Traffic Flow Explorer

Published counts · interactive scenario model

A38 North Petherton traffic flow explorer

Explore how measured traffic, vehicle-mix changes and an M5 junction 24–25 closure could affect flow, capacity, speed and delay on the indicative A38 30 mph town section.

DfT point 16378 2021 counted M5 open 30 mph
Scenario tool, not live traffic: the A38 baseline uses published DfT data, while an M5 closure adds a user-selected share of the separately counted 2025 M5 J24–J25 flow. It remains a screening model, not live traffic or a detailed junction/queue simulation.

The A38 line and M5 junction marker are indicative visualisations. Posted signs, traffic management and actual route choice determine real closure conditions.

Modelled result

Busiest-direction road conditions

—
Two-way traffic — veh/h —
Capacity used —% —
Likelihood of material delay — —% modelled indicator
Estimated added time — — journey · — mph
Busiest-direction capacity —

Northbound

Flow
—
PCU flow
—
Capacity
—
Est. speed
—
Added time
—
M5 diversion
0 veh/h

Southbound

Flow
—
PCU flow
—
Capacity
—
Est. speed
—
Added time
—
M5 diversion
0 veh/h
How the data and estimates are calculated

Measured baseline

The A38 hourly profile is the DfT manual count at count point 16378 on 5 May 2021. The published 2021 AADF is 15,697 motor vehicles per day. DfT’s 2025 A38 value is 17,663 and is marked as estimated; the 2025 option scales each 2021 hourly vehicle class by the corresponding AADF change.

M5 closure input

The closure scenario uses the DfT manual count at M5 count point 56022 between junctions 25 and 24 on Friday 9 May 2025. The selected share is added by vehicle class and direction to the A38 flow. The assignment percentage is deliberately user-controlled because no published count states what proportion of affected M5 traffic actually reaches this A38 section.

Capacity and speed

The default screening capacity is 1,276 PCU/hour in each direction. It converts WebTAG’s 1,200 vehicles/hour small-town reference capacity using the counted 2021 heavy-vehicle share at this DfT point. Cars, vans and motorcycles count as 1 PCU; HGVs and buses count as 2. Speed is estimated with the WebTAG Unit M3.1 small-town relationship, using 80% frontage development and 100% of this modelled section in a 30 mph limit.

Delay and likelihood

Added time is the difference between the modelled journey and the model’s zero-flow journey over the indicative 2.70 km section. With the M5 open, the model uses a one-hour over-capacity period; for an affected direction receiving assigned M5 diversion traffic, it uses the selected closure duration. Additional excess demand is the increase in over-capacity PCU rate versus the M5-open scenario, accumulated over that duration. Recovery assumes the M5 reopens and the pre-closure A38 flow leaves spare capacity. The likelihood percentage remains a modelled indicator, not an observed probability.

Limits of use

This is a corridor screening and engagement tool. The A38 and M5 hourly shapes each come from one manual-count day and should not be treated as every weekday or current live conditions. The closure model does not simulate ramp storage, police traffic management, junction blocking, route-choice feedback, trapped traffic, weather or incident-specific behaviour. It should not be used for navigation, enforcement or engineering design.

DfT A38 point 16378 DfT M5 point 56022 Documented A38 diversion example WebTAG Unit M3.1 Open Government Licence v3.0