hmm, I don't know. State laws seems to say they MUST have a red flashing light (though maybe they do not need to use it unless they are running intersections of something?) and that police are also allowed an additional blue light:

>§ 4571. Visual and audible signals on emergency vehicles.

>(a) General rule.--Every emergency vehicle shall be equipped with one or more revolving or flashing red lights and an audible warning system.

>(b) Police, sheriff, fire and coroner or medical examiner vehicles.--

>(1) Police, sheriff, coroner, medical examiner or fire police vehicles may in addition to the requirements of subsection (a) be equipped with one or more revolving or flashing blue lights. The combination of red and blue lights may be used only on police, sheriff, coroner, medical examiner or fire police vehicles.

So my guess is they probably have a red light also but only use that one when, in addition to flagging down a specific motorist, are also trying to warn traffic around them? Blue is for you and means stop and red is for everyone else and means danger?

edit: I did find this:

>Most police vehicles primarily use red and blue lights – the red part here signifies that an immediate emergency is taking place, while blue police lights signal that police, and not other first responders, are present. In many cases, squad cars also use white lights. What do different color police lights mean?

So it seem, blue=police, red=emergency. red+blue=police emergency

edit2: I also found this:

>In Pennsylvania, the display of only blue light is reserved as a courtesy light for volunteer firefighters when responding to an emergency incident and/or fire house. There may not be any other colors present and no more than 2 flashing or revolving lights/bulbs, further it must have 360 degrees of visibility. Civilians are not required but are generally encouraged to move over when only blue light is present. https://en.wikipedia.org/wiki/Emergency_vehicle_lighting#Blue_4

note: I think "only blue" means no white flashing lights as well

Sounds like a volunteer fire fighter, which typically use blue lights on personal vehicles

Interesting aside: They just passed legislation permitting tow trucks to utilize rear-facing flashing blue lights during roadside recoveries. Apparently, this change was spurred by motorists not paying sufficient attention to amber lights alone.

Cops and fire trucks still have red lights. The rest of the lights are volunteers and local yahoos.

PSP fleet currently has Whelen and they use Trio modules on corners that do R/W/B so it can alternate red/blue on slide 3 and do white for scene and alley lights , local departments vary the next town over from me got a new PIU and got a “good deal” on Federal Pathfinder siren and lighting system ( chinesium baked in ) another department really cheaped out and went code 3 , I have seen some crazy shit on K-9 units running blue ( they haven’t caught shit from homeland YET but rest assured it is coming ) randomly police and fire departments will get wild hairs up there ass and defy there own state laws and lighting conventions , departments are like individuals and try to slide things by and see if they can get away with it ,they get caught and it costs taxpayers stupid money ( back in the early 90’s I know a department that purchased and issued Uzi sub machine gun to each officer , ATF shut that down and confiscated every weapon before anybody got hurt ) blue and white is just stupid game playing risk taking by odd ball departments , as I don’t what agencies cars maybe involved , nothing is a certainty , I have shouted out to agencies that unfit emergency vehicles asking if anybody has done any blue white only but so far nobody in NY,NJ , or PA has built any that I know , if I find anything specific I will update you such as who built them and for who , I know guys from Long Island to Long Beach . There is also the possibility of malfunction or some DIY idiot hacking the brain and not knowing what there doing , that happens a lot , most of the major manufacturers have an encryption lock option and it prevents DIY tampering or corrupting the programming . Since programmable digital flashers you literally program what each LED can flash and when . There is a company in Texas called Feniex that markets a 4 color LED light head unfortunately they are very low quality ,and flawed versions of products get discontinued/orphaned by this company almost monthly .

In the Philly area it is still red and blue.

I don't think there's been any change

Pennsylvania police lights are exclusively to be red and blue. White is optional. Possibly you saw a car with faulty lights. An agency that installs only blue runs the risk of being mistaken for volunteer firefighters, which they wouldn't do for liability reasons. With the advent of LED lights, it's becoming a shitshow, varying quality, color, intensity, placement on vehicle, etc., plus every plow jockey and construction foreman has them installed. Police tend to reuse light systems on newer vehicles to save money, possibly this could explain it? Couldn't replace red obsolete bulbs or something? Might want to call and ask that department.

Gettysburg does red/blue.

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The way our eyes adapt to the dark. Its actually "easier" for human eyes to see blue light spectrum at night. Whereas red light is more pronounced during the day

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Hey OP… where the lights actively on and flashing? Since the lights are LED now, the housing/light bar can be clear and not necessarily the color the light will be when flashing.

What I want to know is why are some cars driving at night with a solitary blue light on the windshield by the rear view. Vehicles look like regular passenger vehicles, not fire police. And often appear to not be in a hurry.

State police all have red and blue where I am

Here is something interesting i found:

In the UK, police cars use blue lights with additional white lights at the front and red lights at the rear to help other road users determine direction of travel. The rear red lights can be switched on and off separately from blue lights to indicate caution.

I thought we figured out that red, and red shifted light can cause aggressive moods and reactions to some senative people so I was expecting to find that they stopped using them, but i have not seen this anywhere that I have researched so it is probably just a me thing, and not supported by evidence.

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I would've thought maybe they were PA Constables, but if it's a fully marked (Livery?) police cruiser, I have no idea.

This is fake news.