This looks good, but can you provide an example in @examples to show what it does.

Imagine when you have different routes it will be useful to return results for each segment for each route type, right? Interested in your use case.

I will try to come up with a simple example.

What I am doing is overlaying trips between bus stops with the number of passengers travelling between each pair of bus stops so I get how many people are travelling on a specific section of a bus route. In my case I am doing this for passengers who take a specific bus route (to see which services they are connecting to/from) and for that reason I want to keep the different bus routes separate even when they travel along the same road (the white line is the "reference" route):

Very cool - so glad it's useful beyond the specific use case for which it was developed: that map looks great. Maybe the following code could form the basis of a simple example that uses this new functionality:

library(stplanr)
routes_fast$group = rep(1:3, length.out = nrow(routes_fast))
rnet_grouped = overline(routes_fast, attrib = "length", byvars = "group")

(For some reason that code breaks using your update of the function - any ideas?)

It seems I forgot to replace the column names I was using to test the function with the variable that was being passed to the function. I've fixed that now and added your example to the documentation.