Officials said on Thursday that Walker had been videotaped on the bus holding his cell phone while he drove, which is illegal for school bus drivers in Tennessee.
Electronic evidence, including readings from a GPS unit and engine monitor on the bus, shows the bus was traveling between "50 and 52 miles an hour," (80 to 83 kph), Chattanooga Police Department traffic officer Joe Warren told the court on Thursday during a court appearance by Walker, who did not speak.
In addition, there were several caution signs saying that the road's curves need to be negotiated at 20 mph, Warren said.
Warren described a video showing Walker holding his cell phone while he drove.