What driving aid did Ray Harroun's 1911 Marmon Wasp famously use?

Ray Harroun’s 1911 Marmon Wasp famously used a rear-view mirror. Mounted on the car, it allowed Harroun to see approaching competitors without a riding mechanic seated beside him.

At the first Indianapolis 500, nearly every driver carried a riding mechanic, who helped monitor the car and watch for traffic behind. Harroun chose to drive alone to reduce weight and improve efficiency. The mirror gave him at least some awareness of cars following, although he later said vibration from the rough brick surface made the view difficult to use consistently.

The innovation became one of the race’s best-known early technical stories. Harroun’s Marmon Wasp won the inaugural 500 on May 30, 1911, and he led 88 of the 200 laps. He was relieved for part of the race by Cyrus Patschke, but the car’s solo-driver layout and mirror are still closely associated with the victory. The device was not a modern camera or radio system; it was a simple cowl-mounted mirror, an early example of a safety and awareness aid in automobile racing.