Industry imports raw materials (oil, ore , forest, agri) and makes goods. If u have your own specialized industry they take raw materials from them. Then they delivery theese goods to commercial with vans. If u produce more than demand they export the rest. To reduce this traffic u have to use cargo trains . If there is cargo station connection from industry to commercial vans and trucks move goods to station and when trains delivers them to the other stations from where the vans and trucks delivers them to commercial. If u connect a cargo station with another city industry exports/imports goods via this connection. So its wise dont connect the local lines with the border lines.
Also traffic comes from people going to their work. U must use metro lines and buses ( carefull coz too many bus stops may result to more traffic) its better have small lines coz buses have only 30 capacity.
U must place offices mixed with residential so they walk to work and reduce traffic even more.