Have you ever seen those black cables stretched across roads? What do you think they are for?   As it turns out, these "cables" are actually tubes filled with air, known as "Pneumatic Tube Detectors." As a car drives over one of the tubes, the air pressure briefly closes a switch located in a box, which is often located on the side of the road. We can estimate the number of cars that have driven over the tubes based on the amount of times the switch has been activated.   Sometimes you see just one tube, and sometimes you'll see two or three. If it's just one, then we can only count the number of cars. If you see two or three, we can compute the speed of each car that passes over them.   Transportation departments in cities and States will deploy Pneumatic Tube Detectors across streets to collect data on traffic (how many cars drive down this street in a day? How fast are they going?). The systems are usually temporary; they will only be set up for a day or a week. Traffic engineers can use this data to determine if speed limits need to be changed, roads need to be widened, or if traffic lights or signs need to be put in place.

Ballistics chronographs are devices used to measure the speed of small projectiles, such as bullets or paintballs. Chronographs use optical sensors at known distances from each other to calculate how fast the object is moving.