Reproduced courtesy of Francis Frith.Whitehall, looking towards Parliament 1880
The first road traffic signals were in operation in London in December 1868, several decades before the invention of the motor car. Traffic jams were not unusual in London's bustling city streets with horse-drawn carts and drays, hansom cabs, omnibuses, and pedestrians clogging the city streets. A journey through London was also complicated by the many road-works for a new sewer system and the overground and underground railways.
As an experiment to assess their effectiveness in dealing with London's traffic, traffic signals (or street semaphores as some called them) were erected just outside the Houses of Parliament at the junctions of Bridge Street, Abingdon Street and Parliament Street. The traffic signal design was proposed by John Peake Knight, an engineer and manager of the London Brighton and South-Coast Railway and based on the signals used on the railway. They were constructed by the firm of Messrs. Farmer and Saxby. Permission to conduct the experiment was authorised by the Home Office and Sir Richard Mayne, the Police Commissioner.
Four policeman already controlled traffic at the junction, but their services were not to be dispensed with until the experiment was over or deemed a success. An additional officer was required to operate the signals. The signals were mounted on an octagonal pillar about 22 feet in height. Lights of red and green were lit by a lamp fed by a gas pipe, and there were three red semaphore arms. The officer by means of a handle, moved the arms up and down, and changed the colour of the lights. The semaphore arm for day-time use, the lights for night-time.
Despite some advance warnings and the distribution of leaflets, one newspaper reported that most drivers ignored the signals particularly when the lights and arms were raised at caution, either through ignorance of the system used in railway signalling, or intent on only following directions of one of the officers who found themselves still having to direct the traffic. Other observers, however, thought it was a success. 
It became a short-lived experiment, for shortly after there was a gas leak and the lamp exploded seriously injuring the policeman. The experiment was not repeated again.
'The last gas explosion that have heard of has been at the street signal-post in Westminster, at the intersection of Great George-street and Bridge-street with Parliament-street. Might have been supposed, naturally enough, that the explosion arose from imperfect construction and arrangements of tbs pillar itself, but that this was the cause there is reason to believe. The roadway all round the pillar for some time had exhaled gas and seemed to be saturated with it. The hollow pillar, at the top of which the new semaphore street signal is worked could sot absorb, but it could receive it a reservoir.When the constable on duty opened the door to turn off the gas from the signal pillar alarming explosion immediately ensued, which it may be supposed arose from the admission of the atmospheric air, and the consequent ignition of the gas accumulated from leakage. The constable's face was badly burned, His helmet was dashed off, and he was partially stripped. The report of the explosion, we are told, was heard Wnitehall, and at the end of Birdcage-walk.
Illustrated Weekly News - Saturday 09 January 1869 on the British Newspaper Archive
 The Morning Post (London), Tuesday, December 15, 1868
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