A former Google engineer has recently addressed an oft-wondered question, revealing - for the first time - just how Google's mapping service calculates data for one its most handy features.

Google Maps does more than provide directions, it also serves up estimated time of arrivals by determining the time it takes to drive, walk, or bike to a destination. Everyone loves this feature, whether they realise it or not, but not many actually know how it works.

Richard Russell, an ex-Google engineer, used Quora, a question-and-answer website where questions are created and answered by its community of users, as a platform to finally detail Google Maps' calculation process. Answering a question about whether ETAs are based on speed limits or actual travel time by previous users, he explained that ETAs come from a variety of things, depending on the data available in a particular area.

"These things range from official speed limits and recommended speeds, likely speeds derived from road types, historical average speed data over certain time periods [...] actual travel times from previous users, and real-time traffic information," wrote Russell. "They mix data from whichever sources they have, and come up with the best prediction they can make."

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However, some companies, which do live traffic, also compare their predictions against actual time in traffic to tune algorithms and data sources, but Russell warned that even the best predictions aren't 100 per cent accurate. Simply put: Calculating ETAs is like predicting the future, and traffic, such as a car crash, is never predictable.

"So, to answer the sub-question in the question details: 'Is it based on speed limits or actual travel time by previous users?' - the answer is 'yes'," Russell added.