When Google Maps figures the mileage between points A and B, how does it estimate the duration of the trip? It’s not based strictly on mileage, so does it go by local speed limits, or what?
Yes. The road information they buy from the suppliers is tagged with speed limit information. I don’t know if they use other factors for the final estimated times.
I think they must use factors other than speed limit. In my state, the two-lane highway speed limit is 70 and, other than in towns, we simply do not have reduced speed zones. There are some very, very wiggly mountains roads with 70 MPH speed limits that you have trouble going more than 50 on. Google Maps’ driving times seem to accurately reflect that you will drive a lot slower on these roads than on the wide-open stretches.
Federal and state DoTs regularly survey all roads and highways for average volume, speeds, and transit times. Google bases their estimates on those surveys, not posted speed limits.
Ah, that explains it then. I have noticed Google Maps seems to be much more accurate than some of its predecessors that did seem to just have set speeds for road types. Old Mapquest for example, had all the two-lane highways averaged at about 45 miles per hour, so it’s travel time estimates were nearly double the actual time in some cases.
Of course, also if you’re in an urban area where Google gets traffic reports, you can chose to factor in the time with the traffic, which is pretty slick.