Driving down the road, what is the first thing that may stop you cold? A red light. Traffic signals, aka traffic lights, are ubiquitous devices about which we (the two of us) knew very little. We decided that we had to acquire at least enough knowledge so that we could chitchat about them while we waited for red lights to turn to green lights.

We found out that the first traffic signal was installed in London in 1868. It was operated by gas and exploded in about a month, killing a policeman. It was not immediately replaced. Not an auspicious start.

A Salt Lake City policeman named Lester Wire invented an electric traffic signal with red and green lights in 1912. In 1920, Detroit policeman William Potts introduced the now ubiquitous red-­yellow-green ­device—­modeled after railroad signals.

For information about Ann Arbor’s own traffic signals, we turned to traffic engineers Cynthia Redinger and Luke Liu. They told us there are 162 signalized intersections within the city’s boundaries. They’re owned and controlled by the city, except for some on state highway business routes run by the Michigan Department of Transportation.

Traffic signals can operate manually, change on a fixed schedule, or detect when cars are waiting. Manual control regularly happens in Ann Arbor only on home football Saturdays, when a police officer takes control of the lights near the stadium.

The time period for fixed time signals is determined by observing traffic on typical days, including weekends. Detection signals identify cars waiting at the intersection using in-ground induction sensors or radar. Pedestrians announce themselves by punching a button located on the box near the crosswalk. (We confirmed that punching the button multiple times will not hurry the process of ­changing—although it may feel good to punch something.)

Detectors and push buttons simply provide data. Every traffic signal has its own computerized controller, but normally, most are controlled by a central computer. This computer has some fairly sophisticated software that times multiple lights on a traffic corridor to minimize delays.

One side effect of this corridor optimization is that cars may be left waiting at side streets for the greater good of the corridor traffic. Nevertheless, the engineers were emphatic in stating that there is no such thing as a two-minute red light.

Although we are not physicists, we have come up with a new theory of relativity: Could it be that the amount of apparent or relative time spent at a red light is directly proportional to the tardiness of the person stopped?