Comprehensive traffic impact assessment A421 Bedford
Effective planning of roadworks is essential to minimise disruption, manage risk, and maintain stakeholder confidence. Following significant flooding on the A421 near Bedford in September 2024, major drainage infrastructure upgrades were required to improve network resilience and enable safe operation of the route.
To support the safe delivery of these works, HW Martin, working on behalf of National Highways, commissioned Immense to undertake a comprehensive traffic impact assessment of proposed Traffic Management (TM) schemes along the A421 corridor.
At a glance
By testing options before implementation, HW Martin were able to:
- Quantify customer delay before works began
- Compare economic impacts across scenarios
- Carry out numerous different simulation
- scenarios before taking action
- Test closures and disruption at different times
- of day or days of the week
- Identify congestion hotspots early
- Inform Design Risk Assessments
- Select lower-risk working windows
- Plan mitigation proactively
The background
The A421 experienced severe flooding in September 2024 due to the failure of a pumping station, resulting in road closures lasting almost three weeks. Replacement works began in early 2025, with further drainage improvements requiring additional Traffic Management measures.
The stretch of road assessed ran on the A421, between the A6 junction at Elstow and Cardington roundabout, and vice-versa.
Once pump replacement works are completed, further closures are required to enable safe removal of old equipment and completion of associated drainage works.
HW Martin required a robust, quantitative assessment of proposed TM schemes to:
- Understand impacts on journey times and delays
- Identify network-wide distribution effects
- Highlight areas of operational risk
- Inform mitigation strategies
- Support the TM design process and client approvals.
The mission
Roadworks inevitably cause delays and disruption to the local areas. The impact of them can be massive, they put pressure on the road networks with bottlenecks and queues. These however can be difficult to predict, and they can be even more difficult to mitigate whilst carrying out the required works.
Using its advanced simulation platform, Immense assessed the likely network impacts of multiple TM options to inform design risk assessments and support evidence-based decision-making.
The solution
The Immense simulation platform enables users to quickly and easily assess the impacts of multiple interventions and alternative Traffic Management (TM) options, analysing “days in the life of” the network under different TM scenarios.
The model was configured to represent 36-hour simulations to capture overnight closure impacts, and 72-hour simulations to capture full weekend closure impacts. This enabled assessment of both immediate and recovery effects across the network.
Each TM option was modelled in detail, and the simulation analysis provided:
- Clear quantification of journey time impacts under each scenario
- Identification of high-risk locations for queuing and congestion
- Insights into diversion route performance
- Comparative analysis of weekend, overnight, and 24/7 narrow lane options
- Evidence to inform mitigation strategies and sequencing decisions.
By modelling each option in advance, the client was able to make informed decisions at the design stage, reducing uncertainty and proactively managing customer impact.
Develop effective traffic management plans
The traffic simulation techniques developed by Immense allow for a realistic representation of the network state, enabling quantitative comparison of impacts of alternative intervention scenarios and identifying potential mitigation options.
Improve customer engagement
The automated generation of output in a friendly format, tailored to expert and non-expert users, facilitates the engagement with users and utility services involved in the process.
Quicker, better planning
By pulling together a wide range of information, the Immense platform provides a holistic picture of various impacts on the transport network, providing the evidence base for enabling better prediction and planning.
The underpinning AWS cloud infrastructure enables quick analysis and comparison of transport options, as scenario runs take hours rather than days/weeks to complete.
The results
Immense worked to inform early-stage optioneering, quantify network impacts, identify operational risks, and support evidence-based decision-making before implementation. By modelling each option in advance, the client was able to make informed decisions at the design stage, reducing uncertainty and proactively managing customer impact.
The study evaluated how different working windows and lane restrictions would affect:
- Approximately 65,000 daily trips directly using the A421 section
- Wider network performance across Bedford and surrounding communities
- Retail parks, schools, and local businesses
- Bus routes operating on diversion corridors.
Through data-driven simulation and structured reporting, Immense supported a safer, more resilient delivery of critical drainage improvements on the A421 corridor.
The future
Immense has developed user friendly traffic simulation tools that enable users to easily and quickly quantify the impacts of roadworks and interventions on the transport network.
The calibrated traffic models and scenario configurations can be used for future option testing by HW Martin, providing ongoing value beyond the initial assessment. Immense also calibrates and validates the models on a quarterly basis, to ensure they run with the most recent traffic data to reduce uncertainty and proactively manage impacts.
Thanks to various layers of information and novel simulation techniques, forecasting the impacts of planned roadworks and unplanned interventions becomes easy.
The friendly user interface and automated output reports that communicate impacts through a few simple figures, allow for an enhanced engagement with stakeholders and impacted users, resulting in an increased confidence in decision making around street works planning
The ability to assess traffic management scenarios before implementation gave us the certainty we needed to plan effectively. The modelling helped us manage risk, minimise disruption and provide our client with clear, evidence‑based recommendations, making the overall delivery process more efficient and predictable.”
Stephen Shaw, Area 6 & 8 Framework Manager, HW Martin (Traffic Management) Ltd.