Estimation: Yearly Time Spent Idling at Red Lights Across the US

Estimate the total time that people in the United States spend sitting in their vehicles while waiting at traffic signals. Agree on what you are counting, build the estimate from assumptions you state and label, and check whether your final number is believable. ```hint Price a single stop first Work out roughly how long one stop at a red light costs, then figure out how many such stops happen across the country. Decide early whether a car with two people inside counts once or twice. ``` ### Clarifying Questions - Does "people" mean drivers only, or everyone in the vehicle, so that two people waiting one minute count as two minutes? - Which vehicles are in scope: private cars only, or also taxis, trucks, delivery vans and buses? - Should the answer cover a day or a year? - Do only traffic lights count, or also stop signs, rail crossings and freeway ramp meters? ### What a Strong Answer Covers - A scope (people, vehicles, kinds of signal and time period) agreed before any arithmetic. - An equation that breaks the total into factors that can each be estimated. - Round, labeled assumptions with a brief reason for each, and arithmetic with the units tracked. - A sanity check by a second route or a per-person comparison, and a view of which assumptions drive most of the uncertainty. ### Follow-up Questions - Which assumption would you most want to replace with real data, and where would that data come from? - How would your estimate change for a dense city compared with a rural county? - If smarter signal timing shortened the average wait, how would you estimate the time saved and what it is worth?