Google also uses information from partner traffic compilers to develop its historical data.

Washington - When you use mapping software for directions, the app typically gives you two trip times: how long it would take to reach your destination driving the speed limit and the time it would take in current traffic.

But how does a computer know what traffic is like?

The data source is rather simple: you, and others like you.

For example, if you have a GPS-equipped smartphone and enable the “my location” function on Google Maps, the company can monitor how fast you’re travelling.

When your smartphone feeds information into the system, Google adjusts its understanding of how long it takes to travel that stretch of road at that time, on that day of the week, at that time of year, under those weather conditions.

Google also uses information from partner traffic compilers to develop its historical data.

Historical data is important because it’s hard to acquire enough real-time information to make traffic forecasts.

Think of the complications involved. There aren’t that many people with smartphones driving down any given block at a given moment, and not all will have enabled their data-sharing software.

Also, some of the phones will be red herrings with respect to traffic.

Some, for example, will be in the pockets of walkers or in the baskets of bicycles.

Other phones may be in a car that has pulled over or is looking for parking.

When you plug a destination into your smartphone, the program’s first estimate of travel time is based on this historical information.

Only then does it begin to add in information from users travelling your route right now.

Different programs use different methods to combine the data that hundreds or thousands of phones are transmitting. Unfortunately, the algorithms are tightly held secrets.

Just as search engine companies won’t talk much about how they analyse a query, traffic mapping companies won’t say how they integrate real-time traffic data into their software.

The companies are willing to reveal a few of their tricks.

When you enter a destination in Google Maps, for example, the app relies more heavily on up-to-date traffic information for the segments that are close to your current location than it does for the distant parts of the trip, since current traffic conditions may not mean much in an hour or two.

Google says its software will offer alerts that recommend a different path when traffic conditions change. This change helps integrate real-time data with information about the more distant parts of a route.

Other apps such as Waze, bought by Google last year but operating as an independent app, use information reported by its users – a crash slowing traffic, for instance.

When other users travel the same road segment, the app asks them to confirm the earlier report.

The whole thing may seem a bit old school – like reporting an accident to a radio station – but it improves traffic forecasts.

Traffic mapping has made substantial progress in recent years, moving from one guy in a helicopter to a crowdsourcing network, but no company has entirely figured out a way to get real-time information to drivers sitting in traffic.

For now, keep your eyes on the road. – The Washington Post