Fermi Estimate: Cars on the US-101 Corridor Linking San Francisco and Mountain View

Estimate the number of cars on US Highway 101 on the section that connects San Francisco with Mountain View. Talk through how you would size it, state every assumption you rely on, and finish with a number you can defend. ```hint Define the count first Decide whether your number describes a single moment or a span of time before you write any formula, because that choice changes which quantities belong in the equation. ``` ### Clarifying Questions - Is this a snapshot of the cars on the road at one instant, or the number of cars that travel this section over a day? - Which time should the estimate reflect: a weekday rush hour, midday, late night, or an average across the day? - Do both directions of travel count, and does "car" include SUVs and pickups but exclude trucks, buses and motorcycles? ### What a Strong Answer Covers - A clear definition of what is being counted (moment versus period, directions, vehicle types), agreed before any math - A top-line equation that breaks the count into factors that can each be estimated - Round, explicitly labeled assumptions, such as road length, lane count and traffic conditions, each with its reasoning - Correct arithmetic and unit conversions, with an acknowledgment of how the answer moves with the time of day - A sanity check from an independent angle and a final range instead of false precision ### Follow-up Questions - How would you convert your snapshot into the number of vehicles that pass a single point on 101 in a day, and which assumption is that conversion most sensitive to? - How would the estimate change on a weekend afternoon, or right after a crash blocks two lanes? - If you could pull one real data source to check your answer, which would it be and why?