Motorway

M4 traffic and collisions

The M4 is a motorway of 196.5 miles in England and Wales, from Hounslow to Carmarthenshire. This page covers its traffic at 81 DfT count points since 2000 and the collisions police recorded on it.

Last updated: 15 September 2026
Data source: Department for Transport
Records analysed: 81 count points in 2025
Methodology: View methodology

Traffic in 2025 6.6bn vehicle miles
Average daily traffic 91,647 vehicles a day, both directions
Busiest section 147,484 J4 to J4B
HGV share 7.9% of vehicle miles
Traffic since 2019 −0.8% same count points
Collisions, 2021 to 2025 1,152 3.8 per 100m vehicle miles

How busy is the M4?

On an average day in 2025, 91,647 vehicles passed each point on the M4, counting both directions. The busiest section is J4 to J4B (Hillingdon, Slough), with 147,484 a day; the quietest listed is J48 to J49, with 50,421.

Busiest sections, 2025

J4 to J4B carries 1.6 times the road’s average flow.

Sections of the M4 with the highest average daily flow, 2025
Section	Vehicles a day
J4 to J4B Hillingdon, Slough · 1.9 mi · HGVs 4.0%	147,484
J4B to J5 Slough · 2.1 mi · HGVs 6.2%	140,828
J5 to J6 Slough, Windsor and Maidenhead · 3.7 mi · HGVs 6.5%	137,329
J3 to J4 Hillingdon · 1.8 mi · HGVs 3.8%	136,321
J20 to J19 South Gloucestershire · 3.1 mi · HGVs 8.7%	128,594
J6 to J7 Slough, Buckinghamshire · 1.8 mi · HGVs 5.9%	127,570
J7 to J8 Buckinghamshire, Windsor and Maidenhead · 3.3 mi · HGVs 8.2%	122,812
J10 to J11 Wokingham, Reading · 5.4 mi · HGVs 7.0%	121,922
J28 to J29 Newport · 2.1 mi · HGVs 7.9%	121,902
J27 to J28 Newport · 1.4 mi · HGVs 7.6%	114,877

Source:DfT,UK Motoring analysis 2000 to 2025 Updated 15 Sept 2026

Route

Hounslow to Carmarthenshire · 196.5 miles

Schematic route: each line joins the junctions at the ends of a DfT count point link.

M4 traffic and collisions by section

51 sections between junctions, in route order. Each is the average of its DfT count points, weighted by length. 8 short link roads and spurs (5.1 miles) count in the road’s totals but are not listed.

Show all 51 sections Tap a column heading to sort

Sections of the M4 in route order, 2025 traffic
#
1	J1 to J2Hounslow	0.8	79,664	2.3%	−5.3%	5	–
2	J2 to J3Hounslow	5.2	97,800	3.8%	−13.7%	46	5.3
3	J3 to A312Hounslow	0.1	136,321	3.8%	+1.2%	11	–
4	J3 to J4Hillingdon	1.8	136,321	3.8%	+1.2%	72	18.2
5	J4 to J4BHillingdon, Slough	1.9	147,484	4.0%	−3.5%	59	11.9
6	J4B to J5Slough	2.1	140,828	6.2%	+8.2%	50	9.8
7	J5 to J6Slough, Windsor and Maidenhead	3.7	137,329	6.5%	−2.0%	52	6.3
8	J6 to J7Slough, Buckinghamshire	1.8	127,570	5.9%	+4.7%	17	4.9
9	J7 to J8Buckinghamshire, Windsor and Maidenhead	3.3	122,812	8.2%	−5.2%	20	2.5
10	J8 to J10Windsor and Maidenhead, Bracknell Forest, Wokingham	7.7	114,322	7.3%	−0.4%	40	2.8
11	J10 to J11Wokingham, Reading	5.4	121,922	7.0%	+5.3%	24	2.2
12	J11 to J12Reading, Wokingham, West Berkshire	4.5	112,156	6.8%	−1.8%	35	4.1
13	J12 to J13West Berkshire	11.6	91,482	7.6%	−4.1%	45	2.6
14	J13 to J14West Berkshire	7.5	88,677	11.8%	−3.4%	45	4.2
15	J14 to J15West Berkshire, Wiltshire, Swindon	12.1	88,613	10.6%	−8.4%	63	3.5
16	J15 to J16Swindon, Wiltshire	5.7	82,860	9.7%	−5.8%	28	3.4
17	J16 to J17Wiltshire	12.5	82,987	8.9%	−12.2%	50	2.8
18	J17 to J18Wiltshire, South Gloucestershire	10.2	80,202	9.5%	+9.2%	41	3.0
19	J18 to J19South Gloucestershire	7.6	93,232	8.9%	−2.3%	34	2.7
20	J20 to J19South Gloucestershire	3.1	128,594	8.7%	−10.0%	31	4.2
21	J20 to J21South Gloucestershire	1.9	81,060	9.2%	+4.4%	16	6.2
22	J21 to J22South Gloucestershire	3.9	56,825	9.2%	+2.9%	8	–
23	J22 to J23South Gloucestershire, Monmouthshire	7.2	84,037	11.8%	+8.3%	18	1.8
24	J23 to J23AMonmouthshire	1.1	97,643	9.9%	+26.8%	8	–
25	J23A to J24Monmouthshire, Newport	4.0	96,934	8.9%	+9.7%	14	2.2
26	J24 to J25Newport	2.7	111,912	8.2%	+7.4%	21	4.2
27	J25 to J25ANewport	0.4	117,507	8.2%	−1.2%	6	–
28	J25A to J26Newport	0.9	83,057	9.9%	−2.7%	4	–
29	J26 to J27Newport	1.8	112,356	7.2%	−8.9%	8	–
30	J27 to J28Newport	1.4	114,877	7.6%	+3.7%	7	–
31	J28 to J29Newport	2.1	121,902	7.9%	−5.5%	12	2.7
32	J29 to J30Newport, Cardiff	3.0	79,133	8.5%	+0.4%	19	4.8
33	J30 to J32Cardiff	5.6	93,859	7.8%	+3.1%	15	1.8
34	J32 to J33Cardiff	3.4	113,578	6.6%	+11.0%	22	3.4
35	J33 to J34The Vale of Glamorgan	2.1	108,134	6.7%	−1.5%	13	3.4
36	J34 to J35The Vale of Glamorgan, Rhondda Cynon Taf	6.3	96,324	6.8%	+7.7%	16	1.6
37	J35 to J36Bridgend	3.7	81,083	6.7%	+9.8%	20	4.1
38	J36 to J37Bridgend	5.5	75,078	7.1%	−1.9%	16	2.3
39	J37 to J38Bridgend, Neath Port Talbot	4.0	72,706	7.4%	+0.6%	9	–
40	J38 to J39Neath Port Talbot	0.9	63,858	7.0%	−5.1%	5	–
41	J39 to J40Neath Port Talbot	1.6	69,078	6.5%	−0.2%	3	–
42	J40 to J41Neath Port Talbot	0.9	66,595	7.6%	−4.8%	5	–
43	J41 to J42Neath Port Talbot	3.1	74,166	6.1%	+1.3%	3	–
44	At J42Neath Port Talbot	0.9	59,526	9.2%	+6.6%	3	–
45	J42 to J43Neath Port Talbot	0.6	93,022	5.3%	+5.8%	11	11.3
46	J43 to J44Neath Port Talbot, Swansea	1.7	91,858	5.8%	+2.4%	6	–
47	J44 to J45Swansea	1.9	79,146	5.9%	+6.7%	11	4.1
48	J45 to J46Swansea	1.9	81,881	5.1%	+6.6%	10	3.7
49	J46 to J47Swansea	1.8	78,991	5.6%	+5.9%	6	–
50	J47 to J48Swansea, Carmarthenshire	3.8	57,038	5.9%	+0.1%	8	–
51	J48 to J49Carmarthenshire	2.7	50,421	6.5%	+7.0%	23	9.7

Vehicles a day is the average daily flow in 2025. “Since 2019” compares the same count points. Collisions are those with the M4 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.

M4 traffic by vehicle type

Cars and taxis make up 75.1% of the vehicle miles on the M4. HGVs account for 7.9%, rising to 11.8% on J13 to J14.

Vehicle mix, 2025

Share of vehicle miles on the M4 by vehicle type
Category	Share
Cars and taxis	75.1%
Vans (LGVs)	16.5%
HGVs	7.9%
Buses and coaches	0.3%
Motorcycles	0.3%

Source:DfT,UK Motoring analysis 2000 to 2025 Updated 15 Sept 2026

HGV share of traffic

HGVs as a share of vehicle miles on the M4, by year
Year	HGV share
2000	10.2%
2001	10.0%
2002	9.6%
2003	9.1%
2004	9.3%
2005	9.6%
2006	9.2%
2007	9.2%
2008	9.2%
2009	8.5%
2010	8.7%
2011	8.0%
2012	7.9%
2013	7.9%
2014	7.9%
2015	8.0%
2016	8.1%
2017	8.6%
2018	8.6%
2019	8.6%
2020	11.4%
2021	10.8%
2022	9.1%
2023	8.7%
2024	8.4%
2025	7.9%

Source:DfT,UK Motoring analysis 2000 to 2025 Updated 15 Sept 2026

M4 traffic trend

Traffic on the M4 came to 6.57 billion vehicle miles in 2025. At the count points present in both years, it is down 0.8% on 2019, before the pandemic, and up 0.5% on 2015.

Vehicle miles a year

Like for like, traffic is up 17.6% on 2000.

Vehicle miles on the M4 each year, the whole road as it was that year
Year	Vehicle miles
2000	5.6bn
2001	5.7bn
2002	5.8bn
2003	5.8bn
2004	6.0bn
2005	6.1bn
2006	6.1bn
2007	6.3bn
2008	6.2bn
2009	6.1bn
2010	5.9bn
2011	6.1bn
2012	6.0bn
2013	6.0bn
2014	6.3bn
2015	6.5bn
2016	6.5bn
2017	6.5bn
2018	6.5bn
2019	6.6bn
2020	4.5bn
2021	5.2bn
2022	6.1bn
2023	6.3bn
2024	6.3bn
2025	6.6bn

Source:DfT,UK Motoring analysis 2000 to 2025 Updated 15 Sept 2026

Average daily flow

Average daily flow along the M4, weighted by link length, by year
Year	Vehicles a day
2000	77,895
2001	79,238
2002	81,579
2003	81,549
2004	83,962
2005	84,946
2006	85,979
2007	87,454
2008	86,167
2009	85,029
2010	83,029
2011	84,675
2012	84,153
2013	84,567
2014	87,765
2015	91,153
2016	91,485
2017	91,398
2018	90,179
2019	92,377
2020	63,193
2021	72,116
2022	84,410
2023	87,565
2024	88,176
2025	91,647

Source:DfT,UK Motoring analysis 2000 to 2025 Updated 15 Sept 2026

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, 60 of the 81 count points were counted; DfT estimated the rest from earlier counts and local trends.

M4 collisions

Police recorded 1,152 collisions with the M4 as the first road from 2021 to 2025, 242 of them fatal or serious, including 36 fatal. That is 3.8 per 100 million vehicle miles, lower than the 4.9 for all motorways in Great Britain. On average there were 1.2 a year for each mile of road.

Reported collisions by year

Up 5.9% from 2021 to 2025.

Source:DfT STATS19,UK Motoring analysis 2021 to 2025 Updated 15 Sept 2026

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.

Collisions per 100 million vehicle miles by section of the M4, 2021 to 2025
Section	Per 100m vehicle miles
J3 to J4 Hillingdon · 72 collisions	18.2
J4 to J4B Hillingdon, Slough · 59 collisions	11.9
J42 to J43 Neath Port Talbot · 11 collisions	11.3
J4B to J5 Slough · 50 collisions	9.8
J48 to J49 Carmarthenshire · 23 collisions	9.7
J5 to J6 Slough, Windsor and Maidenhead · 52 collisions	6.3
J20 to J21 South Gloucestershire · 16 collisions	6.2
J2 to J3 Hounslow · 46 collisions	5.3

Source:DfT STATS19,UK Motoring analysis 2021 to 2025 Updated 15 Sept 2026

A further 113 collisions were recorded at junctions where the M4 was the second road; they are not in the figures above. Fatal or serious uses DfT’s adjusted serious figures. 96.7% of the collisions could be placed on a section. How we count collisions

Council areas on the M4

In route order. Each is the highway authority DfT files the count points under.

Hounslow
Hillingdon
Slough
Windsor and Maidenhead
Buckinghamshire
Bracknell Forest
Wokingham
Reading
West Berkshire
Wiltshire
Swindon
South Gloucestershire
Monmouthshire
Newport
Cardiff
The Vale of Glamorgan
Rhondda Cynon Taf
Bridgend
Neath Port Talbot
Swansea
Carmarthenshire

Planned closures on the M4 in England

National Highways has 63 planned closures on the M4 between 16 September 2026 and 30 September 2026, 62 of them starting in that period. 17 close a carriageway rather than a lane or slip road, and 58 start between 7pm and 6am.

Lane or slip Lane 1 closure

16 Sept 08:00 to 17 Sept 15:00 1 of 3 lanes Road maintenance
Lane or slip M4 both directions Jct 15 to 22 - mobile lane closure

16 Sept 20:00 to 30 Sept 06:00 1 of 3 lanes Road maintenance
Lane or slip M4 eastbound Jct 22 to Jct 21 lane closure 195/1-192/1

16 Sept 20:00 to 17 Sept 06:00 1 of 2 lanes Road maintenance
Lane or slip Single lane running

16 Sept 20:00 to 17 Sept 06:00 Road maintenance
Lane or slip M4 eastbound lane 1 closure (99/4 to 95/4)

16 Sept 20:00 to 17 Sept 06:00 1 of 3 lanes Road maintenance
Lane or slip M4 eastbound Jct 23 to Jct 22 Prince of Wales Bridge lane closure 202/1-195/7

16 Sept 21:00 to 17 Sept 06:00 Road maintenance
Lane or slip M4 eastbound Jct 23 to Jct 22 Prince of Wales Bridge lane closure 202/1-195/7

16 Sept 21:00 to 18 Sept 06:00 Road maintenance
Carriageway M4 eastbound Jct 10 exit slip road closure

16 Sept 21:00 to 17 Sept 06:00 Road maintenance

55 more planned closures in this period are not shown.

Planned closures only: National Highways can cancel or change them, and closures caused by incidents are not shown. Council roads in England are not covered. Roadworks and closures across the UK

Source:National Highways 2026-09-16 to 2026-09-30 Updated 16 Sept 2026

Data sources and methodology

Datasets used on this page
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	81 count points in 2025	15 September 2026
Reported collisions	Road safety open data (STATS19) Department for Transport Derived statistics UK Motoring	2021 to 2025	1,152 reported collisions	15 September 2026
Planned closures	Transport Data Feeds (DATEX II) National Highways	2026-09-16 to 2026-09-30	2,304 planned closure schemes	16 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”.

How we calculate these figures All data sources

Contains public sector information licensed under the Open Government Licence v3.0.

How busy is the M4?

Route

M4 traffic and collisions by section

M4 traffic by vehicle type

M4 traffic trend

M4 collisions

Council areas on the M4

Roads that meet the M4

Planned closures on the M4 in England

Data sources and methodology

Related statistics