Between junctions 3 and 4, the beginnings of slip roads were built on both carriageways for a service area at Abbey Barns between Beaconsfield and High Wycombe.

Late in the 1960s, not long after the first stretch opened, the Ministry of Transport announced the possibility of building a new motorway to link London with Birmingham as an alternative to the M1–M6 route – as well as improving road links to the South Coast ports for The Midlands – but it was not until 1983 that the decision to extend the M40 from Oxford to the south of Birmingham was made.

The preferred route was altered to avoid Otmoor after a vigorous road protest, which included selling over 3,000 small squares of a field to people all over the world.

Beginning in 1997, the motorway was widened to dual-four-lane between J1A and J3 (High Wycombe East) under a Private Finance Initiative.

[5] In 2009 the Highways Agency extended the Active Traffic Management (ATM) system onto the northbound carriageway from J16 to the junction with the M42.

In August 2010 work started on J9, upgrading the southbound exit slip road to three lanes, and similar widening on the connecting A34 and A41 junctions between Oxford and Bicester.

Population figures are based on the 2001 census by the Office for National Statistics[7] Download coordinates as: Data[8] from driver location signs are used to provide distance and carriageway identifier information.

The motorway carries on for another 1⁄2 mile (0.80 km) before it reaches junction 1a, the free-flow interchange with the M25 London Orbital.

It is a partially unrolled cloverleaf, with the smoothest turns from the London-bound M40 (from Oxfordshire, Warwickshire and the wider West Midlands) to the anti-clockwise M25 (London Heathrow, Gatwick, The Channel Ports) and vice versa, since this is the largest exchange of traffic between the two motorways.

The junction used to be a straightforward roundabout interchange with exits for the M40 (west and east), High Wycombe (A404), the A4010, two local roads and the A404 dual carriageway to the south.

During 2007, work was completed which included extra stacking space on the sliproads from the M40, provision for traffic from the A404 northbound to join the M40 westbound slip road without joining the roundabout and provision for the London-bound M40 to skip the section of the roundabout which serves the A4010, High Wycombe, and the A404 north.

Within a mile there is a large road cutting known as the Stokenchurch Gap or The Canyon where the motorway enters Oxfordshire and meets junction 6 with the B4009 for Lewknor, Watlington, and Chinnor.

The A41 dual carriageway serves Bicester and Aylesbury, and both roads meet the motorway at Wendelbury roundabout junction.

This means the A34 is now technically in two halves (it regains status farther up the road at junction 16, although signs on the motorway do not mention this).

The M40 follows a course of almost due north for 5 miles (8.0 km) before reaching junction 10, for the Cherwell Valley services, the A43 and the village of Ardley.

When the A43 (between the M1 and M40) was upgraded to dual carriageway, the junction was redesigned and rebuilt by the Highways Agency to cope with the extra traffic.

The design and execution of the revised design of new junction is greatly derided, mostly because of the three roundabouts giving no priority to the main flow of traffic, (A43-M40 London), and the slip roads off and onto the motorway (except the one accessed via the services) have sharp turns and adverse cambers, which results in lorries frequently tipping over and spilling their loads especially on the roundabout at the end of the northern carriageway.

[citation needed] The motorway then follows a winding route north for 10 miles (16 km) until junction 11, the A422 and A361, serving Banbury.

The motorway does not follow the straight route to the east of Middleton Cheney, meeting with the A422, as it had once been planned, due to a major landowner refusing his land to be cut in two.

Another 12 miles (19 km) north-west along the motorway is junction 12, serving Gaydon and the Heritage Motor Centre via the B4451.

The slip roads join at a roundabout and carry on as the single carriageway A452 to meet with the A452 to Leamington Spa, A425 to Warwick, and the A452 to J13.

This is a large, regular roundabout interchange, and is always busy during peak times due to the various destinations it serves, including The Cotswolds, Stratford-upon-Avon, Coventry, and Warwick.

Pioneer UK is in Stoke Poges and the Barracuda Group (owner of Varsity) is in Marlow, both locations being near the motorway.