Google Maps, accurate?

[Jimrus]

Hi,

We are trying to book ourselves a self drive tour around the Rockies and are finding Google Maps a very useful resource. We find 2 pieces of information particularly useful but... we don't know how accurate they really are... and would greatly appreciate feedback from your experiences, so;
1) Are the point to point mileages accurate?
2) Are the estimated journey times accurate?

Thanks in advance!

[Midwest Michael]

Mileage estimates from Google are very accurate, and probably the best way to get an nearly exact number for a specific route.

The times, not so much. There is a level of guessing on any time estimate, but for long distance trips, they basically assume that you will never have to slow down for traffic or stop for food and fuel. For two lane roads, particuarly over shorter distances, they can actually be off by the other extreme, saying it will take far longer than real world times.

[Jimrus]

Thanks Midwest Michael! Do you know how they calculate the time? Is it based on the speed limit for individual stretches of road? I've noticed it can vary the time quite significantly for similar distances. Does it take account of things like steep and winding roads or would that kind of road have a reduced speed limit anyway?
Thanks!

[CalOldBlue]

Just as one data point; here's a trip I did from Concord, CA to Salt Lake City and back just about one year ago; using I80 to get there and US50 (mainly) returning. I plotted out the Google times beforehand, and recorded the actuals on the trip.

First chart shows Google map times:

Concord-SLC: 10.25 hours
SLC-Fillmore: 2.25 hours
Fillmore-Eureka: 5.25 hours
Eureka-Concord: 7.5 hours

Here's what the timer in my car recorded as actual "ignition on" time:

Concord-SLC: 10.25 hours (straight thru leadfoot driving)
SLC-Fillmore: 3.4 hours (some time lost in SLC looking for a place to eat dinner)
Fillmore-Eureka: 6:00 hours (some time spent driving around inside Great Basin National Park)
Eureka-Concord: 9.25 hours (includes an hour spend idling on a business phone call plus looking for lunch in Fallon and Dinner in Truckee)

Bottom line: for my type of driving (never saw a speed limit I didn't think I could add 5-10mph to) the transit times in Google are pretty good. What messes you up are the stops/side trips/etc's that happen... you need to account for them.

[AZBuck]

Welcome to the RoadTrip America forums!

Every software mapping program is proprietary and won't say how they compute their driving distances and times, but I can guarantee you that they don't worry about the speed limit on individual stretches of road or whether its winding or climbing grades or anything so esoteric. Basically every routine will put every piece of road into one of a very few classes (Interstate, 4 lane divided, two lane rural, two lane urban, and maybe a few more) and assign an average speed to each class. They'll underestimate the time it takes to drive some segments and overestimate others, but in the end it all averages out. The big problem with such programs, as has been pointed out in these forums many many times, is that they don't account for the real world and don't allow for traffic, driver fatigue, or any stops at all for food, fuel, or bathroom breaks. As Michael noted and CalOldBlue documented, they're excellent at giving you mileage estimates, but their time estimates are fantasy. You can do just as well with a couple of rules of thumb that we recommend: 1) You will average about 55 mph, all things considered, over a full day's drive, and 2) 500-550 miles constitutes a full day's drive.

AZBuck

[PNW Judy]

IF I drive a bit over the speed limit and IF I don't make any stops and IF there isn't road construction and accident or rush hour traffic, the times are fairly accurate. That's a lot of IFs and certainly doesn't work for a sightseeing trip when you'll likely stop at scenic viewpoints, need to stop for gas and other breaks, etc. So for anything more than a 2-3 hour sprint down the freeway, they tend to be very overly optimistic. In my experience anyway.

[glc]

To be safe, add 20% to Google drive times. This will allow for things that people normally do.

[RhodyMike]

I agree with the previous posts. Mileage is accurate and obviously isn't affected by other factors like stopping, traffic, weather, etc.

On short trips this is often made up for by the fact that most people drive over the speed limit, but over the long haul the other factors will add more time.


Google a short or medium length trip that you take regularly and compare it with the actual time it takes, this is a good way to guage how accurate it is in relation to your driving.