About 20 percent of Cobb County traffic signals malfunctioned for several hours in the middle of the day Wednesday, causing temporary mayhem along some of the county’s busiest thoroughfares.

Hundreds of calls from drivers flooded Cobb’s 911 center, and county DOT workers had to scramble to manually reset about 110 malfunctioning lights. The affected streets included segments of Sandy Plains Road, Powder Springs Road, Johnson Ferry Road, Atlanta Road and Bells Ferry Road, to name a few.

But it could’ve been much worse. The malfunction happened during spring break, when traffic is unusually light, and in the middle of the workday rather than rush hour, said county spokeswoman Aikwah Leow. The signals malfunctioned shortly after 11 a.m. and were fixed within about two hours.

“We were out there and figured out what we needed to do very quickly,” said Jim Wilgus, interim director of Cobb County Department of Transportation.

Tracey Layman, of Marietta, was running errands on Bells Ferry Road around 11 a.m. when a series of lights along the roadway defaulted to flashing red mode. The lights definitely disrupted traffic but didn’t cause too much delay, Layman said. Still, she watched for impatient drivers who might fly through the lights instead of treating them as a four-way stop.

Mostly, she said, drivers just improvised.

“It was a lot of packs of cars,” Layman said. “Just this pack would go, then this pack would go, and you’d get honked at if you stopped or only had one car go through.”

About 300 calls from drivers about the signals flooded the county’s 911 emergency center, a dispatch supervisor told The Atlanta Journal-Constitution.

It was unclear Wednesday afternoon exactly what caused the lights to go haywire. Motorists reported signals that were blinking red or yellow. Layman said the signal for one of the cross-streets she passed on Bells Ferry Road seemed to be stuck on solid red.

All the affected signals operate on a separate traffic management software called Tactics by Siemens, according to Wilgus. It’s one of several such programs the county uses to control different sets of traffic signals. Wilgus said Tactics is an outdated program that will be replaced in the next few years using Cobb County’s 2016 Special Purpose Local Option Sales Tax funds.

About $12 million from the SPLOST has been set aside for traffic management systems. Wilgus said he didn’t know how much it would cost to upgrade the older software. However, the SPLOST funds will be more than enough to cover it, he said.