Mark Demidovich was headed up I-75 to a meeting when he started seeing the tell-tale signs of Friday’s giant traffic jam on I-285 following a fatal pedestrian accident.

Exit after exit before the I-75/I-285 interchange was backed up with drivers trying to find alternative routes around the accident, which shut down all lanes on east-bound I-285 for hours.

“It seemed like everyone had gotten the message,” said Demidovich, the Georgia Department of Transportation’s assistant state traffic engineer.

Demidovich can’t prove it, but he believes the live traffic maps that many drivers have on smartphones or other devices these days play a key role in helping them avoid such accident scenes.

“We’re a techie city and traffic is bad. That gives me a hunch that traffic apps are used here a lot,” he said.

These days, drivers have a choice of several mobile apps that use a variety of technologies to deliver up-to-the-minute traffic information. They include Georgia DOT's Georgia 511 mobile app, Google's map function and Waze, a so-called "crowd sourcing" app that maps live traffic updates from drivers and recommends detours.

Demidovich said about 170,000 people have downloaded Georgia DOT’s Georgia 511 app for mobile phones since it was launched two years ago, and about 600 new users join each week.

The system piggybacks on Georgia 511’s 19-year-old web site, which uses a system of 3,600 traffic sensors and 800 cameras to give traffic updates every 30 seconds.

He didn’t have data immediately available for the number of users Friday. He said the Georgia 511 web site averages about 40,000 users a week.

But visits to the web site jumped to 120,000 on the day two weeks ago when another pedestrian fatality also shut down I-285 for several hours.

About 230,000 metro Atlantans use Waze each month, making Atlanta one of the app’s 10 most active cities in the U.S., a company spokeswoman said.

Waze spokeswoman Julie Mossler said she didn’t have data on the number of users Friday. Typically, about 90,000 Atlantans use the the app each day, she said.

Google, which bought the Israeli company for $1.3 billion in 2013, also has its own map function for mobile phones that gives live traffic information for much of the country, including Atlanta.

Shannon Wood, of Douglasville, said she used to use other traffic apps, but switched to Waze last year after her husband heard about it from a co-worker.

Wood said she balked at following some routes recommended by Waze that didn’t seem to make sense, only to get stuck in a traffic jam. Now she’s a convert.

Friday, she used it during a shopping trip to Perimeter Mall with her daughter, Branson Wood.

“It told us there was traffic coming so it sent us some crazy little way,” she said. “It’s always right. There’s always traffic when it says there’s traffic.”