I would probably draw in QuantumGIS and export as shapefile. There are many many ways to draw or otherwise generate spatial data. I think it depends on what you mean by 'effectively'?
Edit: Ok, I just re-thought your comment, and perhaps you mean to 'highlight' the existing road paths if they are also bus-routes. That would be trivial if your roads lines have attribute "bus_route". You can also add the attribute, then in QGIS you can select such paths, edit them and set the attribute as 'true', in TileMill you would then add a style to change color for lines with such attribute. Also, if the streets have names and you have a list of such streets, you can do a CartoCSS selection based on the street names attribute.
Edit: Ok, I just re-thought your comment, and perhaps you mean to 'highlight' the existing road paths if they are also bus-routes. That would be trivial if your roads lines have attribute "bus_route". You can also add the attribute, then in QGIS you can select such paths, edit them and set the attribute as 'true', in TileMill you would then add a style to change color for lines with such attribute. Also, if the streets have names and you have a list of such streets, you can do a CartoCSS selection based on the street names attribute.