Why Do Some Car Side Mirrors Have a Dotted Line? Most Drivers Don't Know What It Does

Many drivers have probably noticed a vertical dotted line on the side mirrors of certain vehicles. However, few know what it actually means or why it's there. Auto30 takes a closer look at the purpose of this often-overlooked feature and how it helps drivers stay safer on the road.

The dotted line on a car's side mirror marks the boundary between two sections of glass with different curvatures. This design helps reduce blind spots, but the outer section can distort the apparent distance to other vehicles. Drivers should take extra care when changing lanes or making other maneuvers.

Side mirrors help drivers monitor adjacent lanes, as well as the areas alongside and partially behind their vehicles. Depending on their design, mirror glass can be flat, convex, or aspheric. Each type offers a different field of view and affects how drivers perceive the distance to other vehicles.

Aspheric mirrors are designed with two distinct sections of glass. The wider inner section provides the primary view, while the narrower outer section has a more pronounced curve. This allows drivers to spot vehicles that might otherwise disappear from the main mirror's field of view.

The purpose of this design is to expand the driver's field of vision without making the entire mirror excessively curved. As a result, drivers get a wider view of surrounding traffic while retaining relatively natural proportions in the main viewing area.

However, the additional curvature comes with a trade-off. Vehicles appearing in the outer section of the mirror look smaller and may seem farther away than they actually are. Drivers should avoid relying exclusively on this section when judging the distance of an approaching vehicle.

How to Properly Adjust Your Car Mirrors

Proper mirror adjustment plays an important role in maintaining visibility behind the wheel. Drivers should start by adjusting their seats to a comfortable driving position. Only then should they adjust both side mirrors using the electronic controls typically located on the driver's door or dashboard.

Side mirrors should provide the widest practical view of the adjacent lanes. With a conventional adjustment, only a small portion of your own vehicle should be visible along the inner edge of each mirror. This provides a useful reference point without taking up too much of the available viewing area.