Colnbrook Bypass, A4, Harmondsworth

Date opened/built:



Possibly 540 yards (493m); period source said 600 yards.



Adjoining footway:


Road type:

Rural arterial road.



Both sides of road:

Unknown, but probably not.

Adjacent to social housing:






Period mapping:

OS 1:25,000, 1948 Shows widened road to the west of “MoT Experimental Station.” OS 1: 10,000 surveyed/revised 1930 to 1959, published 1960 Shows experimental stretch of the bypass, to the east of Accommodation Lane. Hatching may indicate period cycle tracks and/or footway.

OpenCycleMap status:

Period maps, newspaper reports, period photographs, Google Earth 1945 aerial.

The period cycle track has disappeared but it’s perhaps instructive that a modern cycleway ends half way along Colnbrook Bypass with an “end of route” sign.

The Colnbrook bypass, an extension of the A4 Great West Road and to the northwest of the future Heathrow airport, was built in 1928 as a “long speed-track from Chiswick to Slough.”

For eight years, these speeds were somewhat curtailed by the introduction of sharp corners designed to deflect motorists on to an experimental 600-yard stretch of road. (Using Google Earth’s 1945 aerial layer I measured the long-gone section to be 540 yards, or 493m.)

“Motorists using the Great West road from today onwards are to be called upon to collaborate with the Ministry of Transport in an effort to discover the ideal method of road construction,” revealed the Daily Mirror in 1929.

“A section of road 20 ft. wide and 600 yards in length has been constructed alongside, and linked up with, the Colnbrook by-pass at the Harmondsworth end. This experimental stretch is composed of many sections of concrete, each of a different specification. Private motorists and lorry drivers will … drive over the new patchwork surface … ascertaining what is the best specification for a concrete foundation for a road.”

The section of road chosen for this test was just metres from the Ministry of Transport Experimental Station at Harmondsworth, forerunner of the Road Research Laboratory, today’s Transport Research Laboratory, and was paid for “wholly at the cost of the Road Fund,” stated the transport minister.

MoT engineers finished their tests on the experimental stretch in September 1937, finding that “concrete surfacing, if carefully laid, can be much thinner than has hitherto been thought possible.”

Three years later, a newspaper report revealed that the Road Research Laboratory also tested cycle tracks somewhere on the Colnbrook bypass where it was found “stabilised soil” could be used for “surfacing for cycle tracks and footpaths, of which the most promising appeared to be cycle track construction.”

(The Road Research Laboratory also used differing slabs of concrete on an experimental post-war cycle track in East Lothian, see LONGNIDDRY, A198.)

As is evident from comparing historic aerial views in Google Earth the Colnbrook bypass has been much reconfigured during its lifetime and nothing remains of the road’s 1930s cycle tracks and nor is it possible to work out where they may have been. It’s perhaps instructive that a modern cycleway ends half way along Colnbrook Bypass with an “end of route” sign.


The Colnbrook bypass, an ... “NEW ROAD TO ASCOT.—The new Colnbrook by-pass ... is to be opened on Tuesday for the Ascot traffic, and will shorten the run by ten minutes.” Daily Mirror, 16 June 1928. “The new road which links the Great West Road with the Colnbrook By- Pass, making one long speed-track from Chiswick to Slough, will have cost, when all the bills have been settled, not less than three-quarters a million. One interesting fact about this road dovelopment is the enhanced value which has been given to adjacent land. Fields and meadows, acquiring building site value, have gone up ten, twenty, thirty and even a hundred-fold in price. An exhaustive inquiry proved that a tax on these increased site values, created with the expenditure of public money, would far to pay for the cost of the construction of the new roads.” Dundee Evening Telegraph, 3 August 1932

For eight years, these speeds ... It would have saved many lives if more stretches of the Colnbrook bypass had been fitted with such speed reducing infrastructure — the road was a notorious “blackspot” with frequent records of fatalities due to speeding and poor driving. Here’s just one, and probably on a stretch without a cycle track:

24 HOURS WITHOUT SLEEP THEN DROVE “ENGINE OF WAR.” Falling asleep while driving home at 6 a.m. from a dance, Lieut. I. W. Gore- Langton, of the Coldstream Guards, Windsor, ran into two cyclists on the Colnbrook bypass, killing one and injuring the other. At Uxbridge Police Court, when he was fined 1:10 and disqualified for six months, he said:— “ I remember turning the bend and was not feeling sleepy. I suddenly awoke with an awful start. “1 had been up all night, and had had no sleep since the night before.” The Chairman—You had gone 24 hours without sleep, and then you took an engine of war on to the road. Going to sleep is no answer. You ought not to have been there. Belfast Telegraph, 21 December 1937

Private motorists and lorry ... Daily Mirror, 11 December 1929.

The section of road chosen ... “Since the appointment of a Technical Advisory Committee on experimental work in 1929 about half a mile of various forms of concrete have been laid on the road attached to the Department’s Experimental Station at Harmondsworth, wholly at the cost of the Road Fund.” HERBERT MORRISON, minister of transport, House of Commons, written answer, 18th March, 1931.

MoT engineers finished their tests ... GREATEST ROAD TEST ENDING WITHIN the next few seeks the most exhaustive test ever carried out by the Ministry of Transport will come to an end. For seven years, traffic on the Colnbrook bypass, near Slough, has been diverted to a stretch of road 600 yards long and 20 feet wide, paved with a special concrete surface [six inches thick, thinner than usual]. Soon the paving will taken up and inspected for wear. “Results of the experiment,” an official told the “Daily Herald” yesterday, “may lead to revolution in road-making. The Ministry’s engineers are finding that concrete surfacing, if carefully laid, can be much thinner than has hitherto been thought possible,” The closing the experimental strip will remove two sharp bends, which have formed one of the biggest danger spots on the London-Slough road. Daily Herald, 01 September 1937

Three years later, a newspaper ... Western Mail, 22 January 1940 It’s perhaps instructive ...

Explore the tracks