Road Signs: The Contrast Edge

→ Dieser Text ist auch in Deutscher Sprache abrufbar.

More visible road signs?

The mandatory use of the Contrast Edge on all German road signs dates back to first motorway construction activities in the 1930s. No Studies are said to be existing on the matter, only the a historic remark stressing the improved visibility of road signs through the implementation of the Contrast Edge (Stephan, P., 20081)). As the Contrast Edge has prevailed until today in the German System, it may be considered as being effective. Germany is not the only country to use a Contrast Edge nation wide– United Kingdom for example.

In order to define meanings of terms (e.g. Contrast Edge, Symbol, Signal Aspect, Border, Ground etc) required for this text, the following illustration (Traffic signs: Definition of technical terms for graphical components2)) is provided.

Contrast Edge and Vienna Convention

By examining the Vienna Convention on Road Signs and Signals3) it becomes clear that the use of the Contrast Edge on road signs is inconsistently covered. In some cases, one is lead to assume that the Convention implies the existence of the Contrast Edge, even if this is not explicitly stated.

To illustrate, sign D, 7 is mentioned, which is used on sign E, 1a and E, 1b, or sign C, 14, used in E, 1c. Diverse other road signs of the Convention bear a Contrast Edge: D, 1b; D, 12b; G, 4b etc.

Figure 2: Use of sign D, 7 and sign C, 14 on other road signs, including an additional Contrast Edge

Since the use of the Contrast Edge is not only relevant for the improvement of perception of signs, but also interesting for its role in the systematic buildup of road signs, it seems highly appropriate to deal with subject in the following.

Possible benefits

Undoing the masking effect & earlier information provision#

Conspicuity (Wertheim A. H., 19894)) is achieved if a Signal Aspect of a road sign can be discriminated from the environment behind through sufficient contrast, in order to prevent the signal to be visually masked by its backdrop.

Typically, a sign is masked by high visual information density, or environments with a predominant colour that only allows for too low contrast to discriminate the Signal Aspect. Lighting conditions play an important role, too. A Contrast Edge on a road sign usually is of white colour, since light reflecting from it is highly visible, partly because the effect of irradiation causes bright coloured elements appear to expand in comparison to those of dark colour.

Figure 3: Irradiation: Even though the strokewidth of both lines is the same, the white line on dark Ground appears wider.

In addition, white used as retroreflective sheeting offers high distance-reflection rates. This can be put to good use for generally improving the conspicuity of road signs, or for early conveyance of first highly relevant components of the Signal Aspect.

The use of the Contrast Edge is expected lead to:

  • Earlier understanding of the existence of a road sign ahead. A longer (and controlled) viewing duration through more focused attention can be expected.
  • Earlier recognition of a primary component of the Signal Aspect (border in the shape of triangle, circular ring or circular area etc.) and understanding of its meaning: warning, prohibition or mandatory etc., while the Symbol is not yet discriminable. This narrows down the possible types of information conveyed by Symbols that can be expected to appear.

Both effects are expected to be put to good use in road traffic, as drivers are burdened by short viewing durations, apparently moving and size changing information (during approach), and changing qualities of lighting conditions.

Masking effect:
Demonstration material – with / without Contrast Edge

In the following, a Signal Aspect from Germany (in its precise from) is presented. The right depiction incorporates the Contrast Edge, the left one does not.


  • In reality, effects of irradiation and (retro)reflection are strong and can not be simulated through this form of presentation. Retroreflection of white light negatively influences the discriminability of the shape of a Signal Aspect’s Ground, while, through retroreflection from the Contrast Edge, the shape of the Border is more discriminable.
  • Since a danger warning sign, due to its triangular shape, only provides very limited space for the Ground to present a Symbol, this type of sign is considered as to be more vulnerable to possible effects on perception caused by the implementation of the Contrast Edge. Therefore, only danger warning signs are used for exemplification.
Figure 4: Long viewing distance – due to the distance, only the white space of the Signal Aspect’s Ground allows to conclude on the existance of a road sign. Too low contrast is provided to discern the left sign from the backdrop. To conclude on the Border’s shape and its meaning via the shape of the Ground seems inappropriate, as this requires additional viewing time during approach until unanmbiguousness is given.
Figure 5: short viewing distance – the additional white of the Contrast Edge makes the whole sign more conspicuous and facilitates faster and unambiguous comprehension.

Visual clutter

In a study by Porathe and Strand, 20115)) it was attempted to measure the conspicuity of signs through the method “conspicuity index”. The investigation included several forms of shielding from visual clutter generated by a high density of visual stimuli in the backdrop of a sign. Clutter is known to decrease conspicuity. The study concluded in the recommendation “that uncluttering of the immediate Ground of signs by using a mask clearly increases the conspicuity of the sign”.

Since it serves as such a mask, the Contrast Edge helps to prevent clutter from influencing conspicuity.

Possible implementation barriers for the introduction of the contrast edge

Example Austria: If the current dimensions of road signs are maintained, it may be suspected that the components of the current Austrian Signal Aspect may be forced to shrink due to the addition of the Contrast Edge. Not withstanding this, the German example’s lesser strokewidth of the Border does not seem to cause perceptual problems, it even provides the opportunity for implementation of the Contrast Edge.

Figure 6: Comparison of Austrian and German danger warning sign


Update 1:
Reduction of Border strokewidth to German measurements

Through this, the space needed by the Contrast Edge is complete compensated for. The Ground space’s dimension, required for the depiction of symbols remains unchanged.

Figure 7: Adjustment of the Border makes room for the Contrast Edge

Update 2:
Increase corner radius of the Border

After adaption that created update 1, the Austrian Ground’s dimension to be used for showing symbols still is smaller than the German road sign’s. This is caused by the size of the corner radius.

In the following, the corner radius implied by the Vienna Convention’s depiction (8,1 % of triangle height) is applied to form update 2. This increases the Ground dimension to 121 % in comparison to the current Austrian model, without changing height or width of the sign. As a result, a corresponding increase of Symbol size is possible, allowing for improved discrimination.

Figure 8: More room for Symbols through increase of corner radius


The general implementation of the Contrast Edge is advisable, since

  • Perception of road signs as a whole improves. The implementation (for Austria and countries with similar Signal Aspect Borders) does not imply any adaption of sign-size(s). There is no influence on the discriminability of Symbols.
  • With the additional implementation of a larger corner radius, more room for the larger depiction of a Symbols is achieved, without the need to change the sign dimension.

→ Dieser Text ist auch in Deutscher Sprache abrufbar.


  1. Stephan P. (2008) Die Straßenverkehrszeichen in Deutschland und seine Nachbarstaaten: Vergleichende Untersuchung der Geschichte und Struktur eines kulturellen Zeichensystems. Berlin.
  2. Egger S. (2015) Traffic signs: Definition of technical terms for graphical components
  3. United Nations (1968) Vienna Convention on Road Signs and Signals
  4. Wertheim A H (1989) A quantitative conspicuity index: theoretical foundation and experimental validation of a measurement procedure. Report C-20 (in Dutch). TNO Human Factors Research Institute, Soesterberg, The Netherlands
  5. Porathe T, Strand L (2011) Which sign is more visible? Measuring the visibility of traffic signs through the conspicuity index method. European Transport Research Review, Volume 3, Issue 1, pp 35-45

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