How Busy Is M4 Junction 4b to 5?
West of the M25 interchange at Junction 4b, the M4 carries traffic that has just joined from the London orbital as well as everything heading out of London towards Slough, Reading and the west.
In the Department for Transport’s 2025 traffic counts it carried 140,828 vehicles a day, making it the second busiest of the 47 stretches between numbered junctions on the M4 that we could compare.
Live traffic speed flow: M4 Junctions 3 to 6
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Latest M4 traffic news
Live traffic news at M4 Junctions 4B and 5
This is the news for Junctions 4B and 5 only, not the whole M4. For the rest of the motorway see all the latest M4 traffic news, the overhead signs and the speed limits in force now.
Updated 02:57 (2:57am) Sun 1 Nov 2026
For everything currently reported on the M4, see our latest M4 traffic news, M4 roadworks and M4 accidents.
Traffic on M4 Junction 4b to 5
140,828 vehicles a day works out at an average of about 5,868 an hour, or roughly 98 a minute, around the clock. That average takes in quiet nights as well as busy rush hours, so at peak times the flow is much heavier.
Split by direction, that count point measured 72,340 vehicles a day eastbound and 68,488 westbound.
Cars and taxis make up about 78% of the traffic. Vans account for around 15%, about 21,553 a day, and lorries for 8,712 a day, 6.2% of everything on the road.
The junctions at each end
Junction 4b serves the M25 interchange. Junction 5 (A4) serves Langley, east Slough and Datchet. Every junction on the motorway, with the service areas in between, is on our map of the M4 junctions, and the places to stop are listed on M4 services.
How it compares with the rest of the M4
The busiest stretches of the M4 are almost all at the London end, between Hayes and Slough, where airport, London and M25 traffic meet. The table shows the ten busiest.
| Rank | M4 stretch | Vehicles per day | Lorries |
|---|---|---|---|
| 1 | J4 to J4b | 156,491 | 6,249 |
| 2 | J4b to J5 | 140,828 | 8,712 |
| 3 | J5 to J6 | 138,753 | 8,436 |
| 4 | J3 to J4 | 136,321 | 5,243 |
| 5 | J19 to J20 | 128,594 | 11,167 |
| 6 | J6 to J7 | 127,570 | 7,577 |
| 7 | J7 to J8/9 | 126,197 | 7,861 |
| 8 | J10 to J11 | 121,922 | 8,549 |
| 9 | J28 to J29 | 121,902 | 9,572 |
| 10 | J25 to J25a | 117,507 | 9,599 |
Traffic figures are annual average daily flows, both directions together, from the Department for Transport road traffic statistics for 2025. On the M4 many stretches are split into more than one count point; where a stretch has more than one, the busiest measured point is shown. The ranking covers 47 stretches between numbered junctions; slip roads, spurs and the complex junctions 41 to 43 are left out. Hourly and per-minute figures are simple averages across the whole day and night. Contains public sector information licensed under the Open Government Licence v3.0.