Engineers Criticise National Highways' Drainage Design Rules
Firms say the Design Palette lacks guidance and contains factual errors, potentially compromising water quality.
Trust 44Craft 67Hype 20How this was reported ▾
The central claim is attributed to a named group of companies, but lacks direct quotes from National Highways.
How well corroborated and evidenced the reporting is. Higher is better.
The article clearly separates fact from comment and provides specific details about the concerns raised.
Context, balance and separation of fact from comment. Higher is better.
The language used is measured and proportionate, with no sensationalism or manufactured urgency.
How far presentation runs ahead of substance. Lower is better.
1 source assessed · methodology
Engineers from nine specialist companies have raised concerns about National Highways' rules for designing drainage systems to manage road runoff. The firms, operating collectively as the Road Runoff Group, expressed their criticisms in a letter dated 2 September.
Road runoff from the strategic road network, including motorways and major A roads, needs management to reduce flood risk and pollution entering water catchments. An investigation in 2024 into ponds around the M25 found potential hazardous waste due to National Highways' inability to confirm when they were last cleaned.
M25 Congestion: Stalled Vehicle Disrupts Dartford Crossing Traffic
Motorists face delays as a stalled vehicle brings traffic to a standstill on the M25.
In November 2025, National Highways identified 182 locations of outfalls and soakaways on its network for pollution remediation by 2030. To support this, WSP was appointed in May 2026 to lead a group delivering the Water Quality Plan, focusing on treating water from the highest risk outfalls.
Concerns over the Design Palette
The Road Runoff Group's letter was sent as part of an ongoing dialogue with National Highways regarding the Design Palette and the Design Manual for Roads and Bridges (DMRB). The group stated that while they understand the distinction between the two, the Design Palette is functioning as the primary design tool for engineers working on retrofitting existing infrastructure. This is because National Highways announced approximately eighteen months ago that the focus would shift from new road projects to upgrading current infrastructure using the Water Quality Plan budget.
M25 Blocked by Truck Incident
Traffic is moving slowly following a vehicle breakdown on the motorway.
The engineers claim that schemes are currently being designed and built using data they have demonstrated to be inaccurate. They requested that factual errors in the Palette be corrected as they are identified, rather than waiting for the next DMRB review cycle.
Critiques on Treatment Efficiencies and Guidance
A key criticism concerns the Design Palette's claim that hydrodynamic vortex separators can remove 15% of dissolved zinc. Manufacturers represented in the group categorically state their devices, which are sedimentation devices, cannot remove dissolved metals. The group also highlighted insufficient design guidance in both the DMRB and the Palette for creating effective swales. They reported that this leads to features being specified as swales but functioning merely as drainage ditches with vegetation, offering no meaningful treatment.
The group further explained that their concern is systemic, with the Design Palette lacking adequate guidance on the sequencing of treatment devices. This can result in devices being placed where they cannot function effectively, such as a vortex separator downstream of a pond, leading to sediment accumulation and compromising infiltration functions.
A spokesperson for National Highways told New Civil Engineer that their Water Quality Design Playbook offers potential solutions for tackling water quality and pollution from road runoff. They reiterated that it does not replace the standards in the Design Manual for Roads and Bridges and was published to help stakeholders understand their plans for mitigating high-priority outfalls and soakaways.
Questions this report answers
+What are engineers criticising about National Highways' drainage rules?
Engineers from nine companies, acting as the Road Runoff Group, have criticised National Highways' rules for designing road drainage. They claim the Design Palette, a key tool for retrofitting infrastructure, contains factual errors and lacks sufficient guidance for effective swales, potentially compromising water quality and flood risk management.
+What specific inaccuracies have been identified in the Design Palette?
The Road Runoff Group states that the Design Palette incorrectly assigns a 15% dissolved zinc removal figure to hydrodynamic vortex separators. Manufacturers confirm these devices are sedimentation-based and cannot remove dissolved metals. They also report insufficient guidance for designing effective swales.
+What is National Highways' response to these criticisms?
National Highways stated that its Water Quality Design Playbook offers potential solutions for road runoff pollution and does not replace existing standards like the Design Manual for Roads and Bridges. The playbook was published to help stakeholders understand their plans for mitigating high-priority outfalls and soakaways.
WE FILE
No invented reporter. No hidden source trail.
The London Travel Desk assembled this report from the outlets listed below. Language-model drafting is reviewed under human editorial supervision, with scoring notes and corrections kept visible.
More Coverage in Roads & Driving
M25 Congestion: Stalled Vehicle Disrupts Dartford Crossing Traffic
M25 Blocked by Truck Incident
M25 Delays After Vehicle Incident Near Staines
