How Accurate Is the Mileage in Google Maps?by Robert Schrader
Google Maps uses publicly available maps and other documents to calculate mileage for routes it gives you between two points. In this way, mileage you receive from Google Maps is only as accurate as the maps the Web service relies on. If the maps are inaccurate or outdated, Google Maps may also be limited in its accuracy.
Google Map Sources
Google Maps derives its information from sources that are publicly available. According to the Google Maps website, the Web mapping service uses map and location data that is available online as well as information businesses and other locations provide Google directly. Since Google depends on the accuracy of its information sources, the accuracy of mileage between locations varies.
When you search for directions between two points, Google provides you several optional routes, each of which requires you to traverse a different distance and results in a different time in transit. The accuracy of mileage for certain routes may be higher or lower than others, although Google always provides you with what it considers to be the most sensible route as the first one.
Type of Transport
The mileage Google Maps lists between two points differs depending on the type of transport you use. Routes on foot may require less distance traversed than if you drive, for example, since pedestrians aren't at the mercy of traffic direction and signals. Likewise, public transit options may require you to travel a distance significantly greater than the direct mileage between two points depending on the trajectory the subway, tram or bus follows.
Changes to Roads
Google Maps constantly updates to reflect the latest available information on road construction and conditions, but new construction and detours may result in temporary mileage inaccuracy. Likewise, Google isn't always able to ascertain whether certain streets are one-way only or intermittent road blockages that result from utility work or public events like festivals or parades.