The route of the A1 in London runs from the northern end of St. Martin's Le Grand in the City to Borehamwood in Hertfordshire, then travels on the northern fringe of Greater London to Bignell's Corner, where it crosses the M25 and becomes a motorway, designated A1(M), which alternates with the dual carriageway A1 as it continues to Edinburgh.
The London section of the road passes through part of the City of London and three London boroughs: Islington, Haringey and Barnet.
The A1 is one of London's main northern routes, providing a link to the M1 motorway and the A1(M) motorway, and on to the Midlands, Northern England and Scotland.
It connects a number of major areas within London, and sections of it serve as the High Street for many of the now-joined villages that make up north London.
Even though it is one of London's major roads, less than 5% of its approximate 60,000 vehicles a day are through traffic – the bulk is local.

The current start of the A1 is the modern roundabout at the northern end of St. Martin's Le Grand where it meets Aldersgate Street, near the site of the now demolished General Post Office, London, the headquarters of the Post Office from 1829 to 1910.
When originally designated in 1921 the A1 started a little further north along Aldersgate Street at Aldersgate Bars, which marked the boundary of the City, though some later maps indicate it starting at the southern end of St. Martin's Le Grand, near St Paul's Cathedral.
 The route runs north from Aldersgate along Aldersgate Street which is a modern dual carriageway, and from the 13th century was known as a wide street with fine buildings and travellers' inns; these were destroyed or badly damaged during the Blitz, and from 1965 to 1976 the 40 acre (162,000m²) Barbican Estate, an arts complex and residential estate, was constructed along the entire eastern side of the street.
At the end of Aldersgate Street stood Aldersgate Bars, which marked the limits of the City of London.
The route of the A1 in London runs from the northern end of St. Martin's Le Grand in the City to Borehamwood in Hertfordshire, then travels on the northern fringe of Greater London to Bignell's Corner, where it crosses the M25 and becomes a motorway, designated A1(M), which alternates with the dual carriageway A1 as it continues to Edinburgh. The London section of the road passes through part of the City of London and three London boroughs: Islington, Haringey and Barnet. The A1 is one of London's main northern routes, providing a link to the M1 motorway and the A1(M) motorway, and on to the Midlands, Northern England and Scotland. It connects a number of major areas within London, and sections of it serve as the High Street for many of the now-joined villages that make up north London. Even though it is one of London's major roads, less than 5% of its approximate 60,000 vehicles a day are through traffic – the bulk is local.
The current start of the A1 is the modern roundabout at the northern end of St. Martin's Le Grand where it meets Aldersgate Street, near the site of the now demolished General Post Office, London, the headquarters of the Post Office from 1829 to 1910. When originally designated in 1921 the A1 started a little further north along Aldersgate Street at Aldersgate Bars, which marked the boundary of the City, though some later maps indicate it starting at the southern end of St. Martin's Le Grand, near St Paul's Cathedral.  The route runs north from Aldersgate along Aldersgate Street which is a modern dual carriageway, and from the 13th century was known as a wide street with fine buildings and travellers' inns; these were destroyed or badly damaged during the Blitz, and from 1965 to 1976 the 40 acre (162,000m²) Barbican Estate, an arts complex and residential estate, was constructed along the entire eastern side of the street. At the end of Aldersgate Street stood Aldersgate Bars, which marked the limits of the City of London.
A1 in London