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Conspicuity and Signs

Road signing

Motorized two-wheel vehicle passing in front of a sign.Road signs play a key role in influencing driver behaviour, orientation and information. In spite of modern electronic driving assistance devices, vertical road signs, strategically placed along roads and supplemented by horizontal markings, will be part of the road environment for many years to come.

At all times, short, clear and immediately understandable messages need to be provided to drivers in time for them to react correctly when driving. First, the sign must be seen early enough (position, size and contrast against the environment). Second, it must be recognised (shape, colour). Third, the message or indications provided need to be read and understood (symbols, letters and characters). Often road signs don't live up to these standards or no longer perform as intended for lack of maintenance or updating.

It is important when designing and installing a sign to ensure it is visible at all times and to prevent it from fading, getting damaged, bending or being hidden behind vegetation. A survey, made by an international working group of the International Road Federation in 1990 for the EU Commission along more than eight thousand kilometres of European Motorways in 11 Member States, established that many vital signs at important "decision points" are simply not visible in time. This is also noted in many other places around the world.

Road sign visibility depends essentially on the luminance of the elements on a sign face.

Luminance is the brightness of an area and is the same regardless of the distance separating it from the observer. Each colour has its own luminance - one notes "dark" colours such as blue, green or red and "light" colours such as white or yellow. Combined, these create the required contrasts which make them show up as clearly as possible for drivers. Often very simple design measures suffice, such as adding a light coloured surround to a dark coloured sign. Daytime and night visibilities of road signs are two essential, separate and basic conditions which have to be considered together when designing road signs. Far too often night time visibility is ignored.

Daytime visibility

Whatever the ambient conditions (e.g. bright sunshine, cloudy, rain or foggy weather), road traffic sign visibility depends on three main basic factors: Contrast, colour and shape.


Daylight illuminates the entire road scene; however the intensity of the ambient luminance influences the visibility of road signing. A sign with a white background and black letters placed against a bright blue sky is less visible than when observed against a dark green forest. For this reason, it is essential to take into consideration the best conditions for an adequate contrast.

The entire sign must be recognised at a glance, by its main features: shape and size. Thus when it has a red border, it is better to add a white frame. In countries where many months of snow prevail, like in the Nordic countries of Europe for example, the background colours of all the danger warning and prescription road signs have been specified in yellow instead of white, the more usual colour, because of the vast surfaces of snow during their long winters which cover the background against which one perceives road signs.

Contrast is also necessary to recognise symbols and read letters and figures on road signs. Black lettering or symbols are to be used on white or yellow sign backgrounds. Signs showing "darker" background colours, such as blue, green or red are required to have white or yellow lettering and symbols. This has been required for many years in the international legal documents on which national law is based. The first international Agreement dates back to 1909 (the first international Convention leading to today's UN "Vienna" Conventions and Agreements in force all over the world). In these Conventions and Agreements, the colours prescribed for road signing take into consideration the basic scientific notions of contrast.


Colour plays an important part in the mission of a sign. International colour codes have been established such that red is acknowledged to signal a danger or a prohibition from doing some-thing. Blue is reserved for mandatory and information signs.

Regarding direction indication signs, four main background colours are prescribed for permanent signing, namely white, yellow, blue and green. Depending on how these particular colours are used on direction signs, authorities can differentiate road categories. This forms an important part of international sign legislation since different or special behavioural rules prevail on motorways, rural roads etc. The background indicates clearly what to do.

Further sign background colours are also coded, for example yellow or orange for temporary deviation signing, yellow for temporary roadwork signs or brown for tourist signs.

Modern electronic variable message signs using LED or light coloured dots may show different contrasts and colours, for example with black as background and white, red and yellow for symbols and written message conveyance.


Different sign shapes enable users immediately to understand and recognise their purpose. International legal practice based on the UN legal documents mentioned above stipulates triangular or diamond shaped (used in Ireland, Australia and United States) signs for warning of dangerous situations. Circular signs indicate either mandatory instructions or a prohibition. Square or rectangular signs provide information and direction indication.

Specific shapes are reserved for the highly important priority signing, such as the octagonal STOP sign, the inverted triangle for loss of priority, or the diamond-shaped sign indicating a priority.


Size and/or dimensions are vital for sign visibility. The higher the speed, the greater the need to provide messages early enough to enable drivers to see, recognise and understand their instructions safely.

Readability of words and symbols on signs defines the initial overall size of a sign. Scientific studies (such as CIE Publications 39 and 72 and numerous scientific studies since 1949) establish the minimum criteria for the choice of letter height, width and shape based on the speed of approach to a sign. The stroke width also provides correct contrasts with the background so that every letter or number on the sign can clearly be distinguished. Symbols have been defined so that they "speak" clear messages thus avoiding many language restrictions.

The current world-wide international "code" for these different categories of signs is described in the legal documents widely referred to as the "Vienna" Conventions, the latest versions of which entered into force in September 1993 (Road Traffic rules) and October 1995 (Road signs and Signals). International technical standards, in particular ISO Standards for signing and symbols and the recent CEN Standards for Vertical Signing and Horizontal Marking done by working group TC 226, can be used as legal reference all over the globe.

Night visibility

Studies in many scientific institutions around the world show that, starting at age twenty, every thirteen years the amount of light required by a healthy human eye to see clearly an object at night doubles. Thus when a person reaches the age of sixty, eight times more light is required to see the same sign identically as when aged twenty!

It is therefore logical to require in any national legislation that road signs must be visible by day and at night in the same way, namely with identical colours, shapes and sizes.

Hence the need for road signs to be equipped with some means to make them visible at all times of the day, whatever the weather conditions, all year round. This is achieved by making them retro-reflective (passive lighting) using specially designed materials (sheeting, not paints) and where necessary also by illuminating each sign using an electrical lighting device (active illumination).

Active illumination

Some authorities have taken the political and costly option to illuminate their signs. This can be done either by external luminaries or internal illumination. These produce the so-called active light. However, such installations require frequent maintenance operations, replacement of light sources and spare parts and power costs. Many alternative solutions are being adopted, in particular with the use of passive lighting on new signs.

Certain signs will always have to be illuminated because of their specific role, frequent bad weather conditions or their position on the road. However, in order to cope with inevitable and mostly unexpected electrical failure in the illuminating device, more often than not, authorities have decided to use a combination of passive lighting on the signs themselves supplemented by an active external illumination device.

This is considerably more advantageous when considering the overall cost of a sign. Indeed maintenance operations are facilitated with this combination. Many experts are convinced of the economic advantages of such a practice: one is free at any time to decide to add illumination to a given retro-reflective sign to answer a local need.

Passive lighting

Passive lighting is provided by retro-reflected light coming from vehicles' headlamps. Retro-reflected light from a sign is observed by drivers in the following way:

Headlamps beam patterns are designed in such a way that the road is illuminated a certain distance in front of the vehicle. They have both a main beam and a dipped (passing) beam. When the light from a vehicle's headlamps illuminates the retro-reflective surface of a sign-face, most of it is directly returned to the source, i.e. the head lamp. Because drivers' eyes are situated close above the headlamps, most of the retro-reflected light also reaches them.

Retro-reflective materials come in all the legally required sign colours. They return the light such that it is seen in the same colour as that of the sign. Thus all original sign colours, shapes and sizes can be clearly seen by means of the retro-reflected light at all times.

Driving conditions are such that most often dipped (passing) beams are used to avoid glare for oncoming traffic, since high beams can only be lit when there is no opposite traffic. In fact, the UN Conventions prescribes the dipped beam as the minimum requirement when a vehicle is in movement at night - even on lit streets - and in bad weather conditions.

The further away from the sign one, is the more retro-reflected light reaches the eyes of the driver. As one gets closer to the sign more light gets directed back to the head-lamp(s) and less to the driver's eyes (principle of the Cone of Retro-reflection). The angle formed between the headlamps, the sign and the observer's eyes gets larger. In other words, the perceived brightness of the retro-reflected light diminishes as the vehicle gets closer to the sign. This explains the fact why it is practically impossible to obtain uncomfortable glare from retro-reflective signs.

This is also confirmed by the fact that more than ninety percent of all road signs are situated at the edge of the road, roughly two metres above the ground. Overhead signs are normally placed some four and a half to five metres above the carriageway.

New technical developments

Vehicle head lamps

New technologies in headlamp design allow for better light emission control. The inventions of new light sources combined with novel light beam patterns provide improved distribution of road and sign illumination. A global dipped (passing) beam has been technically and legally described in 2002 (UN ECE Documents currently under consideration for approval at the World Forum for Harmonised Vehicle Regulations - WP.29). Certain models of motor vehicles are already fitted with such beams. This will lead to common geometrical visibility criteria for road signs.

Retro-reflective materials

The computerised technology leading to the design and manufacturing of retro-reflective sign sheeting has resulted in a considerable improvement in day and night road sign performance. A new generation of high-tech retro-reflective products is now available. These modern materials are based on micro-replication of corner-cubes eliminating the need to use glass beads and their aluminium backing. The new materials are made entirely of plastic materials and provide a far more efficiently distributed and brighter retro-reflected light with richer colours.

Road signs using this high performance sheeting respond well to specific function requirements and provide the best possible performances in relation with their position along or above the carriageway. New families of micro-corner cube retro-reflective materials are designed to allow authorities to consider replacing totally their illuminated signs whilst retaining similar and in some cases even better efficiency. Thereby they are able to enable considerable return of investment as well as create substantial economies over long periods of time.

The UN "Vienna" Conventions consist of a number of documents which can all be found at the UNECE website:

  • "Vienna" Convention on Road Traffic
  • "Vienna" Convention on Road Signs and Signals
  • European Agreement supplementing the Convention on Road Traffic
  • European Agreement supplementing the Convention on Road Signs and Signals
  • Protocol on Road markings

The latest Amendments to the two "Vienna" Conventions are found in docs. TRANS/WP.1/2003/1,2,3,4,5/Rev.4 (5 documents) also at the same address above.

All the pictures in this article are kindly borrowed from 3M who owns the copyright.