NASHVILLE, Tenn. (WTVF) — While announcing that he was upgrading his "safer at home" order from a recommendation to a mandate, Tennessee Gov. Bill Lee said that traffic data and cell phone movement helped him make the decision.

The announcement came on Thursday, at Lee's daily COVID-19 briefing. The order closed nonessential businesses across the state as the coronavirus spread. Lee said he decided to create the mandate after seeing data from the Tennessee Department of Transportation and from a private company called Unacast.

The TDOT data showed a sharp decline in the number of people on the roads as the coronavirus spread. In Nashville, the number of cars on the road dropped by thousands, and at some points there were half as many cars on the road compared to the same day in 2019.

"We're seeing in the four major urban areas a significant drop in traffic volumes," TDOT Traffic Operations Director Brad Freeze said. "What we're seeing now and these volumes, it's definitely new territory."

The real-time, radar data is normally used to help TDOT keep track of incidents on highways and to keep people updated on drive-times.

But those historically low volumes didn't stick around. Starting March 30, the trend shifted into reverse.

"While safer at home measures and further restrictions on business showed a steep drop-off on vehicle movement data...but beginning on March 30th it indicated travel is trending upwards again," Gov. Lee said in his Thursday afternoon briefing.

In that media briefing, Lee also cited statistics from Unacast, a company that used cell phone data to monitor movements.

The Unacast data painted a similar picture: a drop in movement until the end of March, when mobility increased again. Uncast statistics went into deeper detail, showing that urban counties like Davidson and Williamson had less unnecessary movement than more rural ones, which saw a smaller drop.

"Unacast indicates the movement of Tennesseans is trending back to pre-COVID-19 levels," Lee said on Thursday.

Unacast gave Tennessee a D- social distancing rating. NewsChannel 5 reached out to Unacast to learn more about how they use cell phones to gather mobility information, but a spokesperson didn't return our calls.