In 1918 most cars were still open, and many didn’t have windshields. Mirrors were an occasional aftermarket gimmick. The inside mirror finally became standard around 1935, the left side mirror waited till 1960, and the right side mirror until 1990.

Before visual mirrors, at least one inventor considered an audio mirror!

The gadget had a large resonator box with an inlet pointing rearward. The tube carried sound up to the ears of the driver. Supposedly a car or truck approaching from the rear would be audible, though I doubt you could hear it over your own engine and the wind.

The most interesting part of this drawing is the car itself, which seems to be an open bus or jitney. Imagine unfolding and closing the top with a dozen accordion-like folds. It would take at least two strong men standing on platforms or scaffolds.

Some real buses had a similar gadget for a different purpose. With the engine in the rear, the driver couldn’t hear the engine well enough to shift the unsync’d gearbox. A tube led from the engine compartment to the driver’s ear to let him hear the engine.