When you ask for directions in Google Maps, Google not only tells you how to get where you're going, but about how long it should take. It makes this estimate based on speed limit data embedded in its massive geographical database. However, this information isn't always reflective of how quickly you can get from point A to point B in the real world.
How Google Estimates Trip Times
When Google Maps plots directions, it breaks down the trip into individual segments, indicating how long you will travel on each road, street or highway. It multiplies the amount of time on each segment by the speed limit for that segment. Thus, if you're travelling for 75 miles on a road with a 50mph speed limit, it calculates a 90-minute time for that segment. It then adds up the travel time for all segments to generate your estimated trip time.
Where Google Gets Speed Limit Information
Although Google used to outsource the data collection for Google Maps, in 2009 it began building its own teams of local data providers to provide geographic information, including speed limits for road segments. According to Google spokespeople, the speed limit information comes from the speed limit road signs posted for each segment. Thus, the trip time for Google Maps directions assumes you'll be driving the posted speed limit at all times.
Problems With Speed Limit Data
While it's prudent to drive no faster than the posted speed limit, in practical terms it's not often done. On highways in desolate rural areas, the average car may go close to twice the posted limit. On the other hand, traffic on a six-lane tollway during rush hour may be travelling a tiny fraction of the limit. As a result, people sometimes find Google's trip time estimates to be dramatically different from real-world experience.
How to Report Errors
Google actively collects and reviews errors and updates to its map data base. Google Maps displays several "Report a Problem" links, including under driving directions, search results and the maps themselves. The links take you to a form where you can report inaccuracies, which are relayed to the pertinent data collection team.