bridge and had delivered a significant reduction in collisions.
Project obstacles and how they were addressed
The project faced several obstacles during the scoping work. These included the absence of any DMRB guidance for DDIs, uncertainty around how wrong way driving risk would apply to the Strategic Road Network, and UK specific constraints.
No existing DMRB guidance for DDIs, addressed by reviewing CD 122 and CD 123 to understand how requirements and advice would need to change to encompass DDIs. If the DDI concept was adopted in the UK, specific design requirements and advice for the design speed of the minor road and in particular the crossover element including intervisibility and eyebrow (deflection) would be required.
Wrong way driving as a key design risk, addressed by examining international evidence and undertaking a safety risk assessment, which showed how crossover geometry and signing can help limit this risk.
UK specific challenges such as accommodating cyclists and pedestrians (on or off road) with generally less land available, addressed by careful planning, early stakeholder engagement, and context sensitive design to support DDI adoption.
Outcomes and impact
The project delivered the evidence required to understand how DDIs could apply to the Strategic Road Network. The work clarified the safety and operational case using international research, improved understanding of key risks, and set out the standards considerations needed to support a future feasibility stage.
User and stakeholder outcomes
Engagement during the scoping work reflected growing interest in DDIs among sector partners. The SES junction specialist was invited by CEDR to Flanders to understand the concept and practicalities of installing a DDI, and insights from this visit highlighted how the layout had been retrofitted at relatively low cost and delivered a significant reduction in collisions.
International interest was also evident through Ireland’s work to explore a DDI for its own network. These interactions are helping strengthen understanding of how the design has been applied elsewhere and informing the development of the evidence base for the UK..
Key insights and learning
The scoping work provided early insight into how DDIs have performed internationally and what would need to be understood before they could be considered for the Strategic Road Network. Evidence from other countries and wider international research, helped clarify safety potential, key risks and important standards considerations.
Key insights include:
- International evidence shows strong safety potential, including reduced collision frequency and the removal of right turn against oncoming traffic conflicts.
- Wrong way driving remains a key hazard, which will need to be re-examined during the next stage of the project once a potential location has been modelled. The safety risk assessment will be updated following more research.
- Retrofitting is feasible, demonstrated by the Flanders scheme installed on an existing bridge with a significant reduction in collisions.
- A DMRB standards update is essential, as CD 122 and CD 123 currently contains no recommendations or requirements for DDIs and therefore this would require changes to avoid multiple departures.
- Sector interest supports continued exploration, with Ireland considering a DDI and international programmes showing wider applicability.
Future value and next steps
The next phase of the project will move into feasibility work, building on the evidence developed during scoping. With funding for new schemes reducing, an ageing network and targets to reduce carbon yet demands on the networking increasing; there is a possibility that DDIs could improve safety increase capacity, reduce carbon and other environmental impacts at a much lower cost than current junction designs in the UK.
The investment will conclude whether or not DDIs can provide the benefits that are often highlighted in research and could and should National Highways be benefiting from using a different junction format.
This phase will focus on testing how DDIs could be applied to the Strategic Road Network and defining the requirements needed to support any future adoption.
- Identify a suitable retrofit location, model the layout on existing infrastructure, and examine scalability across the network.
- Investigate how risks such as wrong way driving and signal failure can be reduced or removed, supported by modelling and design development, including an iRAP assessment.
- Assess environmental and economic benefits, including land take reduction and carbon saving.
- Provide recommendations for requirements, advice and drawings to inform updates to DMRB CD 122 and CD 123, subject to feasibility findings.
