IN 1868 the City of Westminster introduced a method of mechanical signalling to help the police to control the traffic. A semaphore, having a red and green gas lamp for night use, was employed, but unfortunately an explosion put an abrupt end to this experiment. Early in this century, road signals similar to railway signals were used for controlling a few tramways and also the traffic on Tower Bridge. So back as 1918, colour light signals were used to control street traffic in New York. The Siemens and General Electric Railway Signal Co. (S.G.E.) installed the first modern British traffic signal at a busy road junction in Wolverhampton in 1926. The most recent development of the vehicle-actuated signals is the ‘Autoflex’ system of the S.G.E., a full description of which is given in the Engineering Supplement of the Siemens Magazine for April. It was first brought into use in November 1933 and there are now several installations giving very satisfactory service. In this system vehicles approachmg a road junction pass over pneumatic detector mats, installed in the paths of the various traffic streams, and so notify their movements to an electrically operated controller. The mats are equivalent to the eyes and ears of a traffic policeman. If vehicles leave the intersection on the wrong side of the road the mats are insensitive. The top of the mat is rounded and projects slightly above the road level presenting a good striking face, so that it is not possible for high-speed vehicles or caterpillar tractors to ride over it without registering. If no suitable gap occurs within a predetermined time, the continuous stream is arbitrarily interrupted and the right of way transferred. There is no necessity for long ‘amber’ periods since signal changes take place only when the intersection is clear; two or three seconds are generally sufficient. The power required for a controller is only about 30 watts, which is less than that required by an ordinary lamp.