The rear windshield defrosters seem so much more effective. Is it just the lines across the drivers field of view that makes it decidedly unsafe?
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To do it right would be expensive and piss off the customers.
You run a few extremely thin strips down the windshield. Typically gold. It’s so thin you can’t really see it at all. Connect to a power source. It’s incredibly effective and quick! It’s also not cheap.
The pissed off customer part? You know how you’re driving down the road and hear little clicks of sand hitting your windshield? This makes really small chips. Over time, they add up and are changing how the glass is stressed. If you use air to gently and slowly change the temperature of the glass, it will probably be fine for a very long time. That super hot line of glass? That’s a cracked windshield. You’d also have to build in a safety regulator that wouldn’t allow someone to use the feature unless the glass was below a certain temperature. Otherwise, the strips melt inside the glass, also possibly cracking a windshield.
Ppg makes aerospace windshields like this. I’ve seen probably a hundred pilots crack windshields turning on the anti-ice when they should have used the defrost vents.
Back glass doesn’t need to be as easy to see through, doesn’t get chipped up much if at all, and is typically just tempered safety glass. Windshields are layered and the glass has different qualities. You want to add about 1500 dollars to the cost of the windshield? Most people would prefer the slower air method over the expense and increased likelihood of windshield cracks.
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