Highways S278 Design for Retail Park
Designed and delivered a Section 278 highways scheme for a 6,000 sqm retail park, including a signalised junction and new access road.
Project challenges
The site required a new signalised junction on the A57 to provide safe access for customer and delivery vehicles. The existing road geometry was constrained by a railway bridge 80 metres to the west and a signalised pedestrian crossing 120 metres to the east, limiting the available options for junction placement and layout.
Transport for Greater Manchester (TfGM) required a full junction design compliant with DMRB standards, supported by traffic signal modelling using LinSig to demonstrate that the junction would operate within capacity during peak hours. The highways authority (Manchester City Council) had additional requirements for pedestrian and cycling facilities, including a new toucan crossing and segregated cycle path connecting the site to the existing cycle network.
The project was further complicated by the presence of a major combined sewer running beneath the proposed junction alignment. United Utilities confirmed that the sewer could not be diverted and that any construction over or adjacent to the sewer would require a build-over agreement and restricted construction methodology. This constrained the junction layout and required close coordination between the highways design and the utilities protection measures.
How we solved it
We designed the full Section 278 works package, including the signalised junction, new access road, internal circulation, pedestrian and cycling facilities, and associated drainage, street lighting, and signage.
The signalised junction was designed as a three-arm priority-controlled layout with a dedicated right-turn ghost island on the A57. Traffic signal modelling in LinSig demonstrated that the junction would operate within capacity during the weekday PM peak and Saturday peak periods, with a practical reserve capacity (PRC) of over 15% in all modelled scenarios.
The toucan crossing was located to connect the site to the existing cycle path on the south side of the A57, providing a continuous off-road route for pedestrians and cyclists. The crossing was coordinated with the main junction signals to minimise delays to all users.
To address the combined sewer constraint, we designed the junction with a cantilever footway on the site frontage, avoiding the need for deep foundations above the sewer. The carriageway construction over the sewer was designed using a reinforced concrete raft to distribute loads and prevent point loading on the sewer crown. This solution was agreed with United Utilities through their build-over agreement process and avoided the need for sewer diversion, saving the client approximately £350,000 and six months of programme.
Results delivered
The highways authority approved the Section 278 design after a single round of technical review, with no significant design amendments required. TfGM confirmed that the traffic signal design and modelling met their requirements, and the signals were commissioned within two weeks of installation.
The construction phase was completed on programme, with the sewer protection measures performing as designed and United Utilities confirming compliance with the build-over agreement conditions at post-construction inspection. The toucan crossing and cycle path connection were delivered as part of the main works, providing enhanced active travel connectivity from day one of the development opening.
The retail park opened on schedule, with the highways works completed and handed over to the highways authority for adoption. The smooth approval and construction process was attributed to the early engagement with TfGM and the highways authority, the coordinated approach to the sewer constraint, and the quality of the technical design package.
Project Overview
Section 278 highways works are often the critical path item in commercial development programmes. A well-designed S278 scheme that secures technical approval first time can save months on the overall programme; a poorly designed scheme that requires multiple rounds of amendment can delay opening by six months or more.
This project required a new signalised junction on a busy A-road in Manchester, with the added complexity of a major combined sewer beneath the proposed junction alignment. Our brief was to design the complete highways works package, secure technical approval, and support the client through the construction phase.
Junction Design
The junction location was constrained by the railway bridge to the west and the existing pedestrian crossing to the east. Working within this window, we identified the optimal junction position based on forward visibility, stopping sight distances, and the available road width for a right-turn ghost island.
The three-arm junction was designed to DMRB standards (CD 123 — Geometric Design of At-Grade Priority and Signal-Controlled Junctions), with the following key features:
- Right-turn ghost island on the eastbound A57, providing a dedicated turning lane for vehicles entering the site. The ghost island was 3.5 metres wide with a 25-metre taper, accommodating HGV swept paths.
- Signalised control using a MOVA-compatible controller, with fixed-time fallback. The signal staging provided separate phases for the A57 through traffic, the site access, and the toucan crossing.
- Full pedestrian facilities including tactile paving, dropped kerbs, and pedestrian signal heads at all arms of the junction.
Traffic Signal Modelling
The junction was modelled in LinSig v3 for three design scenarios: weekday AM peak (0800-0900), weekday PM peak (1700-1800), and Saturday peak (1200-1300). Traffic flows were derived from a Transport Assessment prepared by the project’s transport consultant, with turning counts validated against automated traffic count data.
The modelling demonstrated that the junction would operate within capacity in all three scenarios, with practical reserve capacity (PRC) exceeding 15%. The weekday PM peak was the critical scenario, with a PRC of 16.2%. TfGM reviewed the LinSig model files and confirmed that the modelling methodology and results were acceptable.
Sewer Constraint
The 1,500mm diameter combined sewer running beneath the proposed junction alignment was the most significant engineering constraint on the project. United Utilities’ records showed the sewer at a depth of approximately 2.5 metres below existing road level, with a brick construction dating from the early 20th century.
Standard highway construction directly above a sewer of this size and age would normally require either sewer diversion (at significant cost and programme impact) or protective measures to prevent construction loads from damaging the sewer.
We proposed a reinforced concrete raft slab beneath the carriageway construction in the zone above the sewer. The raft distributed vehicle loads over a sufficient area to ensure that the stress on the sewer crown remained within acceptable limits. The raft design was verified by finite element analysis, with the results submitted to United Utilities as part of the build-over agreement application.
This approach was significantly cheaper and faster than sewer diversion. United Utilities approved the build-over agreement within six weeks of submission, and the raft construction added only two weeks to the overall highways construction programme.
Pedestrian and Cycling Facilities
Manchester City Council’s planning conditions required the development to provide enhanced pedestrian and cycling connectivity. We designed a toucan crossing on the A57 approximately 40 metres east of the main junction, connecting the site to the existing cycle path on the south side of the road.
The crossing was coordinated with the main junction signals to provide a linked signal sequence, minimising delays for all users. The cycle path within the site was designed as a 3-metre-wide shared-use route, connecting the toucan crossing to the retail park entrance and internal cycle parking.
Outcome
The S278 technical approval was secured after a single submission, with no significant design amendments required. This was achieved through thorough pre-application engagement with the highways authority and TfGM, ensuring that all design requirements were understood and addressed before the formal submission.
The construction phase was delivered on programme by the client’s contractor, with Aegaea providing technical oversight during the highways works. The junction and crossing were commissioned on first inspection, and the highways works were adopted by the highways authority within three months of completion.