Dartford - Leatherhead
The relationship with the A282 is clear as the first junction after the Queen Elizabeth II bridge is J1. The all-purpose route runs as far as junction 2 with the A2, whereupon the M25 begins. There is a four-lane stretch to J3 with the A20 / M20 at Swanley, after which the motorway runs through the North Downs. J4 is for the A21, which multiplexes with this section of the M25.
Junction 5 is the infamous Chevening Interchange, which is one of the last-minute bodge jobs created when two separate motorway schemes were fused together to make the modern M25. Traffic for the M25 has to turn off to stay on, while going straight on leads to the A21 towards Sevenoaks and Hastings. The M25 merges in at this junction, though there is no access.
The M25 now heads west, now shadowing the route of the A25. This section is now four lanes, but without hard shoulders, and is the only section of high-quality dual carriageway in the area between London and the South Coast. The next junction is with Clacket Lane services, followed by junction 6 for the A22 towards Eastbourne. This is one of three junctions in quick succession; the next two are a fully grade separated four level stack (a rare instance in the UK) with the M23 at J7, followed by a lengthy sliproad up Reigate Hill to meet the A217 at J8. Still on four lanes, the next junction, 9, is a rather convoluted affair with various roads around Leatherhead, including the A243 and A24, with sliproads spaced up to half a mile apart.
Leatherhead - Watford
After the M25 passes Cobham services, it turns northbound following junction 10 with the A3. This section can be one of the most congested in the country, as it meets local roads around Chertsey at J11, the M3 at J12, the A30 at J13 and the A3113 branch to Heathrow Airport at J14. By this point, the M25 is now six lanes wide as it runs west of Heathrow, before meeting the M4 at J14 and the M40 at J15. The motorway then passes under the Great Western & Great Central Joint railway viaduct (constructed some 70 years before the M25) as it heads towards Rickmansworth. This next section, north of junction 17, is one of the oldest sections of the M25, and originally opened as part of the A405. The old road branches off to Hunton Bridge Roundabout at junction 19, while the M25 turns north to bypass Watford. This is the newest section of the motorway, opened in 1986 by Thatcher, and another bodge job to link two disjoint motorway plans.
Watford - Thurrock
Traffic at junction 21 with the M1 has no southbound access, which involves a convoluted route via the A405 at the next junction, 21a. At junction 23 with the A1(M) at Bignells Corner, the M25 runs eastwards towards Potters Bar. There are two cut-and-cover tunnels, first near junction 25 with the A10 at Waltham Cross, then underneath Bell Common (where the original A11, now the B1393, runs on top). The next junction, 27, is with the M11 and is a fully grade-separated whirlpool junction. The M25's course now changes from east to south, as it meets the A12 at junction 28, a frequently congested roundabout, followed by the A13 at junction 30 by Thurrock. Motorway regulations end here, as the final junction, 31, by Thurrock Services, is part of the A282 as it approaches the Queen Elizabeth II Bridge. And this is where we came in, 117 miles earlier....