4 Easy Steps to Choose the Right Size of your DMS

We have created this additional reference to follow our last blog, defining the right size of dynamic message signs In most cases, DMS users already know what they want to display and what messages or graphics will be needed in order to do so. Today, most DMS are full matrix, making it very difficult to determine how many lines and characters a DMS is capable of displaying. This blog helps answer these questions and provides a 4 step procedure to help choose the right dimension of the display area, in addition to some tips like what fonts to use.

 1- Define the  number of lines

Evaluate the length of your messages. Do you need one, two, or three lines of text? Messages longer than three lines of text are not recommended as they make it difficult for drivers to read, understand, and react to the message in time. Remember, shorter messages are best absorbed and that we react to graphics even faster than reading lines of text. Avoid using alternate messages unless they are very short.


2- Define the height of  your text

The height of the text is determined by the viewing distance. The higher the character, the farther the viewing distance you get. The viewing distance is not the only criteria, just because you can see the character does not mean that it is legible to the driver. For example, at 1200 feet you will probably see an 18" character but, it may not be clear enough to differentiate between characters, such as a D or an O. This has nothing to do with the type of LED, the color, or the definition. It is a limitation of the human eye.

To make things easy to remember, a 12" character is legible atapproximately 500 to 600 feet and an 18" character is legible at 1000 feet.


3- Define the font  base on the pixel pitch and character height

The distance between pixels or LED clusters, is referred to as the pixel pitch. The smaller the distance, the higher the resolution and definition. As the resolution increases, so does the price of your sign. When defining your DMS think first about what is really needed in order to display what you intend, then if your budget supports it, you can increase your definition.

The font defines the shape and the size of all characters, numbers, and symbols. Most of the DMS have changeable fonts that can be set remotely and changed via a NTCIP controller. Refer to NEMA TS4 for appropriate fonts and spacing requirements between characters and lines (refer to NEMA TS4 2016 paragraph 5.6 Display character)

This table will help you find the right font:

Character height Font Inter-line Inter-character
20 mm 12" 15x10 8 pixels 3 pixels
20 mm 18" 23x15 12 pixels 4 pixels
33 mm 12" 9x6 4 pixels 2 pixels
33 mm 18" 14x10 6 pixels 2 pixels
66 mm 18" 7x5 3 pixels 1 pixels


4- Define the right display area in pixels

The display area needs to be defined in pixels and will vary for each type of font. You can display 3 lines of 15 characters 18" height in several formats. 

For example:

If you want to display 3 lines of 15 characters in 18" you can select the following signs:

- 28x90 pixels if the pitch is 66mm 

- 54x180 pixels if the pitch is 33mm

- 96x272 pixels if the pitch is 20mm.

The table below can be used as a helpful guide when selecting the right sign:

Display Minimum matrix Available models
      Amber or White Full Color      
Nber of lines Nber of characters Height 12" (46mm) 18" (66mm) 20 mm 33mm WalkIn Front Access Rear Access
1 12 12" 7x75 16x128 18x100   x x
 1  12 18" 32x224 18x150   x x
 1 15 12" 7x90 16x192 18x120   x x
15  18" 32x272 18x180   x x
18 12" 7x110 16x208 18x150   x x
 18 18" 32x336 18x220   x x
2 12 12" 21x75 48x128 27x100   x x
 2 12  18" 64x224 36x150   x x
 2 15 12" 21x90 48x192 27x120   x x
 2 15  18" 64x272 36x180   x x
18 12" 21x110 48x208 27x150   x x
18  18" 64x336 36x220   x x
3 12 12" 28x75 128x64 36x100 x x x
12  18" 96x224 54x150 x x x
15 12" 28x90 64x192 36x120 x x x
 15 18" 96x272 54x180 x x x
 3 18 12" 28x110 64x208 36x150 x x x
 3 18  18" 96x336 54x220 x x x



Defining a sign by its physical dimension is not the right or most cost effective approach. You should first define what needs to be displayed, select the minimum requirements to succeed this, and then if your budget allows, exceed your signs requirments. By following this simple method you can easily get what you want, to improve road safety and driver experience.


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