The A405 is a 4.8 miles (7.7 km) dual carriageway road in Hertfordshire, England, from the A41 at Leavesden Green, near Watford, to the A414 at Park Street Roundabout near St Albans.

From the A412, the A405 travels north, past West Hertfordshire Crematorium and Penfold Park Golf Course, to junction 6 of the M1 at Waterdale (2.3 miles, 3.7 km).

Drivers wishing to access the M25 from the M1 northbound, or the M1 southbound from the M25, must use this section of the A405 as the junction between the two motorways, known as Chiswell Interchange, does not feature the corresponding slip roads.

From Park Street Roundabout, it travelled east, crossing the River Ver and the Midland Main Line before meeting the A6 (now the A1081) at London Colney.

The North Orbital Extension was given the go-ahead in January 1973, from Maple Cross to Hunton Bridge; it was 6.2 miles, and was to cost £6.5m.