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Control of Traffic by Light Signals

    Naturevolume 134pages806807 (1934) | Download Citation



    THE earliest light signals for controlling traffic were operated manually by the police. The next type were operated on a ‘fixed-time’ basis by purely mechanical devices. A later development is to link up a number of local fixed-time controllers with a master controller so that the indications all along a main traffic route bear a definite relationship to one another, with the object of maintaining a steady flow of traffic. An important example of this type is installed in Oxford Street, London. The latest and most popular types of signals are those which are operated by the passing vehicles themselves. There are two systems of this type in general use, and both use a detector mat placed in the roadway. In one system, the compression of the mat actuates an electro-pneumatic contact box placed in the pave ment and only responds when the vehicle is approach ing the crossing. A vehicle ‘parking’ on the mat has no effect on the signal. Suspended signals, which are fairly common abroad, divert the attention of the driver from the road level, and the drivers of modern saloon cars have difficulty in seeing them. The British signals are sometimes criticised because,unlike the ‘clock-face’ or ‘Chinese-lantern’ type of signal used frequently in Switzerland and Germany, they give no indication of the length of time that the red or green indication has to remain. They are objectionable as they induce many drivers to ac celerate to high speeds in order to pass the signals before they change. The uncertainty of the time of change in the usual type forces the driver to go slowly and this leads to safety. It is probable that vehicle-actuated signals will come into general use in England, and this would make ‘clock-face’ signals with their fixed time sequence of events practically impossible. An illustrated article on “Traffic Light Control Systems” appears in Electrical Industries of November 15.

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