A-road
A4251 traffic and collisions
The A4251 is an A-road of 11.9 miles, in Hertfordshire. This page covers its traffic at 11 DfT count points since 2000 and the collisions police recorded on it.
- Traffic in 2025 48m vehicle miles
- Average daily traffic 10,926 vehicles a day, both directions
- Busiest section 19,793 A4146 to B4505
- HGV share 1.3% of vehicle miles
- Traffic since 2019 −6.8% same count points
- Collisions, 2021 to 2025 89 38.7 per 100m vehicle miles
How busy is the A4251?
On an average day in 2025, 10,926 vehicles passed each point on the A4251, counting both directions. The busiest section is A4146 to B4505 (Hertfordshire), with 19,793 a day; the quietest listed is B4506 to Boswick Lane, with 8,692.
Busiest sections, 2025
A4146 to B4505 carries 1.8 times the road’s average flow.
| Section | Vehicles a day |
|---|---|
| A4146 to B4505 Hertfordshire · 0.6 mi · HGVs 1.6% | 19,793 |
| Home Park Mill Link Rd, Kings Langley to Nash Mills Lane, Hemel Hempstead Hertfordshire · 1.6 mi · HGVs 1.1% | 13,206 |
| A416 to B4506 Hertfordshire · 1.2 mi · HGVs 1.1% | 12,679 |
| A414 to A4146 Hertfordshire · 0.5 mi · HGVs 0.8% | 10,364 |
| Nash Mills Lane to A414 Hertfordshire · 1.3 mi · HGVs 0.8% | 10,256 |
| Hall Park Gate to A416 Hertfordshire · 0.9 mi · HGVs 1.0% | 9,468 |
| A41 / B4635 to Boswick Lane, Berkhamstead Hertfordshire · 2.4 mi · HGVs 1.4% | 9,342 |
| B4505 to A41 Hertfordshire · 0.7 mi · HGVs 2.8% | 9,177 |
| A41 to Hall Park Gate, Berkhamstead Hertfordshire · 1.7 mi · HGVs 1.7% | 9,038 |
| B4506 to Boswick Lane Hertfordshire · 0.6 mi · HGVs 1.4% | 8,692 |
Source:DfT,UK Motoring analysis 2000 to 2025 Updated
Route
In Hertfordshire · 11.9 miles
A4251 traffic and collisions by section
11 sections between junctions, in route order. Each is the average of its DfT count points, weighted by length.
Sections Tap a column heading to sort
| # | |||
|---|---|---|---|
| 1 | M25 Jn20 to Home Park Mill Link Rd, Kings LangleyHertfordshire | 13,338 | fewer than 3 |
| 2 | Home Park Mill Link Rd, Kings Langley to Nash Mills Lane, Hemel HempsteadHertfordshire | 13,206 | 14 |
| 3 | Nash Mills Lane to A414Hertfordshire | 10,256 | 20 |
| 4 | A414 to A4146Hertfordshire | 10,364 | 4 |
| 5 | A4146 to B4505Hertfordshire | 19,793 | 4 |
| 6 | B4505 to A41Hertfordshire | 9,177 | 6 |
| 7 | A41 to Hall Park Gate, BerkhamsteadHertfordshire | 9,038 | 6 |
| 8 | Hall Park Gate to A416Hertfordshire | 9,468 | 6 |
| 9 | A416 to B4506Hertfordshire | 12,679 | 14 |
| 10 | B4506 to Boswick LaneHertfordshire | 8,692 | 0 |
| 11 | A41 / B4635 to Boswick Lane, BerkhamsteadHertfordshire | 9,342 | 13 |
Vehicles a day is the average daily flow in 2025. “Since 2019” compares the same count points. Collisions are those with the A4251 as the first road, placed on the nearest section by their recorded position; rates (per 100 million vehicle miles) are shown for sections of half a mile or more with 10 or more.
A4251 traffic by vehicle type
Cars and taxis make up 83.6% of the vehicle miles on the A4251. HGVs account for 1.3%, rising to 2.8% on B4505 to A41.
Vehicle mix, 2025
Source:DfT,UK Motoring analysis 2000 to 2025 Updated
HGV share of traffic
| Year | HGV share |
|---|---|
| 2000 | 2.5% |
| 2001 | 2.4% |
| 2002 | 2.6% |
| 2003 | 2.8% |
| 2004 | 3.3% |
| 2005 | 3.0% |
| 2006 | 3.0% |
| 2007 | 2.6% |
| 2008 | 2.3% |
| 2009 | 2.1% |
| 2010 | 2.0% |
| 2011 | 1.9% |
| 2012 | 1.7% |
| 2013 | 1.7% |
| 2014 | 1.7% |
| 2015 | 1.7% |
| 2016 | 1.5% |
| 2017 | 1.5% |
| 2018 | 1.4% |
| 2019 | 1.4% |
| 2020 | 1.6% |
| 2021 | 1.5% |
| 2022 | 1.5% |
| 2023 | 1.4% |
| 2024 | 1.4% |
| 2025 | 1.3% |
Source:DfT,UK Motoring analysis 2000 to 2025 Updated
A4251 traffic trend
Traffic on the A4251 came to 47.5 million vehicle miles in 2025. At the count points present in both years, it is down 6.8% on 2019, before the pandemic, and down 10.1% on 2015.
Vehicle miles a year
Like for like, traffic is down 11.0% on 2000.
| Year | Vehicle miles |
|---|---|
| 2000 | 52m |
| 2001 | 51m |
| 2002 | 51m |
| 2003 | 50m |
| 2004 | 51m |
| 2005 | 51m |
| 2006 | 51m |
| 2007 | 52m |
| 2008 | 51m |
| 2009 | 52m |
| 2010 | 51m |
| 2011 | 52m |
| 2012 | 51m |
| 2013 | 50m |
| 2014 | 52m |
| 2015 | 52m |
| 2016 | 52m |
| 2017 | 51m |
| 2018 | 51m |
| 2019 | 51m |
| 2020 | 39m |
| 2021 | 43m |
| 2022 | 46m |
| 2023 | 47m |
| 2024 | 47m |
| 2025 | 48m |
Source:DfT,UK Motoring analysis 2000 to 2025 Updated
Average daily flow
| Year | Vehicles a day |
|---|---|
| 2000 | 11,696 |
| 2001 | 11,620 |
| 2002 | 11,481 |
| 2003 | 11,356 |
| 2004 | 11,501 |
| 2005 | 11,612 |
| 2006 | 11,565 |
| 2007 | 11,715 |
| 2008 | 11,564 |
| 2009 | 11,656 |
| 2010 | 11,586 |
| 2011 | 11,670 |
| 2012 | 11,518 |
| 2013 | 11,353 |
| 2014 | 11,658 |
| 2015 | 11,716 |
| 2016 | 11,872 |
| 2017 | 11,641 |
| 2018 | 11,549 |
| 2019 | 11,727 |
| 2020 | 9,010 |
| 2021 | 9,859 |
| 2022 | 10,576 |
| 2023 | 10,780 |
| 2024 | 10,851 |
| 2025 | 10,926 |
Source:DfT,UK Motoring analysis 2000 to 2025 Updated
The chart counts the whole road as it was each year, so new or renumbered stretches move it. The percentage changes compare only count points present in both years. In 2025, 1 of the 11 count points were counted; DfT estimated the rest from earlier counts and local trends.
A4251 collisions
Police recorded 89 collisions with the A4251 as the first road from 2021 to 2025, 31 of them fatal or serious. That is 38.7 per 100 million vehicle miles, higher than the 31.5 for all A-roads in Great Britain. On average there were 1.5 a year for each mile of road.
Reported collisions by year
Up 21.4% from 2021 to 2025.
| Year | Slight | Fatal or serious |
|---|---|---|
| 2021 | 10 | 4 |
| 2022 | 12 | 7 |
| 2023 | 12 | 8 |
| 2024 | 15 | 4 |
| 2025 | 9 | 8 |
Source:DfT STATS19,UK Motoring analysis 2021 to 2025 Updated
Sections with the highest collision rates
Per 100 million vehicle miles, 2021 to 2025. Sections of half a mile or more with 10 or more collisions.
| Section | Per 100m vehicle miles |
|---|---|
| Nash Mills Lane to A414 Hertfordshire · 20 collisions | 84.9 |
| A416 to B4506 Hertfordshire · 14 collisions | 52.2 |
| Home Park Mill Link Rd, Kings Langley to Nash Mills Lane, Hemel Hempstead Hertfordshire · 14 collisions | 36.6 |
| A41 / B4635 to Boswick Lane, Berkhamstead Hertfordshire · 13 collisions | 33.1 |
Source:DfT STATS19,UK Motoring analysis 2021 to 2025 Updated
A further 12 collisions were recorded at junctions where the A4251 was the second road; they are not in the figures above. Fatal or serious uses DfT’s adjusted serious figures. How we count collisions
Council areas on the A4251
In route order. Each is the highway authority DfT files the count points under.
- Hertfordshire
Data sources and methodology
| Figures | Source | Period | Records | Updated |
|---|---|---|---|---|
| Traffic counts and vehicle mix | Road traffic statistics (AADF by count point) Department for Transport Derived statistics UK Motoring | 2000 to 2025 | 11 count points in 2025 | 15 September 2026 |
| Reported collisions | Road safety open data (STATS19) Department for Transport Derived statistics UK Motoring | 2021 to 2025 | 89 reported collisions | 15 September 2026 |
- Annual average daily flow (AADF) is the average number of vehicles passing a point on a typical day of the year, both directions combined.
- Vehicle miles are AADF multiplied by link length and 365, summed over every count point link on the road.
- Sections join count point links between junctions, using the link lines in DfT’s major road database.
- STATS19 covers personal injury collisions reported to the police. Damage-only collisions are not included, and not every injury collision is reported.
- Serious and slight figures are DfT’s adjusted estimates, which allow for police forces moving to injury-based reporting.
- Counts of one or two collisions below Great Britain level are shown as “fewer than 3”.
Contains public sector information licensed under the Open Government Licence v3.0.