Published counts · interactive scenario model
A38 North Petherton traffic flow explorer
Explore how the measured vehicle mix and user-defined changes affect estimated flow, capacity, speed and delay on the indicative A38 30 mph town section.
The line is an indicative centreline for visualisation. Posted signs and traffic regulation orders determine the legal speed-limit boundary.
Modelled result
Busiest-direction road conditions
Northbound
- Flow
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- PCU flow
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- Capacity
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- Est. speed
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- Added time
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Southbound
- Flow
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- PCU flow
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- Capacity
- —
- Est. speed
- —
- Added time
- —
How the data and estimates are calculated
Measured baseline
The 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 value is 17,663 and is marked as estimated; the 2025 option scales each 2021 hourly vehicle class by the corresponding AADF change.
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. Above capacity, WebTAG’s one-hour over-capacity delay relationship is applied. The percentage likelihood is a transparent logistic indicator derived from the volume-to-capacity ratio; it is not an observed probability.
Limits of use
This is a corridor screening and engagement tool. The hourly shape comes from one manual-count day in 2021 and should not be treated as every weekday or current live conditions. It does not include live incidents, roadworks, weather, school-day variation, turning movements, signal timings or detailed queue interaction at junctions. It should not be used for navigation, enforcement or engineering design.
DfT count point 16378 WebTAG Unit M3.1 Open Government Licence v3.0