Ever spot a strange window on the bottom of a semi-truck's passenger door? Here's more about why these trucks have this window - and what it's really for.

For one thing, a semi truck's rear is usually obscured, so a dashboard rearview mirror wouldn't do anything. Regular side view mirrors, meanwhile, are too small to properly reflect the truck's ...

The right side of the semi truck also has a no-zone ... Before changing lanes, look at your rear view mirror to make sure you can see the entire truck, including its wheels.