By on September 30, 2013

Sorry for the tease but to get the full effect of this post you’re going to have to click on Read More. It’s not that we want the additional clicks, it’s just that I’m using a graphic to illustrate this post that is so eye-searing that the layout and graphic designer in me just couldn’t put it on the front page above the break.

Once you do make the jump, you may have trouble focusing on the text in the image below. That’s because of a phenomenon known as chromostereopsis, which the American National Standard Institute (ANSI/HFES-200, Part 5) defines  as “the perception of depth resulting from the close proximity of two colors of disparate wavelengths”. There’s a good explanation of chromostereopsis here. Because of where in our eyes the receptors for different colors are, and how our eyes focus, we perceive different colors as being at different distances. Printers and others who do graphic layout have long known that because they are at opposite ends of the spectrum, it’s not a good idea to use blue letters on red backgrounds and vice versa. Most people perceive blue as closer than red, and as a result the human eye cannot focus on both red and blue at the same time, causing the optical illusion of blurry letters in the graphic below.


I apologize for for the eye strain but I was literally trying to illustrate a point. It could have been worse, I could have made it a flashing, animated GIF.  To remove that visual abomination, click on read more.

Isn’t that better?

Back to the topic.

In addition to  chromostereopsis, as LEDs have proliferated, people have come to realize that its harder to focus on pure blue lights than on any other color. Our retinal receptors are known as rods and cones. Visual acuity comes from rods and is mostly a black and white phenomenon. Color is added by cone receptors. Rods are sensitive mostly to light in the yellow-green part of the spectrum. Pure blue light doesn’t activate rods sufficiently for clear vision.

Flashing blue lights make it hard to focus but flashing red and blue lights together is an even worse idea. To begin with it makes it hard to estimate the distance of an emergency with flashing red and blue lights. More dangerously, when your visual system is being flooded simultaneously with bright red and blue lights, the effect is almost blinding, certainly visually confusing. It’s a problem for motorists but it seems to me it would create an even more dangerous situation for police officers who have to make out shapes and distances in visually confusing lighting situations.

So why do police cars use blue lights in the first place and even worse, red and blue lights together? I suspect the reason is partly historical. In some states police used red lights and in others blue lights. It made sense for manufacturers to offer units with both colors. However, I think the main reason is exactly chromostereopsis. I think the companies selling emergency lights have to be aware of the phenomenon, and they wanted to come up with lights that would surely get your attention. Does the sign above get the attention of your visual system?

A while back I contacted a handful of companies that manufacture and supply emergency lights to police agencies abpuit chromostereopsis but none would comment. It’s not as though the phenomenon is not well known. Just about everyone who works with color knows not to do two-tone with red and blue.

So why do police cars do use red and blue flashing lights?

Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can dig deeper at Cars In Depth. If the 3D thing freaks you out, don’t worry, all the photo and video players in use at the site have mono options. Thanks for reading – RJS

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48 Comments on “Why Do Police Cars Use Red & Blue Lights? They’re Visually Confusing...”

  • avatar

    I thought it worked the other way around? That is, on a dark night when coming up on a parked police car, flashing blue-only lights mesmerizes drivers and makes distance estimation more difficult, that a few nitwits among us suffer from a “moth effect” of mono-color flashing lights (mindlessly drift out of the travel lane and crash into the stationary cruiser), so therefore blue+red is better. (Changing driver’s license requirements to meaningful standards would… oh, but I digress.)

    Or is this really just a ruse by the one or two light bar companies, like the brief era when some fire engines were painted yellow? (All because some paint company published a study “proving” that paint color was more important than the Christmas tree of strobe lights on emergency vehicles).

    • 0 avatar

      That yellow/green color on fire trucks was very effective for visibility in daylight or dark. The color stands out among all the other vehicles and made driving in traffic a little easier as other drivers had more than a siren to go on even with the flashing red lights of the day. HOWEVER, what few realized at the time is that the same color effectively vanished in fog I don’t mean that the mist merely hid it, the color was effectively camouflage that made it impossible to see even at distances where grey and white cars would start to become visible. That traditional red stood out much more clearly. As a result, any environment that was susceptible to fog had the choice of using colors that were highly visible only in ‘ideal’ conditions or colors that were visible in poor conditions. Since the same trucks have to operate in all environmental conditions, most communities returned to the traditional red, now using many more LED/strobe lights for that eye-searing attention grabber.

  • avatar

    The answer is very simple, ITS TO SLOW PEOPLE DOWN… When they are on the side of a road or responding to an emergency the public is supposed to yield to them. Where I live, there is a section in the MV&T laws discussing the different colors and what colors are reserved for what agencies etc. Blue was only reserved for volunteers. Recently studies were shown that blue is easier to see further away therefore there was a change and now police and fire vehicles are allowed to use blue in conjunction with red only out the rear. No forward facing blue allowed except for volunteers. Our society has become too impatient to slow down when they see an emergency vehicle on the side of the road therefore agencies are resorting to new ways too get motorists attention.

  • avatar

    Red was always used when I was a kid. The phenomena of red & blue lights came about – when? late 60’s-early 70’s?

    The police lighting used now, LEDs I suppose, are incredibly bright.

    For example, when driving south on I-75 to go to the ballgame yesterday, on the north side, what seemed fairly close, were flashing blue lights. Someone was pulled over. Thing is, what seemed pretty close was actually OVER TWO MILES AWAY! It’s amazing how bright those things are!

    Almost as bad as those obnoxious blue-tint headlights used on some cars. It just doesn’t seem right to me, and I won’t have them.

  • avatar

    This effect is precisely why I could never buy any VW of Golf IV vintage. The Great Engineer Ferdinand Piech dictated that Volkswagens should have blue numerals in the dashboard (known as “Piech Blue” amongst those in the know), with red indicator dials. What a numbskull. Given that chromostereopsis is generally amplified by glasses (I’m shortsighted), I would spend whole trips in a rental Golf either trying to keep my head still to prevent that generally crucial information from swimming around in circles, or actively trying it out.

    That theme was beautifully carried over to the heating controls (deep red and deep blue colors next to each other). By turning my head I could get the blue to sit outside of the heater dial. Freaky.

    I’m glad to see that this trend was summary stopped and interiors seem to be getting more sensible.

    • 0 avatar

      Additional nominees for the Lighting Hall of Shame are BMW, Pontiac and others for using red instrument panel lights. I heard it was in emulation of aircraft using red lighting to preserve pilots’ night vision–but that was discredited in later studies.

      I think that cop car light bar is an attempt to beat the Close Encounters of the Third Kind spaceship at its own game.

      By the way, unmarked cop cars compound the problem of being recognized at night by motorists coming up on an accident scene. Hoist on the own petard, so to speak.

      • 0 avatar

        I prefer the red dash glow in my Pontiac and also appreciated it in my old Audi, feels very Darth Vader-ey.

      • 0 avatar

        It hasn’t really been debunked. Red light is still the best light for quick adaptation to darkness and sensitivity – the Navy report admits that. For stargazing, it’s still the best.

        The Navy had a particular use case for its sailors: spotting silhouettes in the distance. That doesn’t require maximum sensitivity or adaptation (according to the report, anyway – I would’ve thought it would). And the Navy’s conclusion wasn’t that white light worked better than red for that purpose, but that it was almost as good, and didn’t have any of the negatives that red has (crew comfort).

        Unless you drive without headlights, you don’t need maximum light sensitiviy while driving so there’s little gain in using red – but it’s still more good than bad.

    • 0 avatar

      I hate that you cannot turn dash lights off anymore both for my personal preference and because of the idiots who drive cars with only the front driving lights on at night and then wonder why everyone is trying to perform “a gang initiation” on them by the other drivers flashing head lights at the idiot offender…

  • avatar

    the poor ability of human vision to focus on blue light is why I think anyone who bungs HID capsules into housings designed for halogens should be beaten savagely.

    The French weren’t just being French when they mandated vehicle headlamps have selective-yellow tint.

    • 0 avatar

      HIDs in the proper color temp (i.e. 4300K or lower) aren’t inherently annoying, it’s the nudniks who insist on going higher in the color temp scale into the blues and violets to be “different”. Having said that, I detest HIDs (even the proper color ones) hacked into a headlamp housing not designed for them. All it does is create aiming and glare issues.

  • avatar

    I am surprised that there is not some national standardization about the use of colors — in West Virginia, police vehicles use all blue lights (State police, county sheriffs, local PDs), and fire departments use all red lights. This is governed by state law. Other emergency vehicles are supposed to use amber lights, but more and more I’ve seen amber and white strobes, or in some cases, white only, which is not technically legal. State law also permits purple for funeral cars.

    I too wonder where the idea of red AND blue came about – or red used by police cars – or yellow or lime green fire trucks, they should always be red! :)

    • 0 avatar

      It about freaks me out now to visit states with cop cars running red lights.

      In Tennessee, generally the lighting scheme is blue for law enforcement, red for fire/emergency/ambulance, amber for service vehicles (tow or rollback trucks, other service “accident clean up” type vehicles), and sometimes green for funeral procession lead cars and private security cars (mall security etc). This makes it easy to identify what is going on with the car/truck at the side of the road. That said, a few certain towns do have red/blue for their police vehicles which is off the wall in comparison to general use.

      Where I grew up in NJ, at that time, there were no standards other than green dash or roof magnet mounted strobes for volunteer firefighters in route to a call in their private vehicles.

    • 0 avatar

      In Texas, Blue and Yellow together is for construction/utility vehicles (they use yellow only in other places I’ve lived). I still can’t get used to blue meaning anything other than Florida Highway Patrol.

  • avatar

    In the Commonwealth of Virginia, police vehicles used blue lights only; other emergency vehicles use red lights only.

    France mandated yellow headlamps and fog lamps in the 1930s, but the scientific underpinning of that law has been shown to be faulty.

    See Daniel Stern, an expert of automotive lighting:

    The specific hue of yellow used in headlamps is selective yellow.

    Here’s a YouTube video on white versus yellow headlamps:

    What did we do before the internet?

  • avatar

    I used to wonder why the Matchbox police cars I had when I was a kid had red lights and the fire trucks had blue lights. Then I learned that Matchbox was (at the time) a British company and that was how they looked over there.

  • avatar
    Kyree S. Williams

    With the windows rolled up, I never see or hear an emergency vehicle approaching. I have to take my cue when other motorists start pulling over.

  • avatar

    Interesting.. and why do NY police upstate have red lights only while I see commoners with blue lights affixed on top of their vehicles? I just noticed this over the weekend on a bike ride thru Lake George. Any New Yorkers care to explan?

    • 0 avatar

      Blue lights affixed to the top of vehicles in the State of New York are reserved for volunteer firefighters. All police used to be red with clear out the front and red with yellow out the rear but now can use red with blue out the rear which was a recent change (within the past 2 years or so) Not all departments have switched over yet.

  • avatar

    This is why in New Jersey we have the infamous “move over” law. At night they are absolutely blinding and capable of being seen from Mars. The many times I drove past a patrol car at night and at the last second see the cop or his door open. No wonder they get killed every now and then. The prepubescent adults who design all the blinking license plate lights and mirror lights and grille lights… sheesh! I never had a problem spotting a red gumball in the 60,s. Why do we need 1000 watts of LED strobes to blind oncoming traffic?

    • 0 avatar

      Here in Quebec, we also have the move over law since less than a year, but since this law, every time a see a cop on the road, he is no longer on the side of the road, they basically park in the lane. And also, always after a hill, curve..
      Seems like they are just being dicks about it.

      I agree that they need space, but they are just really overdoing it to the point that I believe is more dangerous than it was before.

  • avatar

    Blue lights affixed to the top of vehicles in the State of New York are reserved for volunteer firefighters. All police used to be red with clear out the front and red with yellow out the rear but now can use red with blue out the rear which was a recent change (within the past 2 years or so) Not all departments have switched over yet.

  • avatar

    Because it’s prettier, flashier, and more shiny than just using one color? Cops seem to take a great deal of pride in showy, flashy things…

    • 0 avatar

      Yeah, and blue is also the most ‘macho’ color. So, it goes without saying that the ‘boys in blue’ are going to go with blue lightbars, regardless of the actual scientific rationale.

      Can you imagine if it were proven that the colors pink and purple were shown to be the most safe and effective in getting people’s attention? Does anyone really believe a police department anywhere in the US would actually install pink/purple lightbars on their cars? Ironically, flashing pink/purple lights on the side of the road would (at least initially) have a huge impact on drivers to slow down, simply due to the ‘what the hell is that?’ effect.

  • avatar
    Joe McKinney

    According to the research I have read, blue lights are more visible at night and red lights are more visible in daylight. Having both colors in a lightbar gives the vehicle maximum visibility in all lighting conditions.

    As far as I know there is no reason for the dazzling, flying saucer light shows seen on many modern lightbars other than the LED and microprocessor technology is now affordable and readily available.

    • 0 avatar
      Joe McKinney

      Here is link to a PDF file showing the results of an extensive study of emergency vehicle lighting conducted by the Florida Highway Patrol. This study led to the adoption of the FHP’s current blue and red LED lightbar.

  • avatar

    That visual example was quite effective! It seems like police cruisers on the side of the road are being hit more often; I wonder whether the combination of the color selection mixed with the overpowering brightness is a contributing factor.

  • avatar

    It really seems you answer your own question–the combo is harsher and forces a reaction from drivers, which would seem to be the desired effect. I’m sure the sound of sirens are designed to be similarly annoying/debilitating.

  • avatar

    LED’s use far less power and produce far more light than traditional bulbs. Red and blue lights have a long history of association with “law” and “emergency” and amber lights with non-emergancy official use.
    Tradition is hard to change I guess.

  • avatar

    Finally one aspect of every day life where my colorblindness renders me a small advantage. maybe. or not. ah hell I can’t tell….

  • avatar

    I agree with your points, but suspect most if not all of the blurriness I see in your example text is JPG compression.
    -fellow graphic designer.

    • 0 avatar


      There’s an example on Wikipedia, and I find it to be perfectly crisp to the eye, although not all that pleasant to look at because it’s super saturated and indeed, blue and red are poor colours to pick for foreground and background colours.

      The JPEG abomination posted on this thread looks blurry because it is. You can zoom in with any editor you prefer and see the sea of JPEG artifacts.

  • avatar

    I agree about the colors. But we’ve been with the traditional red/blue for a long time now, nothing new to see there. What’s changing, for the worse, is the rapid on-off capabilities of the LED lights coming into recent use. The other night I passed a parked police car. Several sets of lights were blinking with furious speed, making a sight so complex that it was difficult to understand even where they were placed on the car. Why? Because they can be. (That’s the same reason that the obnoxious new roadside video billboards are so blindingly bright at night.)

    I place the excessive lighting of cop cars in the same pail of blame with train horns that have grown much louder, by federal regs. Nobody ever lost their job for specifying more safety, with louder warnings, etc. That must be the reason, right? Because otherwise, the cops must be intent on creating fear and panic wherever they go…

  • avatar

    It is absolutely blinding to be anywhere near those lights. It is also impossible to see if there is a trooper near their door, or stepping out into the road etc, at night. I have no idea who thought these lights were a good idea.

    I’ve had a trooper come up behind me at night when I was passing in the left and he flashed his lights. I thought the god damned world had exploded. He was lucky I didn’t slam on the brakes.

  • avatar

    Another interesting fact is that during the day, the color red is much easier to see. At night, blue is much easier to see.

  • avatar

    Funny, some fire departments are putting a single “gumball” blue light on the left rear corners of their trucks. Odd, but not terribly annoying.

  • avatar

    I’ve noticed the Ohio Highway Patrol going to all-blue lighting, especially on the newer Chargers. (I may have seen one or two of the last of the CVPIs with that as well.)

  • avatar

    Okay Ronnie what other colors would you give them? Cops cars also have seatbelts – they don’t get used except pursuit. Probably beyond my time when sensors clear traffic. Light & sound may become way tamer. Back to a quaint digital recording of electric bells for pedestrians & cyclists.

    Matchbox? British was always all blue. No red except Scottish police boxes.

  • avatar

    On the railroads, a red flag was a signal to stop, while a blue flag was used as a safety warning to protect workers on the line.

    Traffic light color choices came from the railway flags, and I would presume that the color choices of emergency vehicle lights share the same origins.

  • avatar

    Not only the human eye is impaired when faced to red and blue. The latest camera sensors are also challenged by two colors so far apart in the spectrum delivered via fast switching LEDs getting brighter and, most recently, surface mounted to emit their beams at the most effective angle with no need for reflectors. Cameras in my truck perform miserably around cop cars. HID headlights (even in halogen housing) do not pose a problem as long as the camera features Wide Dynamic Range(WDR). If you tell a cop you’re almost blind because of the flashing lights you’ll get the question: Have you been drinking tonight? I thought they were immune to their own lights, like snakes to their own poison…

  • avatar

    Back in the day, Tulsa PD ran reds only on their cruisers, which were black with white roofs and doors. This ultimately resulted in a couple of pretty nasty crashes where drunks tried to drive between the red lights, thinking they were, well, red lights (why the drunk wasn’t STOPPING for the red lights is another matter, probably related to them being drunk…), and crashing heavily into the stopped cruisers.

    Outcome: TPD switched to red & blue lights, and painted the deck lids white.

    In Michigan, it was pretty simple: red only = fire, ambulance, or State Police (at least in the rural areas where they run the trademark single overhead rotating lamp). Red/blue = local police/sheriff. Yellow = wreckers and DOT/maintenance vehicles. Purple(!!) = lead car in a funeral procession.

    When I moved from Michigan to Texas 5 years ago, had to recalibrate (something I’m still doing, frankly), as TXDOT and other road-maintenance vehicles ALSO use red/blue lights.

    And yeah, Ronnie, they’re not designed to be focused on, they’re designed to get your attention (irritation does that) and prompt you to slow down/move over/both.

  • avatar

    Every day I look at the piles of used light bars removed from our various Police , Fire and other Fleet vehicles and wonder : who’s getting rich on all this junk ? .

    We change them out faster than they can change the color specs on us .

    For a few years , night time road construction signs had blue lettering that turned yellow when hit by the rising sun , they were dropped fairly fast but they were really easy to read from far off , much more so than the normal yellow signs used at night .

    I wonder why they had to stop using them .


  • avatar

    The article does express the fundamental nature of the the color combination but it also leaves out many key points. Red is a Hot Color, Blue is a Cold Color. When the two colors are intensely flashed asynchronous, the two hemispheres of the brain become disoriented, which creates an emotional fight or flight response. This intense “Strobe” light effect has also been known to cause seizures in many people which makes absolutely no sense whatsoever if you don’t want motorists susceptible to having a seizure, driving a motor vehicle into the back of your parked cruiser. The next suggestion is that scientists have concluded that the RED/BLUE color scheme can play a major roll in social perceptions and Psychology, this is why in politics you will notice the Blue-Democratic and Red-Republican plastered all over the nation prior to an election. Thirdly the colors red and blue are utilized in 3D glasses and affect the brains understanding of what it’s viewing. These people have been tinkering with this stuff so long they know exactly what frequencies/Durations to use and for what situations…it’s very complex and that is most likely why “They” won’t share a great deal of information.

  • avatar

    Terrible article. The actual truth is that studies have shown red lights are most visible during the day and blue most visible at night and amber the best compromise. A mix of red/blue is the best all around. Not being able to judge the distance to the source (even if that were true) is a non-issue to any driver who isn’t a moron. As soon as emergency lights are seen you should start slowing down and be prepared to stop.

  • avatar
    Peter Burton

    This article is wrong in so many ways. As ‘TheJB’ wrote, red has been proven to be the better color in the daylight and blue the best color for night. Amber is the best ‘all round’ color but it’s a warning color for work vehicles, not a color for emergency vehicles. Where I live ALL ambulance, fire and police vehicles have been using red/blue beacons for well over ten years under
    Workplace, health and safety reasons..

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