People will react first to that which is most visible
and colour plays a central role in allowing us to
detect objects quickly, thus allowing more time to react to and avoid a hazard.(Solomon1, p71)
In countries the world over there are different traditions and influences that determine the base colour used on emergency vehicles.
In the United States the colours were red or red/white for fire vehicles and black & white for police cars.
In Australia ambulances used to be a creamy beige colour - they are now white.
Police vehicles in the UK are often leased so they are ordered in metallic silver colour to take advantage of a 10% increase in resale value at the end of the lease.
England and EEC countries have now agreed to paint their vehicles yellow/green RAL 1016, however each country will retain their different liveries and markings.
Whatever the traditional reason for emergency vehicle body colour, there are some sound technical reasons for choosing particular vehicle colours. In Australia where every delivery van on the road looks like a white Mercedes Sprinter ambulance, agencies rely heavily on coloured livery and markings to announce the vehicle's presence.
The role of colour to increase vehicle visibility
Colour safety revolves around the levels of contrast sensitivity in the human vision system. We see an emergency vehicle most easily when it contrasts against the local background at that particular location. Solomon1 p68 describes earlier studies from the National Space and Aeronautics Administration (NASA) showing that bright colours, which are rarely present in nature, are most attention getting. The colours of both rural and municipal settings have, on average, been judged to be dark. Against these dark backgrounds a light object is more easily seen than a dark object.
In road traffic situations there are many coloured objects within the visual field.
What are the properties that will draw attention to one important object, even if it is outside the area of central vision and located in the fringes of the peripheral fields? Listed here are the factors (described in general terms) that most affect contrast:
The target object is relatively large in size
The target possesses high levels of brightness & colour contrast
The object is moving, flickering rapidly or the object suddenly appears within the field of view
The target shape is easily recognised - this is based on the previous experience of the viewer
There is no need for the viewer to recognising the individual colour schemes or markings of the different agencies operating emergency vehicles. A driver cannot use valuable reaction time to determine which emergency organisation he has to avoid. He simply reacts to avoid the oncoming vehicle he sees before him. He may realise that the shape of the vehicle is a fire truck and then experience tells him to slow and wait or move around it.
It should be remembered that vehicle body colour is discussed here in terms of light
reflecting from a surface - not as transmitted light emitted from a lamp source.
Colours in the visible spectrum have inherent differences in brightness. The human visual system will detect an object initially as a brightness (without colour) as its image enters the peripheral rod system. After more of the peripheral cones are stimulated, the colour value will be begin to be detected gradually. As the object image moves closer to the higher cone populations in the region of central vision, the visual intensity of the colour will increase.
The spectral sensitivity curve shown below illustrates the regions of maximum visual sensitivity of the eye for the light wavelengths in different colours (the reflected vehicle body colours).
Wavelengths or colours that stimulate the outlying peripheral photo-receptors earlier all fall within the greenish-yellow to the yellowish-green regions.
The blue hues are detected next.
The red regions are the last colours to be detected.
Red has poor reflectivity and is not detected well in the periphery during daylight hours. The intensity of red in daylight only appears once the object is visualised within the central field of view. Red is perceived even less during darkness and under adverse lighting conditions, both in central and peripheral vision.
our human vision is red/orange blind:
For low light conditions
Under scotopic vision (involving the Purkinje shift)
Red colour variations are perceived as black, often merging with the regions of darkness in the visual field at night.
Read a discussion on the advantages and disdvantages of the many different vehicle colours - CLICK HERE
So what is the best body colour for emergency vehicles? Without doubt, a single body colour with high brightness levels and high colour contrast when it is viewed under varied light and weather conditions is safer than two tones of colour or the use of a single dark colour.
The majority of the vehicle bodywork should be painted in one of the colours shown below in order of preference. A white base colour requires the careful addition of a second highlight colour to approximately 10% of the total surface area to increase visibility. The vehicle paintwork should be regularly polished to keep reflectance values high.
Yellow - green
A number of research studies have confirmed the effectiveness of yellow-green as the most conspicuous colour for emergency vehicles (Auto and Road User Journal, pp1-5). Solomon and King conducted a study in 1984 using nine cities and 750,000 fire trips - using American fire departments with large fleets and high mileage provided the ideal research base.
A later follow-up study in Dallas was run over four years. Using a colour mix of appliances with yellow/green and white versus all red or red and white appliances the earlier results were confirmed. During the course of the study, red appliances responded to 153,348 calls and yellow-green appliances responded 135,035 times. Only the visibility related accidents were considered in the study (not accidents with parked cars or fixed objects).
Of a total 36 accidents, 20 were studied - with yellow-green appliances accounting for only 4 out of 20 accidents. Solomon and King concluded that the risk of visibility related accidents was three times greater for red/white appliances and the likelihood of injury or tow away damage was reduced for yellow-green appliances.
The lower impact speeds of yellow-green accidents were due to the enhanced visibility in the face of danger (providing greater warning and reaction time for the impending accident which had become unavoidable). The lesser visible red appliances were perceived at a later stage and in general much closer to the final point of impact.
In another research paper De Lorenzo & Eilers (p1333/83) state that the multicoloured ambulance while distinctive, may suffer decreased conspicuity because of the effects of camouflage. This research applies to the different colours like red & white as well as the corporate patterned markings used to identify emergency vehicles.
Exterior lighting can affect the reflected colour of emergency vehicles. This changes the visual appearance of the vehicle and this single factor of outdoor scene lighting may compromise safety. There are five basic types of artificial light used on streets or in building carparks:
Incandescent and halogen lights have little effect on a vehicle’s colour
Fluorescent lamps have almost no effect on colour
Low pressure sodium lamps producing narrow-band yellow light turns most colours grey, brown or black. Under sodium lamps yellow-green changes little and may appear as off-white. White appears orange and yellow is only marginally affected by sodium lamps.
Mercury vapour lamps may exhibit similar changes to LP sodium lighting.
Metal Halide and high pressure sodium lamps produce the best colour rendition and vehicle colours are close to the normal colour appearance of daylight.