Hey
Is it possible to make reflector material like police cars/officers and fire trucks have?
Like this:
It starts glowing when you spot light on it at night, and at the day it looks like normal white.
Like this:
Thanks for help 
nobody knows?
It’s called a retro-reflective material, and it’s key characteristic is that it reflects nearly 100% of the light it receives. Unlike a chrome or glass material however, a retro reflective one is designed to scatter this reflected light, such that the material appears to glow when viewed from a vehicle (with lights on) or taken from a camera (with flash on).
I haven’t tackled this in UE4, but I imagine you’d either overdrive the base color so that diffuse lighting is pronounced, or somehow get a shadowed albedo term to drive emissiveness.
If my math is correct (disclaimer: it probably isnt), PBR materials such as they are in UE4 reflect 10% of the light that hits it.
So you have good news and bad news there. The good news is that UE4 has a built-in multiplier for this 10% reflection: specular. You can multiply this between 0-1, where 0.5 is the default value. The bad news is that, unlike the emissive material input, it’s clamped to 0-1. That means to get the output you want you’d have to get in to the source code and un-clamp the specular material input and then plug (I’m guessing) ‘500’ in to specular.
Don’t take me at my word though.
I guess if you were smart about your Eye Adaptation and your ambient lighting conditions you could fake it with an emmisive material that takes in a fresnel function to mask out side view angles.