Map showing traffic load on roads

Hi everyone.

I want to show on a map how much cargo is being transported on different roads within a region. This could measured i.e. in tons or amount of trucks/deliveries. Is it possible to show this as different thickness on the roads, depending on the amount being transported? I want to show the route itself - as a line (from point A to B), not just as an area with a circle. 

Does anyone know an easy way to do this?

Do I have to define the roads myself as different points on the map, and draw lines between these points?