Estimation: How Many Miles of Road Does Google Maps Cover?

Estimate how many miles of road Google Maps covers. Count linear miles: each stretch of road is counted once however many lanes it has, so a four-lane road one mile long counts as one mile. Explain your equation and assumptions, and check the result. ```hint Anchor, then scale Estimate one country you can reason about from its area and how densely different kinds of land are covered by roads. Then decide how the rest of the world compares with it, instead of estimating countries one by one. ``` ### Clarifying Questions - Should the estimate cover the whole world, or one country such as the United States? - Which roads count: public roads only, or also private roads, service roads, parking-lot lanes and unpaved tracks? Are footpaths and trails excluded? - A divided highway is often drawn as two separate carriageways. Does it count once or twice? - Is an order-of-magnitude answer enough, or do you want a point estimate with a range? ### What a Strong Answer Covers - Pinning down geography, road types and the treatment of divided highways before estimating. - A clear equation that segments where road density really differs, such as urban versus rural land or richer versus poorer regions. - Round, labeled assumptions and arithmetic that is easy to follow and correct in its units. - A sanity check through a second method or a known anchor, plus a range and the assumptions that drive it. - A link back to Maps: how the exact figure could be computed from map data and what it would be used for. ### Follow-up Questions - How would you compute the exact figure from the map's own road data, and what could make that number misleading? - How would you estimate how many of these miles have street-level imagery? - If the mapped total grows from one year to the next, how would you tell new construction apart from existing roads that were newly mapped?