Legibility of LED based variable message signs
af Kai Sørensen
This report describes a number of tests of the legibility of variable message traffic signs that were carried out in a period from the spring of 2008 until the autumn of 2009. A variable message traffic sign is called VMS in the following.
Persons involved in the planning and execution of the tests include Lene Herrstedt (Trafitec), Sara Nygårdhs (VTI), Sven-Olof Lundkvist (VTI), Belinda la Cour Lund (Trafitec), Puk Kristine Andersson (Trafitec) and Esben Raahauge Nielsen (DELTA Light and Optics).
All the tests involve presentation of a number of prearranged messages on a VMS, representing variation of some parameters supposedly related to the legibility of the messages, to a group of test persons. In the early tests, the criterion for the legibility was the legibility distance of each of the messages for each of the test persons. In the later tests the criterion was rating of the legibility at predetermined distances.
By legibility distance is meant the largest distance at which the legend can be correctly read by a test person. The maximum legibility distance is obtained when the message is presented with good conditions regarding luminance and contrast, so that the visual acuity of the test person is the limiting factor. It is a general rule of thumb, which is also supported by the tests, that for persons with normal visual acuity the maximum legibility distance expressed in metres can be estimated as 7 times the character height expressed in centimetres.
Some basic information is provided in section 2. The early tests are described in section 3 for the purpose of explaining the background for later tests. These are described in sections 4, 5, 6 and 7 for respectively the luminance of the VMS needed for legibility, the preferred luminance, the quality of character legends and the quality of traffic signs. By quality is meant aspects affecting legibility.
Conclusions are provided in section 8 and also listed below.
Annex A is an operational interpretation of the results of the project. Sections A.2 and A.3 are largely based on the results, while section A.4 represents general procedures as accounted for in the CIE TC-4-40 draft technical report “Performance evaluation of retroreflective traffic signs”, draft 2009 or in national road standards such as Danish road standards.
Conclusions regarding regulation of the VMS luminance:
- the apparent luminance of characters with thin strokes depends on the pixel stroke width
- accordingly, the VMS nominal luminance, as defined in EN 12966-1 “Road vertical signs – Variable message traffic signs – Part 1: Product standard”, has to be set in view of the stroke width of the characters
- the VMS nominal luminance has to be regulated in view of the ambient illumination on the VMS. A suitable regulation curve called “L3 continuous”, or just L3, that provides luminance as a function of illuminance is introduced
- a luminance index LI is introduced in order to include both of the above-mentioned aspects of VMS luminance
- an LI value of 0,25 provides almost maximum legibility distances, while an LI value of 0,5 provides the preferred luminance in rating tests
- the ambient illumination on the VMS is best described by a weighted illuminance on the front and the back of the sign with weights of respectively 75 % and 25 %.
Conclusions regarding legibility of legends:
- when the VMS luminance and the presentation of legends is adequate, the legibility distance expressed in metres can be 7 times the character height expressed in centimetres as corresponding to normal visual acuity defined by 6/6 vision
- use of more pixels to form the strokes of characters lead to better ratings of the legibility at short to medium distances, but not at distances close to the maximum legibility distance. The general conclusion is that the pixel spacing S should be sufficiently small compared to the shortest distance D at which the legend is to be read. It is proposed that S is maximum 0,0004×D (or that D is minimum S/0,0004)
- city names can be formed with good legibility with a character pixel height of 8
- city names with a capital letter followed by small letters are more readable than city names with capital letters only
- a less dense packing of letters forming city names leads to higher ratings of the legibility than a dense packing. The general conclusion is probably that the line spacing should be approximately 40 % larger than the letter height, when using capitals only, and approximately 50 % larger than the letter height, when using leading capitals followed by small letters. Additionally, the gap between letters should be approximately 25 % of the letter height
- the three last-mentioned conclusions are assumed to be valid not only for city names, but for all character messages using letters and digits
- the legibility of some often used warning signs with pictograms for “queue”, “road work” and “danger” is not really good, when presented on a VMS with 48 times 48 pixels on which pixels can only be on or off. A VMS with more pixels can present the pictograms with more detail and would probably make them more legible. It is probable that the technique of “smoothening” by individual setting of the luminance of each pixel can lead to improvement of the legibility, but this was not tested.
Annex B provides a worked example for a LED based VMS.
The project has been organized by the NMF “Nordic Meeting For improved road equipment”. Refer to www.nmfv.dk.