The perception of light signals : the effect of mixing flashing and steady irrelevant lights.

A previous experiment (Research Note No. RN/3609) has shown the danger of increasing the number of irrelevant lights in a drivers field of view and that, given a certain number of irrelevant lights, the signal most easily seen is one which flashes while the irrelevant lights are steady. However, a flashing light which may be an important signal to one driver may be irrelevant to others in the vicinity. When driving it is in fact common to find a relevant signal light, flashing or steady, beside other lights irrelevant to oneself, some of which flash while others are steady. A second experiment with the same general conditions as before has therefore been carried out to find the effect of a mixture of flashing and steady irrelevant lights as a background to an essential signal. In the tests the signal light was always amber in colour: the irrelevant lights were ten in number, 5 green and 5 red. These irrelevant lights changed their positions at the same average rate as the signals appeared; they were either `steady', 'nine steady and one flashing', `eight steady and two flashing' and so on to `all flashing'. Both steady and flashing signals were used and so a total of 22 test trials, each consisting of the responses to 100 signals, were required. These were given to each of four subjects in a balanced sequence. It was found that the advantage gained by the use of a flashing light so a signal was lost if even one other light in the background was flashing. It was a definite disadvantage to have the signal flashing if three of the ten irrelevant lights flashed and when the number flashing was more ten four, the ability to perceive flashing signals was seriously impaired.From the form of the curve of mean time of response to signal plotted against number of similar irrelevant lights (i.e. flashing if signal is flashing, etc.) it is concluded that when there was a choice between recognising a signal by its colour or by its flashing, the flashing aspects is used. These results are of importance in any consideration of visual perception.