Time-Distance Diagrams

After I was in a car accident a few years ago, I contacted the city Traffic Control Engineer to see if I could get a copy of the signal timing sequence for the intersection of the street where the accident occurred. The information they provided allowed me to construct a time-distance diagram to relate the path the car traveled to the 90-second traffic signal cyles for several streets. 

The time in seconds can be read down the side of the diagram and the distance can be read across the bottom. Stationary objects (like the traffic lights) show up vertically on the chart while moving objects cross the chart at different slopes depending on their speed. The blue line represents the path taken by a car which starts from a complete stop at one intersection and accelerates to a speed of 35 miles-per-hour in a very leisurely 9 or 10 seconds. Note that the line crosses the final intersection during a green light.

What I liked about this diagram was how easy it was to show a series of timed lights and the effects that different average speeds had on the outcome. I was reading Edward Tufte’s The Visual Display of Quantitative Information at the time and his section on train schedules was very inspirational. Despite my efforts, however, the other driver sued for injury and my insurance company settled out of court. Oh well, at least I was able to get this great chart out of the process.

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