Why Do Semi Truck Passenger Doors Have A Little Window At The Bottom?

You've probably spotted that tiny window at the bottom of a semi truck's passenger door cruising on the highway. Most people confuse this for an aesthetic finish or even a pet window. Well, this misconception couldn't be further from the truth. It has nothing to do with pets inside the semi but rather plays a critical role in road safety. That tiny little window at the bottom is known as a peeper window. 

While it would make the perfect spot for a "doggo" to catch all the action outside as the semi-truck drives, it's a safety accessory that solves one of the biggest blind spots on a semi. Semi-trucks sit high and don't have a clear view of objects right below the cabin. Both the driver and passenger have a blind spot on each side of the semi, but the passenger side is much bigger. That tiny window at the bottom is designed to increase the truck driver's visibility in areas where they'd be forced to lean over to see instead.

These lower-door windows give semi-truck drivers a better view of what's happening near the curb, especially when pedestrians and cyclists are too close to be noticed or don't appear in the mirror due to the truck's ride height. While semis have a 200-foot blind spot at the rear (with a trailer) and another 20-foot blind zone right in front (in front of the hood), the widest blind spot on a semi-truck is on the passenger side door. Without the down-view window, the driver wouldn't be able to see anything next to the passenger door extending multiple lanes to the right.

Therefore, if a semi-truck is trying to switch lanes and your car or bike is in the right lane next to the passenger door, you'd be invisible to the driver without that little window. The down-view window is also a bonus for inexperienced drivers trying to circumvent heavy traffic, adding to their field of view. Several car brands that make semi-trucks have also attempted to address this blind spot by installing a fisheye mirror that sticks out of the truck's right-hand side fender as an alternative to the window.

As you've probably already noted, not all semi-trucks have this window. Older American-made trucks may not feature this tiny door window unless they were retrofitted or upgraded. Newer trucks, on the other hand, come stock with the peeper window — especially those manufactured in Europe. Even though newer tech offers better views of "no zones," with features like 360-degree cameras, the tiny passenger door window is still necessary and works better when integrated with safety features like auxiliary cameras and mirrors. 

In cities like London, heavy goods vehicles need a permit called the DVS (Direct Vision Standard) to drive in Greater London, with a star rating (minimum of 3 stars out of 5) given based on the visibility of the cab. Fitting a peeper window automatically moves a semi from a zero to a one-star rating on the DVS. In the U.S., the FMCSA (Federal Motor Carrier Safety Administration) also offers guidance on driver awareness and safe maneuvering of large vehicles in no-zones. Regardless, keeping an eye out for any furry faces in these windows the next time you pass one may be a fun endeavor.