Today, you can buy an app for your smartphone that can measure braking distances. Back in the day, this was measured with blanks or shotgun shells packed with chalk or paint.

Back in the 1930s, an anonymous inventor came up with a device designed to show how reaction times translated in to distance traveled. It was called the “brake detonator”. All humans take between .8 and 1.1 seconds to hit the brakes from when a obstacle is detected.

To measure the distance traveled, some genius came up with the idea of putting two blanks shells packed with paint on the bumper of the car (some used chalk). The first blank would be triggered by the passenger. This would mark the pavement as well as the driver’s underwear. A second shell or blank was fired when the brakes were applied.

Some police departments used the device to demonstrate why a driver should pay attention. Some cab companies used the device randomly during a driver’s hiring process to measure their reaction time.