To place a traffic path, first select the traffic type icon in the top of the left Traffic panel. There are 5 options, Vehicle, Rail, Bike, Run and Walk.
After selecting the traffic type, click the 'Create New' button and there will be a half-dome white element on your cursor. This will track elevation changes if the terrain goes up or down.
Click points as necessary to place the traffic path and press Esc when you are done with your path. You can later adjust the path nodes as well as the path attributes such as traffic volume, number of lanes and lane width.