ArticlePDF Available

THE SCIENCE OF WARNING LIGHTS

Abstract

Literature would support the uniform use of warning lights on vehicles that require motorist to perform similar actions. Familiarity of warning light colour and colour combinations, the vehicles that use each, and the response expected allows for an automated response by a driver when the colour combination is detected in the environment. Scientific evidence suggests that red and blue combinations offer the best visibility in both low and normal light conditions, and in adverse weather, and standardization of law enforcement warning lights may be prudent. Variations in warning light use are demonstrated by providing an overview of warning light use in Canada.
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 1
JCJR
The Journal of Criminal Justice Research
RESEARCH ARTICLE
THE SCIENCE OF WARNING LIGHTS
Gregory S. Anderson1
University of the Fraser Valley
Darryl B. Plecas
University of the Fraser Valley
ABSTRACT
Literature would support the uniform use of warning lights on vehicles that require
motorist to perform similar actions. Familiarity of warning light colour and colour
combinations, the vehicles that use each, and the response expected allows for an
automated response by a driver when the colour combination is detected in the
environment. Scientific evidence suggests that red and blue combinations offer the best
visibility in both low and normal light conditions, and in adverse weather, and
standardization of law enforcement warning lights may be prudent. Variations in
warning light use are demonstrated by providing an overview of warning light use in
Canada.
Keywords: Emergency warning lights, Color, Intensity, Flash rate, Canada
Author Biographies
Dr. Anderson is a professor in Kinesiology and Physical Education. Trained as an
applied physiologist he has an interest in occupational physiology and bona fide
occupational requirements in physically demanding occupations.
Dr. Plecas holds the RCMP Research Chair and is the Director for the Center for
Criminal Justice and Public Safety Research in the School of Criminology and Criminal
Justice at the University of the Fraser Valley.
1 Correspondence should be forwarded Gregory S Anderson, PhD. University of the
Fraser Valley, 33844 King Rd., Abbotsford, BC V2S 7M8, CANADA
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 2
INTRODUCTION
The purpose of a warning signal whether visual (light) or audio (siren) is to alert the
driver to the presence of an emergency vehicle and inform them as to the appropriate
action to be taken (Ullman, 2000). To be effective, the system must be detectable to
drivers in all conditions, including low light, high noise, and adverse weather conditions
(Rubin & Howett, 1981, 5). An effective warning system should elicit a five step
response from the driver: signal detection of the audio and light signals (audio between
20 - 20,000Hz and light 400 700 nm); signal processing as the driver notices a
disruption of the normal driving routine; signal interpretation at higher brain centres
based on familiarity of the signal and previous experience; decision making, determining
the most logical response to the stimulus (yield, stop or continue without action); and
final, execution of the action which would involve maneuvering the vehicle along the
decided course (Rubin & Howett, 1981, 6-7). Inefficiencies in a warning signal may
come at any of the five levels of response from detection to response.
While there is a paucity of peer reviewed journal articles investigating optimal light
sources for warning signals, the literature that does exist is dated and comes primarily
from the United States Department of Commerce, The National Bureau of Standards,
and the Journal of the Illuminating Engineering Society. The literature does, however,
establish that the association or interpretation a motorist associates with a given signal
is based upon his or her previous experience. With regards to warning lights, what is
clear is that the interpretation of visual warning signal not only is based on colour, but
on the configuration of various colour patterns. In September of 1978, the US
Department of Commerce issued a report entitled Emergency Vehicle Waning Lights:
State of the Art (NSB Special Publication 480-16). This report was released to establish
standards on the best colour combination and general configuration for emergency
vehicle warning lights (p.1). The report clearly articulated concern over the lack of
uniformity of warning light use in the United States. The variability in warning light use
between emergency and law enforcement agencies across the United States was a
source of confusion among motorists and pedestrians:
…because of the proliferation of lights having a similar appearance, yet
calling for different behaviours depending on the particular vehicle on
which they are mounted, and the particular locality in which they are
used….Moreover, since the meaning of the signal (coding) varies among
different states and localities, an out-of-town driver may react to a local
emergency signal inappropriately… (Howett, Kelly, & Pierce, 1978, p.3).
The use of various patterns of lights by different enforcement agencies such as police
departments or other enforcement agencies can be confusing to motorist. For example,
in “Blue Light Syndrome” Nixon (1998) offers a recent example of motorist confusion in
Illinois when two different police agencies working in the same area were using different
colours of warning lights. When the Illinois State Police took over the responsibility for
patrolling the interstate highways lying within the Chicago city limits, the State Police
had problems with motorist compliance when trying to pull them over during routine
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 3
traffic stops or for the purpose of citations following traffic violations. The problem
stemmed from both a “recognition” and “processing” problem, as the traditional police
services (City of Chicago) used only blue lights, while the State Police of Illinois used
only red lights. The people in and around Chicago recognized red lights as emergency
vehicles and purposefully drove from one lane to another to yield the right of way, and
did not formulate a decision that included pulling over and coming to a stop;
The people of Chicago didn’t know who was behind them and drove
from one lane to another to yield the right-of-way. State Police vehicles in
this division are now equipped with a mixture of red and blue lights, and
the problem seems to have been solved (p. 46).
A recent study performed by Ullman (2000) for the Texas Transportation Institute
(What’s Flashing Before your Eyes?) evaluated specifically the effects of different colour
and combination of warning lights on motorists reactions. Using an opinion survey,
motorist were asked to assess how different warning light colors and color combinations
were related to the perception of the motorist as per the nature of the problem that lied
ahead, and their appropriate driving response. The results demonstrated a distinct
difference between the motorist’s perception and response to varying warning light
colour. The results suggest that:
…motorists can and do associate different flashing warning light colors
with different types of vehicles, and also associate different appropriate
driving responses to them based on those vehicles. More importantly,
though, motorists do not appear to focus strictly on any one color but
rather on the specific color combination used to identify a specific vehicle
type (i.e. construction or maintenance, police, fire, ambulance) (p. 47).
Combinations of lights were assessed and processed, compared to past experience
and previous knowledge, and responses formulated which were specific to the light
combination. Motorists clearly made associations between light combinations and
appropriate driving behaviours.
To follow up on the survey results, data was then collected by the Texas Department of
Public Safety by monitoring the traffic response to vehicles sitting on the shoulder of the
road displaying various lights or combinations of lights. Interestingly, all colour
combinations caused the motorists to slow down, but there were no significant
differences between light colours. However, the addition of blue lights did increase the
number of brake applications observed, suggesting the motorist were preparing for an
encounter with a law enforcement vehicle and were preparing to slow down in advance
of the vehicle parked on the side of the road.
Functions of the Eye
The center part of the retina is known as the fovea and provides the greatest ability to
see colour; within the fovea, there are two types of photoreceptor cells: centre cones
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 4
working best in daylight, and rods working best at nighttime (Howett, 1978, 45). The
cones are the denser in the centre of the fovea, with the rods extending to about 20
degrees from the centre of the fovea (45). The cone system is able to mediate colour
vision because there are three different kinds of cones: one absorbing long wavelength
light (red); one absorbing medium wavelength (green); and one absorbing short
wavelength light (blue) which is the strongest (p. 45). In daylight, we move our eyes
until the image is focused in the central fovea; whereas at night time, we look about 15
to 20 degrees away from the light so the image is formed in the rod rich area (45). The
peripheral part of our vision is important in the detection of visual warning lights in
emergency vehicles particularly because emergency vehicle approach from different
angles towards a target vehicle (42).
CONSPICUOUSNESS OF EMERGENCY WARNING LIGHTS
To be effective, warning lights on emergency and law enforcement vehicles must be
patterned to cause maximum reactivity leading to detection of the signal, being of great
enough distraction to catch the driver’s attention. The degree of conspicuousness of a
flashing light depends on a number of factors. Among these are:
1) flash intensity
2) flash rate
3) ratio between the time the light is on compared to the complete flash cycle
4) variation of the intensity of the light flash (“pulse shape” or “waveform”)
5) colour of the light source
6) angular dimensions of the beam
7) area of the light-emitting surface(s)
8) motion of the light source (up and down or side to side)
9) number and spatial arrangement of the lights, and
10) pattern of light flashes (“phase relations”) from the various lights in a multi-
light configuration. (Rubin & Howett, 1981, p. 5).
Colour Use of Warning Lights
The National Bureau of Standard research indicates that the highest intensity of light
from a fixed electrical device is an unfiltered white light; that is, a white unfiltered light is
the light that can be seen at the greatest distance (Rubin & Howett, 1981, 13).
However, use of a pure white light on emergency vehicles is not feasible because of the
multitude of surrounding sources of white lights and an alternating colour is necessary
to distinguish the vehicle as being an emergency vehicle. Because yellow lights are
associated with slow moving and/or maintenance vehicles (Rubin & Howett, 1981;
Ullman 2000) and a “prepare to yield or merge” (Smith, 1991) message, the use of
yellow (amber) lights is not necessarily practical for emergency or law enforcement
vehicles, even though amber lights are “twice as bright as red and four times as bright a
blue” (Smith, 1991, 57). Therefore, an alternate colour such as red or blue has been
used with the white to distinguish the vehicle as an emergency vehicle (Rubin & Howett,
1981; Nixon, 1988). To use the favorable light qualities of yellow light, amber is often
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 5
associated to indicate yield or to provide directional signals and should not be
overlooked as an adjunct to be used in conjunction with red, blue, or white warning
signals (Smith, 1981, 57). However, due to the engrained historical use of amber lights,
amber lights are not useful as a primary colour for law enforcement agencies, as are
associated with traffic behaviours that do not have the motorist pull over or yield the
right of way.
The use of alternating red lights on emergency vehicles has several distinct advantages
due to light qualities and associations. The colour red is most associated with “stop”,
“danger” or “emergency across borders and ethnicity. Further, in fog or smog red
penetrates fine particulates of the haze better than other colour lights of equal intensity
(Rubin & Howett, 1981). During the daytime, red is more visible against a blue sky than
an all light system, while it maintains its appearance of red near the “border line of
visibility, whereas, all other colors tend to fade into white” (Rubin & Howett, 1981, 14).
Blue lights, or alternating blue lights on law enforcement vehicles, have several
advantages. First, it is the standard use of blue lights on many of the international
police vehicles, and the engrained response to the blue light vehicles (Rubin & Howett,
1981, 14). Blue is related to the international policing image, from the colour of lights to
the colour of uniforms. Further, blue light offers distinct advantages during low light
conditions. First, there are few blue lights in the nighttime environment, but the eye
itself favors the detection of blue over red during low light conditions. The rods of the
retina are associated with night vision, and detect blue light more easily than red light.
Conversely, under adequate light conditions, the cones are associated with daytime
vision and twice the amount of blue light energy is required to be perceived as equal to
red light (Smith, 1991). This suggests that the best combination of lights would be red
and blue red for daytime and blue for night time viewing.
Effective Intensity
The greatest determining factor in the complicity of warning lights is intensity (Rubin &
Howett, 1981, 17). Previous National Bureau of Standards research has set the
optimum daytime intensity for primary warning lights on emergency vehicles as follows:
at least 2000 candela2 (cd) for white, at least 1000 (cd) for yellow, at least 400 (cd) for
red, and at least 200 (cd) for an all blue light system (Rubin & Howett, 1981, 17). As
previously mentioned, blue lights are relatively ineffective during daytime, with red and
white lights providing the greatest visibility; therefore, the visibility of the vehicle may
serve the purpose more than the usage of a higher powered blue light (having a highly
recognizable colour pattern on the vehicle itself). Adding red or white light to the light
combination would also increase the visibility of the vehicle. Adding coloured lights
becomes somewhat problematic as Howett et al., (1978) reports that a white 2000 (cd)
incandescent light covered with a plastic or glass filter will only transmit a portion of that
true filtered light colour; a yellow filter will reduce the transmitted light by as much as
50%, a red filter will reduce the light by as much as 20%, and blue filter will reduce the
2 Candela (cd) refers to the unit of luminous intensity of a light source.
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 6
transmitted light by 6%. For light equality, a filtered light will then require a stronger light
source which is not always feasible.
Flash Rate and Duty Cycle
Rubin and Howett (1981) suggest that a flash rate of 90 flashes per minute (fpm), ± 15
fpm for red and white lights is desirable, while a flash rate of 60 fpm ± 10 fpm is
desirable for blue lights. Their recommendations suggest the synchronization of the two
outside lights with a central red and white alternator would be desirable for greater
effect. The emergency warning lights should concentrate their brief flash to 20% of the
total on-off cycle as “a short flash gets more visual effect from the same amount of
energy” (Rubin & Howett, 1981, 16). Non-emergency warning lights should be on
longer than they are off (on 50 to 70 % of the cycle) as such lights are perceived as
flashing off rather than on, connoting a lesser sense of urgency. In the case of law
enforcement, the primary enforcement colours should use a short flash, while amber
warning lights and light bars should use a longer on phase.
Strobe Light and Incandescent Warning Lights
Howett et al., (1978) reported results of studies examining different colour and types of
light. In their research, they indicate that blue lights generates a considerably greater
true blue light with xenon strobe than for the incandescent light, while red and yellow
filtered lights using a xenon strobe rather than an incandescent light provide better
colour. The reason blue light using a xenon strobe works better is because it has a
higher colour temperature of around 5,000 to 6,000 degrees Kelvin which has more blue
light than the incandescent light with a temperature in the 2,000 to 3,000 Kelvin range
(Smith, 1991, 58). In fact, when a red filter is placed over a xenon strobe, most of the
light energy is absorbed by the red filter (Smith, 1991), therefore allowing an
incandescent light operating at a temperature between 2,000 to 3,000 Kelvin to contain
more red light, thus making it more visible.
The use of strobe light systems is effective in long distance warning and poor weather
conditions, but is harder to track visually. Whereas rotating signals provide strong
primary and secondary signals off mirrors, with the light bouncing off of the surrounding
buildings, they provide a longer flash duration allowing the light source to be tracked
more easily (Smith, 1991). This allows the revolving light to provide enhanced visibility
in urban and intersection areas. An additional warning light located 45 degrees in front,
on both sides of the vehicle, is recommended for alerting drivers to the presence of an
emergency vehicle (Smith, 1991).
WARNING LIGHTS USED BY ENFORCEMENT AGENCIES IN CANADA
Familiarity of warning light colour and colour combinations, the vehicles that use each,
and the response expected allows for an automated response by a driver when the
colour combination is detected in the environment. For this reason, as was reported in
the United States (Howett, et al., 1978), lack of uniformity of warning light use by
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 7
agencies of similar scope and mandate become a source of confusion among motorists
and pedestrians.
To examine the use of various light combinations in Canada, various non-police
enforcement agencies were contacted across British Columbia and Canada. The
results of these findings are contained in Table 1 of this manuscript. The general
conclusion is that enforcement agencies that require vehicles to stop, such as
conservation, fish and wildlife, and fisheries model the colour of warning lights used by
police forces. The colour of warning lights seems to be associated with governmental
policy or provincial regulation of legislation.
The predominate colour usage of police and enforcement warning lights, with the
exception of Ontario that uses red, is blue and red sometimes with an amber light
used to alert vehicles to the presence of an emergency vehicle, often used as a traffic
warning indicator. This seems to be consistent with the literature reviewed and what is
recommended in other reports. Consistency across agencies that provide enforcement
is important, as the general public associates the message of certain light combinations
to certain actions. For example, an amber warning light may be associated with caution
or slow down, whereas, a blue and red warning light may be an indication to “pull over
and stop.” The example provided concerning law enforcement in the City of Chicago
best emulates the difficulties enforcement agencies may have if different colours of
warning lights are used by different law enforcement departments and agencies. An
additional example on the confusion of the public may occur on the issuance of blue
lights on snow ploughs in Ontario and Yukon (blue and amber) regions. The general
public, not necessarily aware of various warning lights used by various agencies, may
become easily confused if there is not a standard colour light system used within a
given region.
CONCLUSIONS
Literature would support the uniform use of warning lights on vehicles that require
motorist to perform similar actions. Typically, emergency vehicles have standardized
this in Canada with the use of red and red and white lights, indicating that motorist need
to yield the right of way to such vehicles. However, there is no uniformity among law
enforcement agencies. While most law enforcement agencies use red and blue lights,
motorists expect to pull over and stop when confronted by a vehicle with red and blue
flashing lights. Scientific evidence suggests that red and blue light combinations offer
the best visibility in both low and normal light conditions, and in adverse weather. As
such, the standardization of law enforcement warning lights may be prudent in Canada.
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 8
REFERENCES
DeLorenzo, R.A. & Eilers, M.A. (1991). Lights and siren: a review of emergency vehicle
warning systems. Ann. Emerg. Med. 20:1331-1335.
Dixon, B. (1994). Befogging the brain. British Medical Journal, 308: 725.
Douglas, C.A. (1957). Computation of the effective intensity of flashing lights.”
Illuminating Engineering. 52:641
Howett, G. L. (1979). Some Psychophysical Physical Tests of the Conspicuities of
Emergency Vehicle Warning Lights. Washington, DC: US Printing Office.
Howett, G. L., Kelly, K. L., & Pierce, E.T. (1978). Emergency Vehicle Warning Lights:
State of the Art. Washington, DC: US Government Printing Office.
Nixon, R. A. (1988). The blue light syndrome. Law and Order, 36 (10), 44-47.
Rinalducci, E.J. & Beare, A.N. (1974). Losses in night time visibility caused by transient
adaptation. Illuminating Engineering. 3(4): 336-345.
Rubin, A. I., & Howett, G. L. (1981). Emergency Vehicle Warning Systems. Washington,
DC: US Government Printing Office.
Smith, A.G. (1991). Effective warning lights. Law and Order, 39 (7), 57-62.
Ullman, G. L (2000). What’s flashing before your eyes? Roads & Bridges, 38 (7) 46-49.
Retrieved March 10, 2004 from Academic Search.
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 9
Table 1. Enforcement Agency and Emergency Warning Light Configuration
Resource Agency
Red & Blue
Warning
Lights
Any other colour of
Warning Lights
Previous Colour
of Warning Lights
Federal or
Provincial
Policy
Province of B.C.
Conservation Service
Red/White
Red/Blue
Provincial
Department of
Fisheries and Oceans
X
Amber
Red Alone
Federal
Parks Canada
X
Red/Red
Federal
Customs Canada
Provincial Sheriff
Services
Red/White
Provincial
Provincial Corrections
Commercial Transport
Red/Red/White
Provincial
Provincial Forestry
Red (fireball)
Provincial
Province of AB
Commercial Transport
X
Provincial
Natural Resources
X
Provincial
Province of SK
Conservation Service
X
Provincial
Commercial Transport
(Highways)
X
Red
Provincial
Provincial Corrections
Provincial Sheriffs
Province of MN
Conservation Service
X
Red
Provincial
Provincial Corrections
Red (limited)
Provincial
Research Article Anderson & Plecas
Copyright 2010 - The Journal of Criminal Justice Research (JCJR) - Volume 1, Number 2 Page 10
Commercial Transport
(Highway Inspectors)
X
Amber (light bar)
Red
Provincial
Province of ON
Provincial Corrections
Commercial Transport
(Highway Inspectors)
Red/Red/Amber
Provincial
Conservation Service
Red/Red
Provincial
Province of N.B.
Conservation (Ranger)
X
Red
Provincial
Provincial Corrections
Province of N.S.
Provincial Corrections
Provincial Sheriff
Service
X
Amber
Provincial
Province of NF
Provincial Corrections
X
Red/Red
Provincial
Provincial Sheriffs
Future
Red/White
Provincial
Conservation Service
X
Provincial
N.W.T.
Provincial Corrections
Sheriff Services
Commercial Transport
X
Amber
Yukon
Conservation Service
X
... This study also revealed that amber light falls somewhere between red and blue lights in terms of effectiveness. In another study, blue lights were found to be easily detectable during nighttime and that rates of braking are higher when flashing blue lights are used (Anderson and Plecas 2010). ...
... In 2010, psychologists from Fraser Valley University in Canada reported that habituation or prolonged exposure to certain types of warning lights impacts the compliance rate (Anderson and Plecas 2010). As a result, perennial exposure to amber lights mounted on the construction work fleet might prove to be unfit to promote work zone speed reductions over a long period. ...
Article
Vehicle speed in work zones is a significant concern to state transportation agencies and their construction partners. Prior research indicates that law enforcement vehicles within a work zone with active flashing lights result in reduced vehicle speeds. Placement of flashing blue lights on construction equipment during nighttime construction work has also been identified as a potential control measure to reduce speeds. The goal of this research study is to determine the impact of flashing blue lights on construction equipment on the speed of passing vehicles in mobile work zones. The research design consisted of a controlled experiment involving flashing blue lights mounted on the rear of a paver during mainline paving operations on three case study projects on high speed roadways at night in Oregon. Vehicle speed data were collected during multiple work shifts for each case study, both with and without the flashing blue lights on. The experimental results reveal that vehicle speed is affected by the presence of flashing blue lights during nighttime work. With the blue lights flashing and at distances upstream of the paver where the driver can see and react to the blue lights, mean vehicle speeds tended to be 4.3-25.7 km=h slower than when the blue lights were off. Closer to, immediately adjacent to, and downstream of the paver, the difference in the mean speed with the blue lights on was typically less or not significant; in some cases and mostly downstream of the paver location, the mean speeds were higher with the blue lights on than with the blue lights off. The speed differential (i.e., the speed prior to the work zone minus the speed in the work zone) was measured as 4.8-11.3 km=h greater with the blue lights turned on than with the blue lights off. The results support construction contractors and state departments of transportation for decision making about the use of flashing blue lights on equipment in work zones. The researchers recommend using flashing blue lights on paving equipment during nighttime operations if allowed by local laws to reduce the speed in the work zone and increase worker and driver safety. This research contributes to the construction industry by providing quantitative evidence of the extent to which flashing blue lights mounted on paving equipment impact vehicle speeds in work zones. Recently, all stakeholders, including the Oregon Department of Transportation, law enforcement, and contractors, have agreed to use the flashing blue lights on paving equipment in Oregon during the 2021 construction season.
... Amber light falls somewhere in between red and blue light in terms of effectiveness (Howell et al., 2019). Another study reported that, blue lights are easily detectable during nighttime (Anderson & Plecas, 2010) and that rates of braking are higher for flashing blue lights. ...
... In 2010, psychologists from Fraser Valley University in Canada reported that habituation or prolonged exposure to certain types of warning lights impacts rate of compliance (Anderson & Plecas, 2010). As a result, the researchers conclude that perennial exposure to amber lights mounted on the construction work fleet may prove to have reduced effect on speed reduction over a prolonged period of time. ...
Technical Report
Full-text available
Vehicle speed in work zones is a significant concern to ODOT and construction partners. Prior national research shows that law enforcement vehicles located within a work zone with active flashing blue lights result in reduced vehicle speeds. Placement of flashing blue lights on construction equipment has been identified as a potential control measure to further reduce speeds. The goal of this research study is to conduct an initial evaluation of the impact of flashing blue lights located on construction equipment on the speed of passing vehicles in work zones. The research design consisted of a controlled experiment involving flashing blue lights mounted on the rear of the paver during mainline paving operations on three case study projects on high speed roadways in Oregon. Vehicle speed data was collected during multiple work shifts on each case study, both with and without the flashing blue lights on. The experimental results reveal that vehicle speed is affected by the presence of flashing blue lights. Speed differentials between the road work ahead sign and the first exposure to the paver resulted in greater speed reductions in all three case studies when the flashing blue lights were on. Additionally, within the active work area at distances upstream of the paver where the driver can see and react to the blue lights, mean vehicle speeds tended to be lower when the blue lights were on. Closer to, immediately adjacent, and downstream of the paver, the amount of reduction in mean speed was typically less or none at all with the blue lights on; in some cases the mean speeds were higher with the blue lights on. Generalization of the results to all projects with a high level of confidence is limited given the low number of case study projects and the presence of confounding variables.
Article
Research on transient visual adaptation is concerned with the quick changes in sensitivity which the eye makes as it adjusts to variations in luminance levels. Investigations reported here provide a description of transient adaptation effects at luminance levels comparable to those found in nighttime highway lighting conditions. Findings indicate that for downward changes from a low luminance to an even lower one, smaller losses were observed than those found at higher luminance levels. Visibility loss breaks down at the lower limit of 0.002 to 0.02 fL (0.0069 to 0.069 cd/sq m). Individual differences in the amount of visibility loss for a given luminance change, whether upwards or downwards, were found to be large. Within the limits examined, the size of the target has no effect on the degree of visibility loss as measured by the interval between the beginning of the transition from one background to another and the onset of the test-letter presentation.
Article
I've just returned from a round trip on the M25 around London, a total distance of 100 miles (160 km) or so from junction 16 to junction 28, which I negotiate at least once each week. It was foggy, exceedingly foggy in places, while elsewhere there were patches of mere mist and several areas with clear air.The motorway is well served with fog warning lights, and someone somewhere was obviously busy at the controls. The problem was this. The illumination of the signs bore only the most random relation to the density of the fog. There was warning after warning in regions of sunshine and clarity, yet nothing whatever amid murky areas with the poorest visibility of all. Occasionally the fog happened …
Article
Emergency medical services providers routinely respond to emergencies using lights and siren. This practice is not without risk of collision. Audible and visual warning devices and vehicle markings are integral to efficient negotiation of traffic and reduction of collision risk. An understanding of warning system characteristics is necessary to implement appropriate guidelines for prehospital transportation systems. The pertinent literature on emergency vehicle warning systems is reviewed, with emphasis on potential health hazards associated with these techniques. Important findings inferred from the literature are 1) red flashing lights alone may not be as effective as other color combinations, 2) there are no data to support a seizure risk with strobe lights, 3) lime-yellow is probably superior to traditional emergency vehicle colors, 4) the siren is an extremely limited warning device, and 5) exposure to siren noise can cause hearing loss. Emergency physicians must ensure that emergency medical services transportation systems consider the pertinent literature on emergency vehicle warning systems.
Computation of the effective intensity of flashing lights
  • C A Douglas
Douglas, C.A. (1957). Computation of the effective intensity of flashing lights." Illuminating Engineering. 52:641
Some Psychophysical Physical Tests of the Conspicuities of Emergency Vehicle Warning Lights
  • G L Howett
Howett, G. L. (1979). Some Psychophysical Physical Tests of the Conspicuities of Emergency Vehicle Warning Lights. Washington, DC: US Printing Office.
Emergency Vehicle Warning Lights: State of the Art
  • G L Howett
  • K L Kelly
  • E T Pierce
Howett, G. L., Kelly, K. L., & Pierce, E.T. (1978). Emergency Vehicle Warning Lights: State of the Art. Washington, DC: US Government Printing Office.
The blue light syndrome
  • R A Nixon
Nixon, R. A. (1988). The blue light syndrome. Law and Order, 36 (10), 44-47.
Emergency Vehicle Warning Systems
  • A I Rubin
  • G L Howett
Rubin, A. I., & Howett, G. L. (1981). Emergency Vehicle Warning Systems. Washington, DC: US Government Printing Office.
Effective warning lights. Law and Order
  • A G Smith
Smith, A.G. (1991). Effective warning lights. Law and Order, 39 (7), 57-62.
What's flashing before your eyes? Roads & Bridges
  • G Ullman
Ullman, G. L (2000). What's flashing before your eyes? Roads & Bridges, 38 (7) 46-49. Retrieved March 10, 2004 from Academic Search.