Truck windshields are vertical, and car windshields are tilted.
The simple reason car windshields are tilted is to reduce air drag. Air flow hitting the wide, flat surface of a vertical windshield would greatly reduce the car’s ability to move forward—the engine would need to work harder, burning much more gas, to allow the car to maintain speed. When it hits a slanted surface, air flows up and over the car.
A tilted windshield helps the car cut through the wind efficiently.
Another benefit of a tilted windshield is gravel or any other road debris which hits the glass is likely to be directed upward, causing only a glancing blow, not a direct hit. The result is less damage the glass.
They are made of two pieces of tempered glass sandwiching a polyvinyl core so they don’t break up into dangerous shards if shattered. The windshield is designed to hold together even if it’s cracked. The windshield’s tilt, like its construction, is a safety feature.
It’s Different for Trucks
So if reducing drag is so important, why don’t trucks have tilted windshields like cars?
Surely it’s important they move through the air efficiently too. This is true, but it’s more important that truck drivers have a clear view when they are behind the wheel of a big rig. Angled (and curved) glass creates visual distortion, which truck makers want to avoid.
In a car, with its smaller windshield, this distortion is relatively minor—not enough to be considered a hazard. But a big truck has a big windshield. So the resulting visual distortion could be significant. Distances would be deceptive and depth perception compromised. Moving objects would be harder to track.
It’s simply safer for a truck driver to look through a vertical piece of glass than a large, angled windshield.
Air drag on trucks is usually reduced by an airfoil on top of the cab, above the windshield. This helps deflect air up and over the vehicle. Road debris isn’t quite the same problem for trucks as it is for cars, since the truck is much higher off the ground. But truck windshields are thicker and stronger than those on cars, so a dangerous breakage is highly unlikely.