# When a maps app/site or GPS gives an estimated time of arrival for a destination, in what way do they calculate for traffic lights?

Asked by jca (36043) October 22nd, 2015

A friend of mine and I were discussing today a possible dinner destination. I suggested a restaurant that, when I checked on Google Maps, is the exact same distance, time-wise, from my house and her house. It shows 38 minutes for me to get home and for her to get home, even though it’s 10 miles longer for me (probably due to highway and her’s is local roads). When I emailed her about the location suggestion and told her about the 38 minutes for each of us, she stated that her time will be longer due to traffic lights (which I took to be she’s making a negative out of what I considered a positive).

I am just curious, how do map programs and GPS calculate for time for traffic lights? Do they go on the assumption that all lights will be green, all lights will be red, or 50/50, or something else?

Observing members: 0 Composing members: 0

I’m going to make some assumptions here about Google’s algorithms based on my experience with using them as a consumer. (Note: I also develop GPS/mapping/routing software, but we do not use Google. We had to write our own, using a different API.).

As @Apparently_Im_The_Grumpy_One‘s link points out, there are many sources of data Google can use. But one of the big ones is the actual live data provided by phones that are actually driving the routes. My guess re: the traffic lights is that Google doesn’t know how long the lights are programmed for. Rather, it has a set of data that it can use to calculate an average time. It can also incorporate current traffic conditions and speed along these roads. It would be trivial to calculate adjustments to incorporate this historical data with current conditions to provide a fairly accurate estimate.

I have learned to just trust in my phone’s directions. I have a busy schedule, and all appointments are in my phone. It constantly monitors current traffic conditions and travel times so that it can tell me precisely when to leave. When an accident happens in front of me on my route, it redirects me before it affects me. Recently I was told to get off the highway even though there was no traffic. As the redirect sent me along some smaller, slower roads, I caught a glimpse of the highway up ahead and saw that it was a parking lot.

One thing to notice (if you’re interested) is what the traffic view looks like. Enable it on your phone and take a look at the color-coded traffic indicators. They are extremely accurate. As you drive the speed limit, you’ll see that you’re driving along green (no traffic) sections of your route. When things slow down due to traffic, you’ll see yourself driving on orange or red sections (depending on the severity of traffic). And when you see your car indicator on the map approaching yet another green section of road, it’s amazing to see how accurate it is when you’re suddenly out of the traffic and traveling normal speed again. This is possible because every car driving this road is a data point.

DoNotKnow (3002)

Lotsa ways and sometimes not accurate. Mine adds ten minutes to a five minute trip, no stop lights and other times a twenty mile trip on a highway will be calculated nineteen minutes.

I don’t know but whatever way they are using it is wrong. If I am passing through Austin, no matter what the time of day, I need to add at least 20 minutes to the estimate. Unless it is on I35 when I only need to add 10 minutes, 50 if it is Friday after 4 pm.and that is a frikken interstate highway!

rojo (24159)

I always assumed it’s the same as how my car navigation does it. It uses average speeds for certain types of roads.

JLeslie (59781)

Good question. When I wondered the same thing recently, my Google search found this.

Pachy (18592)

@DoNotKnow What app are you using?

2davidc8 (9701)

^ Android. Google. All opt-in re: location settings.

DoNotKnow (3002)