Back in the late 1950's the Chicago Police dispatched on a radio frequency of 1500 KHz. This frequency was receivable on most AM radios. You know the kinds, the ones that have Police marked on the dial.  Now a days you won't find anything there but it was a busy channel.

Listening one afternoon, I heard the dispatcher advise "ALL CARS, effective immediately your roof top light will be activated for the next 24 hours for traffic safety".

Seems  that an effort was made by the department to reduce accidents during holidays by letting everyone know of the police presence.

So here are a few hundred cars driving around for 24 hours with the red roof lights flashing.  You will never guess what happened.

Eventually, drivers began ignoring the emergency vehicles with activated lights when they were on real emergency runs.

When O.W. Wilson took charge in 1960, things changed. He realized the problem that RED lights didn't have the effect they should have, the populace was 'dumbed' down by their mis-use. He also wanted something to distinguish the CPD from other Departments.   

Hence a law was passed in the state legislature allowing a Police Dept of a municipality of 500,000 or more to utilize BLUE lights.   Lets see, Ooop's only Chicago fits that definition...how convenient.

That law stands to this day.

OW made some other changes for his department, one we are interested mostly here is the pulsating siren. Not the common electronic one heard now a days, but a mechanical siren which pulsated. A non-electronic Whoop Whoop siren which in my opinion was better than any electronic siren heard today, and of course for OW's purposes immediately identifiable as a Chicago Police Vehicle, not an ambulance or fire vehicle...but a CHICAGO POLICE vehicle..

Another item, not associated with vehicles but just as interesting was the popular checkered hat.  At the time there were many people wearing the common police hat, security guards etc often mistaken for Police Officers. 

OW had a municipal ordinance passed which allowed only members of the Chicago Police Dept to wear hats with checkers. He got the idea while visiting England and observing their police uniforms. Since it was a municipal Ordinance and not a state law, several suburban departments have adopted the checkered hats for their uniforms.