195002 – Riddle – Road Traffic Signalling with Special Reference to Vehicle Actuation

There are not many fundamental principles which are common alike to Road Signalling and Railway Signalling. Indeed beyond the general Pact that both are employed for preventing two or more vehicles attempting to occupy the same place at the same time, there is very little similarity.

On the other hand many obvious differences present themselves at the outset. The railway signalling scheme can rely upon the trains it controls running upon a pre-set route. The road vehicle cannot (except in rare instances) be directed, but can merely be stopped before crossing a potential collision point. Continuous detection of the presence and passage of a train over a route or through a section is easily provided. The road vehicle can at the best indicate its movement towards a point of intersection by its passage over a, detecting device fixed in its path of approach.

It was perhaps the lightly accepted (even unconscious) assumption that the difficulty of detecting the passage of a road vehicle was insurmountable that led to the use for some years of Road Signalling Systems which took no account of the approach of traffic, but blindly switched the right
of way first to one road and then to the crossing road. The optical arrangement of the Railway and
Road signals is again different: the former calling for accurate focussing of a limited light-source
to form a beam which need be no broader than the deviation of the driver’s eye position from the
optical axis of the signal. The road signal must give a full indication to vehicles over at least half the width of the road, and as seen from the low-seated sports-car, or the high seat of a bus or
truck.

Again the implication of a green signal at a street crossing has some differences from that of
the Railway signal. The latter is a true right-of-way (or as popularly rendered-“right away”) signal; whilst the green signal given to the road user still keeps him very much on the alert for vehicles turning in front of him and for pedestrians crossinq his path and, if he is turning to the right, of taking full responsibility in negotiating the latter part of his movement. This may not
explain why the arrangement has been adopted in Road signalling of putting the green closest to
eye-level in contradiction to Railway practice, but it does serve to bring out the differences of approach in the two fields.