This is worth the read, we'll hit the points after the jump:

Effective January 1st 2017, examination of tinted or shaded windows will be required during motor vehicle inspections.

• The windshield and front side windows cannot block more than 30% of the light.
  • Seventy percent or more of the light from the outside must pass through the window.
• This law also applies to the rear window unless the vehicle has outside rear-view mirrors on both sides.
  • The mirrors must give the driver a full and clear view behind the vehicle.
• Rear side windows must also allow at least seventy percent of light from the outside to pass through the window if the vehicle is classified as a
  • station wagon
  • sedan
  • hardtop
  • coupe
  • hatchback

That's direct from NYS DMV, as linked. So what's it mean? Well, technically the law has always stated that tint outside of the mentioned range, is illegal. However, prior to 1/1/2017, it wasn't evaluated during a NYS Inspection. Now it is, so once a year, you'll get scrutinized for having it.

Most vehicles have between 15-25% light blocking tint from the factory. Plenty of larger vehicles like, say, a Tahoe or Suburban, will have much darker factory tint. You'll notice a few stipulations in the above law:

Tinted Rear Window- "...unless the vehicle has outside rearview mirrors..."

• Every vehicle for sale in the US in the past 50yrs has sideview mirrors. So you can definitely tint the rear facing glass.

Rear Side Windows - "...vehicle is classified as a..."

• You'll notice that registration class 'SUBN' isn't listed there. A common classification for vehicles with 3rd row seating, allowing for darker tint on the windows behind the driver and passenger fronts. Just check your registration.

What this means, in practice, is that you can legally add tint to your factory glass, so long as you don't breach the above specs. If you happen to have tint that's darker than that, there's high likelihood that you'll need to remove it in order to pass inspection. The NYS DMV does make allowances for medical exemption, but you'll need to have the MV80W form filled out by your doc.

So, why would someone opt to run darker tint than legally allowable, if its against the rules? Setting aside the non-moving violation potential and commentary of 'law-abiding citizens', or 'government overreach'.... we get into:

• Privacy - reducing the chance of a break-in while parked in a public lot, for example
• Reducing sunlight intensity affecting driver eye strain
• Reducing in-car temps
• Eliminating harmful UV rays hitting the kids in car seats
• Adding a layer of safety in the chance of a crash, by reducing the chance of glass going flying into the cabin.
• Good looks. Lets be real, cars are like fashion for many people. Gotta 'look back at it' and like what you see.