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A1(M)/South Mimms - Baldock

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South Mimms - Baldock
From:  South Mimms (TL224001)
To:  Baldock (TL233364)
Distance:  24 miles (38.6 km)
Meets:  M25, A1, A414, A505, A507, A602, A1000, A1001, A1081, A6129, B195, B197, B656
Former Number(s):  A1
Primary Destinations
Highway Authorities

National Highways


Hertfordshire • Middlesex

Route outline (key)
Arrownorth.jpg North Baldock - Alconbury
A1(M) South Mimms - Baldock
South Arrowsouth.jpg Central London - South Mimms
Junction List
Junc Northbound Southbound
1 Start of Motorway A1;M25, A1081
Services symbol.gif South Mimms services (at J1)
2 A1001 No Exit
3 A414 A414, A1001
Tunnel Hatfield Tunnel
4 A414 A414
Bridge Stanborough Bridges
5 No Exit No Exit
6 A1000 A1000
7 A602 A602
8 A602 A602
9 A505 A505
10 A1; A507 Start of Motorway
Services symbol.gif Baldock services (at J10)



The A1(M) starts off underneath Junction 23 of the M25 as a continuation of the A1. The northbound A1 is D3 to the south of the M25, with a lane drop at the interchange. The A1(M) heads north as D2M for four miles to Hatfield, Junction 2. Approximately one mile further north, it becomes, at Junction 3 (the A414 west), a D3M and tunnels underneath 'The Galleria', a shopping mall. The tunnel is approximately 1,237 yards long and was upgraded in 2011 to include disabled egress in case of emergencies.

Roughly ¾ mile from the tunnel's northern portal, Junction 4 is encountered, the junction with the A414 eastbound and the A6129 to Welwyn Garden City. The road now passes between the village of Stanborough on the west side and Welwyn Garden City on the east side, and as it does so, it starts to climb up a relatively steep hill. Cresting the hill, it drops down to pass on the eastern side of Welwyn village and Junction 6.

This junction is a spread out affair. Northbound, from the slip road exit to the slip road entry is a touch over a mile! When first built, there was a lane drop halfway between the exit/entry slip roads. In 2015, the lane drop was moved to the northbound exit slip. The northbound on-slip joins the D2M main line at the bottom of another hill, and while moving the lanedrop has helped traffic flow at the exit, the climb up the hill remains congested in peak periods.

Underneath the motorway at junction 6 are the remains of a Roman bath house, and to preserve it, a dome had to be built over the baths with the motorway passing over the top.

The road now climbs again quite steeply, before descending down to pass on the west side of Knebworth village. Knebworth House is on the western side. Junction 7 is now encountered at the southern edge of Stevenage and the A1(M) now swings round northwestwards, then northeastwards, to skirt the town, and at the northern end of the town is Junction 8. The main road here (other than the A1(M) itself, of course) is the A602 heading westwards to Hitchin and Luton, and in the opposite direction, heading back into Stevenage. There are also a couple of minor roads leading to small villages either side of the A1(M).

Just after Junction 8, the A1(M) yet again becomes a D3M for a little over 2 miles until it reaches Junction 9, the A505 east and west. It now gently weaves its way between Letchworth (on the west side) and Baldock (on the east side). 1.6 miles after passing underneath the Cambridge to London Kings Cross railway line, Junction 10 is encountered and the motorway now ends, continuing northwards as a D2.


Early improvements underway in the 1950s
Main Article: A1 Early Improvements

A series of bypasses where constructed through the 1950s and 1960s around the new suburban commuter areas of Middlesex and several market towns in the Midlands. These schemes massively reduced travel times on the Great North Road and provided vital relief for traffic-choked towns.

1962: Stevenage Bypass

Junctions 6 to 8 - the 7.5 mile initial section was opened on 26 July 1962 by Ernest Marples, Minister of Transport. Cost £2million.

1967: Baldock Bypass

Junctions 8 to 9 Letchworth Gate opened on 15 June 1967.

Junctions 9 to 10 Stotfold opened on 23 August 1967. This joined the A1 dual carriageway section northwards


1979: Hatfield-South Mimms

More details required.

1986: Hatfield Bypass

More details required.

Notable Locations


Hatfield is the birthplace of the de-Haviland Comet aircraft. Now home to the University of Hertfordshire. Hatfield House, a large house and gardens often hosting concerts and the like.

Main Article: Hatfield

Hatfield also has a shopping mall above the A1(M), the Galleria.

Main Article: Hatfield Tunnel


Stevenage is 50s/60s new town. Plenty of green spaces and a boating lake. The first Inter-City station out of Kings Cross on the ECML.

Main Article: Stevenage


National Highways state that the section between junctions 5 and 9 is in the top ten busiest sections of the A1 route between London and Leeds (east) and is therefore proposing to make the existing two-lane section between The Clock and Coreys Mill Interchange into a three-lane smart motorway, increasing capacity by around 50%. This would make the A1(M) three lanes in each direction from the A414 Roehyde interchange to the A505 Letchworth Gate interchange. In addition to the A1(M)'s role as a major north-south motorway, the A405-A414-A1(M)-A505 corridor provides an alternative from the north-western section of the M25 to the M11 and onto Cambridge.

In August 2016 a strategic report on the route was published. In March 2020 the project was paused to allow National Highways to focus time and resources on implementing the actions from their review of existing smart motorways.


Roads UK

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Welwyn Garden City
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A20 • A21 • A22 • A23 • A24 • A25 • A26 • A27 • A28 • A29 • A30 • A31 • A32 • A33 • A34 • A35 • A36 • A37 • A38 • A39
A40 • A41 • A42 • A43 • A44 • A45 • A46 • A47 • A48 • A49 • A50 • A51 • A52 • A53 • A54 • A55 • A56 • A57 • A58 • A59
A60 • A61 • A62 • A63 • A64 • A65 • A66 • A67 • A68 • A69 • A70 • A71 • A72 • A73 • A74 • A75 • A76 • A77 • A78 • A79
A80 • A81 • A82 • A83 • A84 • A85 • A86 • A87 • A88 • A89 • A90 • A91 • A92 • A93 • A94 • A95 • A96 • A97 • A98 • A99
Motorway sectionsA1(M): (South Mimms - Baldock • Alconbury - Peterborough • Doncaster Bypass • Darrington - Birtley)
A3(M) • A8(M) Baillieston spur • A38(M) • A48(M) Cardiff spur • A57(M) • A58(M) • A64(M) • A66(M) • A74(M) • A92(M)
DefunctA1(M) Newcastle CME • A2(M) Medway Towns Bypass • A4(M) • A5(M) • A8(M) Renfrew bypass • A14 • A14(M) • A18(M) • A20(M) • A36(M)
A40(M): (Westway • Denham -Stokenchurch) • A41(M) • A42 • A46(M) • A48(M): (Port Talbot bypass • Morriston bypass) • A62(M) • A88 • A99
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