A38 Cumulative Traffic Report Vr 1.5B Aug 2026

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Combined Traffic Impact of Current and Proposed Housing
Developments Within the North Petherton / Huntworth
Corridor
This is a live technical evidence document and will be updated as new Transport Assessments,
modelling data, traffic counts or planning information become available

© Version 1.5B August 26

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Abstract
This report presents a comprehensive technical assessment of cumulative traffic impacts associated
with existing and proposed residential developments along the A38 North Petherton – Huntworth
corridor. The study considers both individual and combined effects on network performance, highway
safety, and policy compliance, in accordance with the National Planning Policy Framework (NPPF), the
Sedgemoor Local Plan (SLP), and Somerset Local Transport Plan 4 (LTP4).
The assessment reflects a total development quantum of 2,730 dwellings. Findings indicate material
capacity constraints, increased congestion potential, and elevated safety and air-quality sensitivities
along key sections of the corridor. These outcomes highlight the importance of coordinated transport
planning and strategic infrastructure intervention to ensure long-term policy compliance and
sustainable network operation.

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Executive Summary
This cumulative traffic assessment confirms that the current and planned residential development
proposals — totaling 2,730 dwellings — would generate a severe, non-mitigatable impact on the A38
corridor, the principal link between North Petherton and M5 Junction 24.
Current Applications (1,330 Dwellings)
Forecast modelling indicates a 48.28% increase in daily traffic volumes relative to the updated DfT
2021 baseline data (16,530 vehicles). This contravenes Sedgemoor Local Plan Policy D13, which
requires development to minimise adverse traffic impacts. Within one to five years of occupation,
the corridor is projected to experience routine peak-hour congestion and network stress.
Cumulative Scenario (2,730 Dwellings)
When combined with future allocations, daily traffic flows are expected to increase by approximately
99.09% relative to the updated DfT 2021 baseline data (16,530 vehicles). This engages NPPF TR6(3)(4): cumulative impacts must be assessed, and proposals refused where severe adverse
capacity/congestion impacts remain after taking account of mitigation and wider network
improvements.
Safety and Health Implications
The projected increase in vehicle movements conflicts with Somerset LTP4’s Vision Zero
commitment, exacerbating existing high-risk conditions along the A38 and worsening local air quality.
Persistent queuing and slow-moving traffic are expected to elevate exposure to NOₓ and PM₂.₅,
posing long-term public-health risks.
Policy and Mitigation Context
Taken together, these findings indicate that the cumulative development scale cannot be
accommodated within the existing highway network without strategic intervention. While localised
junction optimisation may marginally improve efficiency, meaningful mitigation would require
corridor-wide infrastructure upgrades. Continued monitoring and phased delivery aligned to
demonstrable transport improvements are recommended to ensure future policy compliance.

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Contents
Abstract ................................................................................................................................................... 1
Executive Summary ................................................................................................................................. 2
Part 1 – Impact from Current Applications (Updated 1,330 Dwellings) .................................................. 4
Part 2 – Cumulative Scenario (Updated 2,730 Dwellings) ....................................................................... 7
Part 3 – Policy, Safety & Health Analysis ................................................................................................. 8
Part 4 – Key Findings & Recommendations ........................................................................................... 10
Appendix U (Update) — Applicant TRICS Consistency Review .............................................................. 12
Appendix T (Update) — DfT TAG Screening Appraisal........................................................................... 14
Revisions / Updates ............................................................................................................................... 16

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Part 1 – Impact from Current Applications (Updated 1,330 Dwellings)
Overview
This section analyses eight live planning applications within the North Petherton study area,
representing a combined total of 1,330 residential units. The assessment quantifies traffic generation,
identifies network implications, and evaluates compliance with adopted transport policies.
Application Summary
Application Name

Application No.

Dwellings (Units)

Park Lane

37/25/00044

64

Gateway (Phase 1)

37/25/00055

15

Gateway (Phase 2)

37/25/00042

346

Vistry Gateway

37/22/00126

150

Stafflands Farm

37/25/00023

190

Daws Lane

37/25/00018

175

Dancing Hill

37/25/00088

140

Shovel Lane

TBC

250

Total

—

1,330 units

Traffic Generation and Baseline Data
Calculation Parameter

Detail

Adjusted Baseline Traffic Adjusted DfT 2021 A38 corridor baseline *
New Daily Trips

1,330 units × 6.0 trips/unit/day

Result
16,530 vehicles
7,980 trips/day

48.28 %
% Increase on Baseline

7,980 ÷ 16,530 × 100
increase

*Adjusted working baseline: DfT 2021 A38 link (A361 junction to A38 spur/M5 J24), uplifted for 235
dwellings in post-2021 major committed developments not represented in the 2021 count.

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Trip Rate Justification and Sensitivity Context:
The main assessment adopts a trip-generation rate of 6.0 two-way trips per dwelling per 12-hour
day, consistent with TRICS residential averages for small towns and semi-rural locations with
moderate accessibility. This reflects typical car-ownership levels and commuting patterns within
North Petherton.
To confirm robustness, a sensitivity test was applied using 7.2 trips per dwelling (equivalent to 0.6
two-way trips per hour), representing a higher-intensity scenario sometimes applied in national or
urban contexts. The outcome indicates that while numerical uplifts increase proportionally, the
overall policy interpretation and impact classification remain unchanged.
Sensitivity Note: Applying 7.2 trips/unit/day yields 9,576 trips/day and a 57.93% increase on the
same baseline; this does not materially alter the interpretation of network stress or policy context.

Individual Application Traffic Contributions (12-hr day; 6 trips/dwelling; baseline 16,530)
Application

Units New Trips/day

% Total New Trips

% Increase on Baseline

Park Lane

64

384

4.81%

2.32%

Gateway Phase 1

15

90

1.13%

0.54%

Gateway Phase 2

346

2,076

26.02%

12.56%

Stafflands Farm

190

1,140

14.29%

6.90%

Daws Lane

175

1,050

13.16%

6.35%

Shovel Lane

250

1,500

18.80%

9.07%

Vistry Gateway

150

900

11.28%

5.44%

Dancing Hill

140

840

10.53%

5.08%

TOTAL

1,330

7,980

100%

+48.28% overall

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Policy and Guidance Position (1,330 Dwellings)
Although this scenario alone is not assessed here as meeting the NPPF TR6(4) “severe adverse
impact” refusal test, it demonstrates inconsistency with local transport-policy objectives and
materially worsens network stress.
Policy / Guidance

Reference

Relevance

Policy D13 – Sustainable A ~48 % traffic increase fails to minimise adverse
Sedgemoor Local
Transport and

impacts, increasing congestion, air-quality pressures

Movement

and safety risk.

Sedgemoor Local

Policy D14 – Managing

Anticipated congestion at M5 J24 compromises safe

Plan (SLP)

Transport Impacts

and efficient SRN operation.

Plan (SLP)

Conflicts with Vision Zero and sustainable-travel
Somerset LTP4

Vision and Objectives
principles.

Time-Based Network Performance Projections (1,330 Dwellings)
Timeframe Projected Impact on A38 / SRN Network

Key Policy Conflict

Frequent network stress; recurrent peak-period

SLP Policy D14 – Network efficiency

congestion; M5 J24 approaching capacity.

undermined.

10–15

Persistent capacity deficit; traffic diversion to

SLP Policy D13 – Failure to minimise

Years

unsuitable roads; elevated collision risk.

adverse impacts.

1–5 Years

Long-term congestion and progressive air-quality NPPF TR3/TR6 – sustainable
20 + Years
deterioration.

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transport/network impacts.

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Part 2 – Cumulative Scenario (Updated 2,730 Dwellings)
Overview
This cumulative scenario combines the eight current planning applications (1,330 dwellings) with the
proposed project of approximately 1,400 dwellings at Walford Cross, producing a total of 2,730
residential units.
The assessment models overall trip generation, cumulative network stress, and long-term policy
compliance.
Traffic Generation and Baseline Comparison
Calculation Parameter

Formula / Data Used

Result

Baseline Traffic

Adjusted DfT 2021 A38 corridor baseline *

16 530 vehicles

2,730 × 6.0 trips/unit/day

16,380 trips/day

(16,380 ÷ 16,530) × 100

99.09 % increase

Total New Daily Trips

% Increase on Baseline

*Adjusted working baseline: DfT 2021 A38 link (A361 junction to A38 spur/M5 J24), uplifted for 235
dwellings in post-2021 major committed developments not represented in the 2021 count.
Sensitivity Note: Using a 7.2-trip rate produces 19,656 trips/day (+118.91%), confirming the same
severe-adverse-impact conclusion under NPPF TR6(4), subject to mitigation and wider network
improvements. The use of 6.0 trips/dwelling provides a representative mid-range scenario aligned
with TRICS data, while the 7.2-trip test provides a robust upper-bound sensitivity check.
Time-Based Network Performance Projections
Timeframe Projected Impact on A38 / SRN Network

Policy and Compliance Implications

1–5 Years

Systemic capacity failure; chronic gridlock
along A38/A361; M5 J24 breakdown;
restricted emergency access.

NPPF TR6(4) – severe adverse network
impact; SLP D14 – strategic network
compromised.

10–15
Years

Prolonged congestion; degraded air quality
(NOₓ, PM₂.₅); unsafe active-travel conditions.

SLP D13 – Cannot minimise adverse
impacts; Somerset LTP4 – Vision Zero
conflict.

Persistent corridor congestion; economic
20 + Years constraints; requirement for major
infrastructure (bypass or junction upgrade).

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NPPF TR3/TR6 – sustainable
transport/network capacity.

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Interpretation and Severity Assessment
The near-doubling of traffic is assessed as a severe adverse transport-network impact under NPPF
TR6(4), including cumulative impacts.
Expected effects include:
•

Network-wide congestion and delay

•

Operational failure at M5 J24

•

Increased collision risk

•

Worsening air quality and public-health harm

No realistic mitigation package exists that would maintain network performance or policy compliance
at this scale.

Part 3 – Policy, Safety & Health Analysis
Policy and Guidance Compliance Summary
Policy / Guidance
National Planning
Policy Framework
(NPPF)

Reference

Compliance Position

TR6(3)-(4)

Cumulative impacts assessed as a severe adverse
network impact, subject to mitigation and wider
network improvements.

Sedgemoor Local Plan
Traffic growth challenges objectives to minimise
Policies D13 & D14
(SLP)
adverse impacts and maintain safe operation.
National Highways

SRN Sensitivity to
Growth Principles

Forecast traffic uplift exceeds typical strategic-network
growth acceptance bands without major mitigation.

Somerset LTP4

Vision & Objectives

Growth reduces scope for sustainable-travel and safety
improvement programmes.

Detailed Impacts on Health, Travel, and the Strategic Road Network (SRN)
Public Health Implications:
Projected long-term queuing within North Petherton’s residential corridor will increase exposure to
airborne pollutants (NOₓ and PM₂.₅). This condition constitutes a chronic public-health hazard,
directly undermining the social and environmental pillars of sustainability defined by the NPPF.

Impact on Travel and Network Efficiency:
The A38 serves as a critical link in Somerset’s Strategic Road Network, connecting regional traffic to
the M5. Functional failure of this corridor would have cascading effects on both local and national
movement, affecting logistics, commuter traffic, and emergency access. Diversion of vehicles onto
unsuitable local roads is expected to increase accident exposure and reduce residential amenity.

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Accident Data and Safety Context
High-Risk Corridor:
The A38 already exhibits an above-average accident frequency, with certain sections recording
incident rates exceeding twice the national mean.
Incident
Description

Date

Location

Injuries / Outcome

04 August
2024

A38 Taunton Road, Bridgwater
Serious collision
Fatal – motorcyclist died at
(between Huntworth Business Park
between
scene; road closed for
Rbt and Showground Rd /
motorcycle and car. investigation.
Stockmoor Dr Rbt)

04 October
Walford Cross (junction with A361)
2024

Tractor–car
collision.

Serious injuries; air
ambulance deployed; full
road closure.

13
December
2019

A38 North of Huntworth
Roundabout, Bridgwater

Three-vehicle crash.

Non-fatal; caused
significant delays.

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November
2019

A38 at Thurloxton (between Walford Motorcycle–car
Cross and Moon Lane)
collision.

Fatal – motorcyclist
sustained fatal injuries;
road closed for
investigation.

Observed A38 Traffic Volume and Speed Data (Pegasus Group / Daws Lane)
Measure

Evidence

Traffic flow

5-day: 11,100; 7-day: 10,300 178 buses/coaches (weekday)

85th %ile speed NB 39.47 mph

Context

SB 39.66 mph

Pegasus's April 2023 A38 ATC recorded a weekday average of 11,100 vehicles/day, including 178
buses/coaches, with 85th-percentile speeds of 39.47 mph northbound and 39.66 mph southbound. This
provides contemporary local validation of traffic intensity and operating speed alongside the adjusted DfT
corridor baseline.

Existing Network Conditions
The A38 / M5 Junction 24 corridor experiences recurrent congestion even under baseline conditions.
National Highways and RAC monitoring regularly report “severe delays” during peak and seasonal
traffic periods. These findings confirm that the network currently operates near or beyond its design
capacity, rendering it highly sensitive to additional growth.

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Part 4 – Key Findings & Recommendations
Key Findings
The cumulative development quantum (2,730 dwellings) results in an approximate 99.09% increase in
daily traffic on the A38/A361 corridor versus the revised 2021 baseline.
Key observations:
•

Increased congestion on the A38 and M5 J24.

•

Forecast demand beyond operational capacity at multiple junctions.

•

Limited potential for safety and mode-shift programmes.

•

Increased exposure to air-quality sensitivity zones.

Network Performance Outlook
Proportional modelling indicates:
•

Recurring peak-period delays during early occupation.

•

Consolidation of queuing in later phases.

•

Extended journey-time variability for local and strategic movements.

Policy Alignment Position
The forecast demand presents challenges for:
•

NPPF TR6 cumulative transport impact test.

•

SLP D13 & D14 transport policies.

•

SRN growth-sensitivity guidance.

•

Local authority mode-shift and Vision Zero objectives.

Mitigation Context

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•

Local junction works alone will not offset demand pressures.

•

Strategic-scale intervention (e.g., junction reconfiguration or bypass) likely required.

•

Funding and timing must be defined before counting as deliverable mitigation.

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Conclusion and Recommendations
The cumulative scenario indicates traffic demand exceeding the capacity of the A38 corridor and M5
J24.
This provides a technical basis for identifying cumulative operational constraints and the need for
strategic infrastructure planning.
Recommendations:
1. Seek strategic-transport planning input on corridor-wide solutions.
2. Phase development to match demonstrable capacity improvements.
3. Implement ongoing corridor performance monitoring to inform future planning decisions.

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Appendix U (Update) — Applicant TRICS Consistency Review
U1. Purpose
This appendix reviews applicant-submitted Transport Assessments to test whether the trip
generation assumptions used in the Cumulative Traffic Report (August 2026 Ver 1.5B) are reasonable
when compared against locally submitted TRICS evidence.
The review uses only the following applications:
•

Park Lane (37/25/00044) — Miles White

•

Daws Lane (37/25/00018) — Pegasus

•

Bridgwater Gateway Phase 2 (37/22/00126) — Peter Evans Partnership (using National
Highways benchmark rates)

No independent TRICS analysis has been undertaken.

U2. Applicant Peak-Hour Trip Rates
Site

Dwellings AM Peak two-way (veh/dw/hr) PM Peak two-way (veh/dw/hr)

Park Lane

64

0.503

0.510

Daws Lane

175

0.523

0.489

Gateway Phase 2 150

0.560

0.585

U3. Methodological context
Park Lane (Miles White)
Recent TRICS surveys (2022–2024), small sites (40–100 dwellings), servicing vehicles excluded and
inclusion of some edge-of-centre locations. This produces a lower-bound benchmark.
Daws Lane (Pegasus)
Large TRICS sample (2014–2024), edge-of-town and suburban car-based locations. The applicant
explicitly describes this as a “robust / maximum trip generation” scenario.
Gateway Phase 2 (PEP)
Uses National Highways-required benchmark trip rates derived from earlier strategic TRICS surveys.
These are stated to be higher than more recent TRICS averages and therefore represent a strategic
upper bound.

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U4. Locally evidenced envelope
Across the three applicant Transport Assessments a consistent envelope emerges:
•

AM Peak: ~0.50 – 0.56 veh/dw/hr

•

PM Peak: ~0.49 – 0.59 veh/dw/hr

This envelope is entirely derived from applicant-submitted evidence.

U5. Sites Not Yet Incorporated into Appendix U
Appendix U reviews only those Transport Assessments currently examined as part of this report,
namely Park Lane, Daws Lane and Bridgwater Gateway Phase 2.
At the time of publication, additional transport evidence may exist for other sites within the
cumulative assessment area; however, these documents have not yet been reviewed as part of this
appendix and are therefore not included within the locally evidenced trip-rate envelope established
in Section U4.
For cumulative assessment purposes, where site-specific trip-generation data has not yet been
reviewed, it is reasonable to apply a proxy trip rate based on the locally evidenced envelope:
• Central proxy: mean of Park Lane, Daws Lane and Gateway Phase 2
• Robust proxy: upper bound (National Highways benchmark)
This approach is consistent with DfT / TAG practice when site-specific data is incomplete or has not
yet been incorporated into the cumulative assessment.

U6. Implication for the Cumulative Traffic Report
The Cumulative Traffic Report uses 6.0 and 7.2 daily trips per dwelling.
When translated into peak-hour equivalents, these fall within or below the locally evidenced TRICS
envelope above.
Accordingly, the cumulative modelling assumptions are reasonable and not optimistic when tested
against applicant Transport Assessments.

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Appendix T (Update) — DfT TAG Screening Appraisal
T1. Purpose
This appendix applies DfT TAG (WebTAG) screening to the TRICS-validated demand established in
Appendix U.
Two cases are considered:
•

Central case: mean of Park Lane, Daws Lane and Gateway Phase 2

•

Robust case: National Highways upper bound (Gateway Phase 2)

T2. Peak-hour demand used
From Appendix U:
•

Central peak rate: ~0.53 veh/dw/hr

•

Robust peak rate: ~0.58 veh/dw/hr

For 2,730 dwellings:
Case

AM peak two-way flow

Central

~1,447 vehicles/hour

Robust (NH) ~1,583 vehicles/hour
These flows would feed directly into the A38 / Huntworth Roundabout / M5 J24 system.

T3. TAG screening interpretation
Under WebTAG Unit A1.3:
•

Flows of this scale on a corridor already operating near capacity result in:
o

Severe delay

o

Queue propagation and reliability failure

o

Network instability at M5 Junction 24

These impacts trigger TAG “large adverse” user impacts.

T4. Economic, safety and environmental implications
Using DfT standard values of time and accident valuation, flows of this magnitude produce:
•

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Multi-million-pound annual journey-time losses

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•

Increased collision risk on the A38 and SRN approaches

•

Higher NO₂ and PM₂.₅ exposure from persistent queuing

These represent material adverse impacts under TAG Units A1 (user impacts), A3 (environment) and
A4.1 (social/safety).

T5. TAG conclusion
Using applicant-validated TRICS demand (Appendix U), the cumulative development scenario results
in:
Severe residual transport impacts under DfT TAG.
This supports the NPPF TR6(4) conclusion in the main report that the cumulative scheme would have
a severe adverse network impact unless effective strategic mitigation changes that outcome.

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Revisions / Updates
Version Date Revision Details

1.3

Updated baseline traffic to reflect post-2021 major residential developments (Royal
Nov View, Chaucer's Meadow, Batts Meadow and Carrots Farm). Trip generation values
2025 revised to reflect the location and settlement characteristics of assessed
developments in accordance with TRICS guidance.

1.4

Jan
Added Appendix U (Applicant TRICS Consistency Review) and Appendix T (DfT TAG
2026 Screening Appraisal).

1.5

Updated to include the proposed Shovel Lane residential development (250
dwellings) within the cumulative assessment area. Current application totals revised
from 1,080 to 1,330 dwellings and cumulative development totals revised from
2,480 to 2,730 dwellings. Updated all associated trip-generation calculations,
June
percentage traffic increases, cumulative scenario forecasts, DfT TAG screening
2026
outputs, summary tables and cumulative impact assessments. Revised Appendix U
to clarify the status and review coverage of available Transport Assessments and
supporting transport evidence. Updated Executive Summary, Key Findings and
associated figures to maintain consistency throughout the report.

1.5A

Data clarification only. Retained the adjusted 16,530-vehicle DfT corridor baseline and
Aug all existing cumulative calculations. Clarified baseline provenance and replaced the
2026 derived speed-band summary with direct Pegasus 2023 ATC evidence for local traffic
volumes, bus/coach flows and 85th-percentile speeds.

1.5B

Aug NPPF reference update only: superseded §116 references replaced by August 2026
2026 NPPF TR3/TR6. No traffic data, modelling or calculations changed.

Disclaimer (Errors & Omissions)
This assessment is based on information available at the time of preparation. It is provided for
technical and planning purposes only and does not constitute a legal document or precedent. While
every effort has been made to ensure accuracy, no liability is accepted for any errors, omissions, or
reliance placed on the contents beyond its intended scope.

August 2026 Ver 1.5B

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