Chalfont Viaduct seen from the M25 (in 2009)
|OS grid reference|
|Carries||Chiltern Main Line|
|Official name||Chalfont No. 1 Viaduct|
|Other name(s)||Misbourne Viaduct|
|Named for||The Chalfonts (Chalfont St Giles, Chalfont St Peter, Little Chalfont)|
|Material||Blue/black engineering brick|
|Total length||104 metres (114 yd)|
|Height||12.5 metres (41 ft)|
|No. of spans||5, each 15.5 metres (51 ft) wide|
|No. of tracks||2|
|Track gauge||4 ft 8 1⁄2 in (1,435 mm) standard gauge|
|Designer||James Charles Inglis, R.C. Sykes|
The Chalfont Viaduct (also known as the Misbourne Viaduct) is the first of two five-arch brick railway viaducts on the Chiltern Main Line in south-east England. Located between Gerrards Cross and Denham Golf Club stations, it spans the M25 motorway between junctions 16 and 17 at Gerrards Cross near Chalfont St Peter, from where the bridge gets its name. The bridge is known as Chalfont No. 1 Viaduct; the longer Chalfont No. 2 Viaduct is a short distance to the west and spans the A413. 
The Chalfont Viaduct is built of blue and black engineering brick with additional decorative brickwork. The bridge is approximately 12.5 metres (41 ft) high, although it varies in height due to changing ground level, and it has five semi-elliptical arches, each 15.5 metres (51 ft) wide. It was constructed between 1902 and 1906 by the Great Western Railway (GWR) to carry trains on the Great Western and Great Central Joint Railway between London and High Wycombe across the River Misbourne. It was designed by James Charles Inglis, chief civil engineer of the GWR, and assistant engineer Robert Cherry Sikes.
In the mid-1980s the construction of the M25 motorway required the Misbourne to be diverted via underground concrete culverts. The route of the motorway was aligned to pass through the arches of the Chalfont Viaduct, leaving the viaduct largely unaltered apart from the raised ground level and the addition of concrete supports and crash barriers. The viaduct is the only brick-built bridge on this section of the M25.
For many years the Chalfont Viaduct has borne a graffiti slogan, "give peas a chance" painted in large white letters on the south-facing parapet, visible to M25 drivers heading in a clockwise direction. The north side of the bridge bears the painted legend "peas06".
Originally, the graffiti on the south parapet simply read "peas". It is thought that "Peas" was the tag of a London graffiti artist which can be seen on other locations. The words "give" and "a chance" were added later. The altered slogan may refer to the artist being repeatedly arrested, and it may also be a play on words in reference to John Lennon's popular 1969 song, "Give Peace a Chance".
The graffiti has become a popular local landmark and is regarded affectionately by motorists as a reassuring location marker. In 2018, when the graffiti was partially removed and repainted in 2018 to read give helch a break!!!, it prompted media commentary.
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