I would also extend this idea to windows in the home. I have windows that face west that would benefit from an auto-tint.

There is a "window treatment" that involves a certain gas substance between two panes of glass. The gas is activated by a button/switch that allows the user to fill up the glass with gas, making it opaque, OR trap the gas in a hidden compartment, making the glass clear.

If it were possible to make the window almost opaque, it might be possible to keep the car (or home) interior cooler on a sunny day, saving energy that would be used on air conditioning. Also, it might be interesting if the windows could be opaque from one direction and clear from the other.

Laws against tinting windows have almost nothing to do with the driver's ability to see, because as the window tint becomes darker, the light level inside the car is reduced, allowing greater visibility through the tint (to a point). Rather, saftey for law enforcement officers is the primary focus. Criminals with tinted windows can shoot a police officer through heavily tinted windows without the officer being any wiser. Thus, most states have heavy restrictions on tint.

I like this idea, especially Voltmers suggestion. However, any tinting of a car's front windshield is illegal. Automotive side windows can be tinted to certain degrees of darkness. I assume you meant the side windows.

If you make the glass dark, it will tend to absorb light and heat the interior indirectly as infrared. If you make the glass clear, light transmits unhindered and heats the surfaces within. Another alternative is to create a series of layers that increase the reflectivity index for the infrared and ultraviolet spectrums, while allowing visible light to pass unhindered. So you see clearly, without the ultraviolet or infrared components. See

http://www.ocioptics.com/coatings.html

This is just one source of such optical coatings.