How to highlight the number of overlayed lines in ArcMap?

Using ArcMap 10.2, I have a feature class composed of several lines representing bus routes for the same transit line. There are different routes according to schedules. In the image below, each route is represented by a different color. Most route have common sections.

What I need is to highlight the number of overlayed route in different sections, with one color whose lightness varies.

I thought about representing the layer in red line, then apply a 80% alpha, hoping overlaying lines become more opaque. But it doesn't work in ArcMap.

Any suggestion?

I don't have Spatial Analyst extension.