Ambulance Visibility
Emergency vehicle conspicuity research on livery, warning lights and
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Reflective Markings Reflective Markings Prado-www.ambulancevisibility.com-ACT Ambulance-John Killeen

 

 

Overview
Retroreflective markings on emergency vehicles have been in use since the early 1950's. This section looks at the markings used on emergency vehicles to enhance the vehicle safety and the safety of people around the scene. The benefits of different designs, patterns and colours, along with descriptions of the types of marking materials available are discussed below. The current generation of emergency vehicles present new problems for the designer and the fixing of suitable reflective materials to the difficult shapes & compound curves of modern body panels. Contemporary vehicle designs have large areas of mottled-finish plastic trim around the car body - a surface that will not easily accept the adhesive on most reflective films, increasing the difficulty of marking-up new vehicles.
(Photo - ACT Ambulance Intensive Care Paramedic Single Response Unit with high visibility reflective livery - 4WD Toyota Prado)
 
The geometry of retroreflectivity
The action of retroreflectivity is, ...the principle of returning reflected light to the source by using minute reflectors embedded in the surface of a support film. During daytime visibility is a result of brightness and colour contrast against the background. Visibility at night is a function of brightness (or luminance-contrast).
  
At night, light reflected back toward the light source from a retroreflective surface (eg. a sign) is returned in a cone shape within an angle of approximately three degrees of the source. The amount of light returned depends on the geometry of the situation and the retroreflective properties of the materials being used on the traffic control device (Chrysler, p1).
 

 
Two major angles influence the amount of reflected light output when the markings are seen from any particular position.
  Entrance angle
  • The entrance angle (or offset angle) - formed by the angle between a line drawn outward at 900 from the face of the film and the line of the light source. The entrance angle will  change continuously changing when the vehicle is moving.   

  Entrance angle2Artwork courtesy of  3M                     

  • The observation angle - formed between the light source, the retroreflective film and the observer. This angle is also changing constantly in a moving vehicle and will change depending on the driver's eye height in cars, 4WD's, trucks & buses.     

                                                                                                                                                          
 
If the reflective film is on a moving vehicle and the observer is in a second moving vehicle, the angular calculations can become very complex.  The perceived brightness or luminance of retroreflective films is dependent on several factors;
  • The light source (headlights or flashlight)  
  • The type and cleanliness of the retroreflective film
  • The observer's location (viewing angle), height and visual acuity       

A major problem with perceived brightness is the driver's age which involves the contrast factor. As the driver ages, their ability to perceive brightness decreases and older drivers require at least three times the luminance of younger drivers to see a sign at the same distance (Chrysler, p3). Another factor is the background contrast and how it affects the visibility of retroreflective films. Urban area environments with high ambient light levels always decrease conspicuity.

Retroreflective colour
Reflective films have been used on emergency service vehicles for many years. Retroreflectivity is passive and the reflective colour is "switched on" by the headlights of an approaching vehicle. The film supplies a steady, non-flashing panel of reflected light. This provides a relatively unchanging target within the acceptance zones of the reflective material. There are a myriad of designs, colours and patterns used to distinguish each emergency service from their companion organisations. A wide range of reflective colours are available and many vehicle liveries are designed by professional graphic artists displaying the individual corporate identity of the parent organisation. 
 

Brightness and Luminance
In measuring reflected light from retroreflectors and fluorescent materials, the terms are:
  • Luminance - refers to light that has been measured by electronic instrument. 
  • Brightness factor - the human eye's perception of the light levels and this is very much influenced by colours.
  • The difference = Brightness: Luminance Ratio - the two factors can be very different for the one material and are heavily influenced by the entrance and observation angles.  
Recent research on new generation materials (especially prismatic fluorescent retroreflective materials) shows that a coloured reflector (yellow) can appear as bright to the human eye as a white reflector with a measured luminance more than three times as great as white. This same phenomenon can be seen in daylight, since a vivid colour like fluorescent lime-yellow can be more conspicuous than white. Dark coloured retroreflective films such as blue and red still require the use of larger surface areas to maintain equivalent visibility values at night. The brightness factor of the most popular colours are in order:
  1. White 84% & Yellow 57% (depends on product, see above)
  2. Red 13%   
  3. Green
  4. Blue 8%   
 
Chromatic aberration affects different colours used together
The darker colours can also be affected by chromatic aberration. When they are mounted on the vehicle in close proximity to each other the eye struggles to focus on the different wavelengths of light. This is most pronounced when white is used alongside the darker colours . Chromatic aberration can affect the perception of the reflective form, has a direct affect on human distance judgement and when used in patterns can influence response time and braking distances.
 

 
Reflective Contrasts-www.visibility.com-ACT Ambulance-John Killeen
Increasing personnel safety
at night with reflective colours
At night, retroreflective tape sewn onto clothing can effectively contrast personnel against the reflective vehicle markings located around a visually cluttered accident scene. The body movements of personnel plus the differing yellow and white reflective colours provide greater contrast and safety for personnel.  ACT Ambulance and ACT Fire Brigade use yellow horizontal markings on vehicles and white reflective tape on staff trousers and reflective vests (photo).
 


The use of design and pattern in retroreflective materials
The single thin horizontal retroreflective stripe affixed to vehicles has been considerably enhanced during the last few years by the influence of good visibility design principles. Unfortunately many designers and emergency service organisations still have not carefully considered their design approach to reflective films, along with the on-scene viewing factors and way their choices are interpreted within the human cognitive viewing process.
 
Corporate design teams and graphic artists have continued to produce some unusual and confounding designs. Sometimes these are decorative, occasionally full body murals and more often-than-not, non-functional artistic license. The use of diagonal stripes, zigzag and chequered patterns may be visually spectacular at night but these same patterns will present as camouflaging effects when seen in daylight. In addition, different colours displayed side-by-side within the patterns may actually decrease conspicuity. 
 
Reflective patterns and vehicle shape
All exotic patterns and designs disturb the vehicle's silhouette and reduce the visual angle (the vehicle's apparent width and height) by breaking up the dominant vehicular shape into many smaller unrelated segments. When used together or with white, the reflective colours of red and blue may also display the disconcerting visual effect of chromatic aberration.  The viewer has difficulty in perceiving the colours and this effect can delay recognition and increase reaction time. The eye and the brain are subconsciously forced to adjust to the different light wavelengths visible within the scene.
 
Horizontal vs vertical
Micro-nystagmus, a horizontally biased scanning pattern in the eye ensures that the rods and cones cannot fixate on the focused image....Thus objects that display vertical stripes as part of a visibility package can enhance safety (Solomon, p78).
 
Solomon is referring to vertical lines outlining or set at appropriate points within the vehicle's shape. Reflective patterns, diagonal or candy-stripe designs confuse the eye & brain, thus slowing object recognition and wasting precious reaction time. This is the very reason that candy-stripe reflective tape is being replaced by solid colours and outline stripes on Australian emergency vehicles.
 
Some important points about horizontal and vertical lines should be elaborated upon. A single thin horizontal stripe along the vehicle waistline may not be seen at night as a barrier. It is similar to the thin rail between two high-jump posts in athletics. Over long and medium distances in darkness, the narrow stripe can appear to be a minor obstruction (and of little importance) that when seen from a distance is not very high off the ground. The approaching driver may not see the dark vehicle glued to the back of the thin glowing reflective stripe until he is too close to stop (see photo on left).
  
Thin reflective line-www.ambulancevisibility.com-John Killeen
 
 
ACT Rural Fire Tanker with old narrow stripe (left) and new Ford Ranger Command vehicle with wide stripe and white outline (right)
Note: the finish on the plastic resin canopy prevents any type of reflective material adhering to the burnished surface.
 
The factors of shape, size, vehicle outline and orientation are just as important at night. Some European countries completely cover the side panels of their vehicles with red reflective material so they stand out against snow during the day and present a unified coloured vehicle side profile at night.
 
Making the narrow stripes wider!
If that same thin horizontal stripe is widened, it will be more effective and better visualised during the approach. A wide stripe can be seen through the side window of cars as the emergency vehicle moves alongside in traffic. The extension of that same wide stripe forward to the headlights attracts the attention of cross traffic when nosing into intersections past stationary cars or other vehicles alongside.  If a combination retroreflective/fluorescent film is used, this warning is effective both day and night.
 
Additional stripes or panels may be added to the bonnet and rear doors if required. Visibility can be increased further by adding a second band of film at full height if possible, encircling the entire vehicle just below the roofline. The red/orange HiVis stripe seen at the bottom of the ACT Ambulance vehicle doors does not intrude too far into the larger white space of the base colour. This secondary colour has been added onto ACT vehicles to provide additional visibility at extreme distances (greater than 600 meters) and to stand out in "white" conditions during daylight, especially fog & snow.
  ACT Intensive Care Ambulance reflective livery-www.ambulancevisibility.com-John Killeen
ACT Intensive Care Ambulance-www.ambulancevisibility.com-John Killeen 
  
  
  
 
 
 
 
 
 
 
ACT Intensive Care ambulance with fully enclosed reflective markings - note the white tape is almost invisible during daylight.
  
The outline of any vehicle should be emphasised with vertical retroreflective stripes. The object vehicle should now be seen as large with a definitive vehicular shape. The vehicle's perimeter should be clearly defined by a closed circuit of reflective tape. If these reflective vertical lines are the same colour as the vehicle body, they will be visible only under illumination at night. During daylight hours the vertical stripes will remain unseen, therefore not corrupting the vehicle's shape or colour. Any gaps in the outline should be avoided if possible.....incomplete coverage is discouraged as it may contribute to the camouflage effect (De Lorenzo and Eilers, p1333/83).
 
 
1.  Solomon, Stephen, Emergency vehicle accidents - prevention and reconstruction, 1999
3.  Chrysler, Susan, Luminance: What the driver sees - Discussion Paper, 3M Transportation Safety Product Center, 1999
 
 
 
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