Develop a new function modified from dodgr_distances to accept one additional parameter specifying a binary "flag" variable. Use this to aggregate two distances, rather than just one in current dodgr_distances, with the second variable only aggregated along edges with the flag variable set to TRUE.

Use cases:

Generate a sample of journeys between random points in a given area/city, and calculate things like:

1. Average proportional distance that pedestrians are forced to walk along major motorways
2. Average distance of journeys along streets named after men compared with streets named after women.

The latter would provide more insight into equalstreetnames (with more cities via github repo linked at top of that page). While those websites are really cool, it's also very obvious that streets named after women tend to be both peripheral and minor. To work out the exposure of actual people traversing a city to female versus male streets requires a realistic routing of random (or all) journeys through a city. That in turn could be achieved with this functionality.

ping @Robinlovelace @mem48 Any thoughts on the usefulness of this functionality for your work? Should be pretty easy to implement, if you think it would help.