[citation needed] The early M1 had no speed limits, crash barriers, or lighting, and had soft shoulders rather than hard.

It started at the Watford Bypass (A41), which runs south-east to meet the A1 at Apex corner, and ended on the A5 at Crick.

The M10 spur motorway connected the M1 to the North Orbital Road (A405/A414, a precursor of the M25) where it also met the A5 (now renumbered here as the A5183) and, 2 miles (3.2 km) to the east via the A414, the A6, which subsequently became part of the M25.

A £1.5 million contract was given in May 1958 for the most southerly section, from Aldenham to Beechtrees (the M10 junction), for two lanes of reinforced concrete, to open in November 1959.

Diverging from the A5, the motorway takes a more northerly route through the East Midlands, via Leicester, Loughborough, Nottingham to Sheffield, where the M18 splits from the M1 at junction 32 to head to Doncaster.

The new road involved the construction of a series of new junctions, bridges and viaducts to the east of Leeds.

The second phase continued east to Scratchwood (London Gateway Services, which occupies the location of the missing junction 3, from where an unbuilt spur would have connected to the A1 at Stirling Corner to the north-east).

The M1 then runs south alongside the Midland Main Line towards Hendon, where it meets the A1 again at junction 2 via a tightly curved flyover section.

Before the completion of junction 2, southbound traffic left the motorway via a slip road which passed around the back of the now disused Homebase and under the A41/A1 Mill Hill Bypass, and looped round to join it at Fiveways Interchange.

There the motorway meets the North Circular Road (A406) at a grade separated junction and roundabout.

Recent concerns about accidents and deaths on the former hard shoulder have led to a halt and review into extending all lane running which reported in July 2021.

[33] The upgrade work for this final phase of the plan, the section running from junction 13 to Eagle Farm roundabout, started in September 2018 and was completed in December 2020.

[33] Work began on the 10-mile (16 km) section between the M25 and Luton (J6A-J10) in 2006 and opened in 2009, which included the construction of new parallel collector-distributor lanes between junctions 7 and 8.

The M10 spur was built as a motorway because it inevitably led to the M1, but as non-motorway traffic could now travel between the A414 at Hemel Hempstead and Park Street Roundabout without having to access the M1, the M10 was downgraded to an A road, and designated as part of the A414 to allow for this.

[35] A variable mandatory speed limit system was installed, making this the first smart motorway scheme on the M1.

Work to introduce dynamic hard shoulder running on approximately 15 miles (24 km) of motorway between Luton and Milton Keynes (J10-J13) was completed in December 2012, at a total cost of £327 million.

[39] Work to widen the 15-mile (24 km) section from Nottingham to Mansfield (J25-J28) to four lanes each way began in October 2007 and was completed in May 2010, at a cost of £340 million.

[43][44] National Highways converted the existing 23-mile (37 km) section of the M1 between Milton Keynes and Northampton (J13-J16) into an all-lane-running (ALR) smart motorway consisting of four lanes running in both directions without a hard shoulder, with the project's cost being £373 million.

Construction began in January 2018, with the scheme opening in stages until 9 March 2023, when the project was fully complete.