This paper describes the results of laboratory experiments in order to determine the just-noticeable deviations in the grid structure of matrix signs. The deviation at which 75% correct (50% in practice) was scored, was defined as the just-noticeable deviation (jnd). The maximum allowable deviation, invisible for 85% of the road users, for white, red, and green lightpoints, is 0.0811.R(super 1.71).d(sub ave)(super -0.71) mm. "D(sub ave)" is used for the average lightpoint distance in mm. "R" stands for the viewing distance in mm. No difference was found between lines and areas. Similar results are predicted for curved lines and other grid types. The jnd for blue lightpoints was a factor 2.2 larger. There was no difference between the presentation times of 1 and 5 seconds. The jnd for a luminance ratio of 1:10 was 20% larger than for a luminance ratio of 1:560. No difference was found for deviations at the edge or in the middle of areas. There was no difference between crosswise and lengthwise orientation of the lightpoint displacement in lines. No evidence was found for a lightpoint size effect on legibility. The paper also presents a practical design example of a matrix sign for signalling the maximum speed limits of 50 and 70 km/h on motorways.
Abstract