When I was reading chapter 8, I noticed that among the options to enrich the routing problem there was none that suggested anything about adding a public transportation system to it. I think it would be an interesting way to illustrate the incentives to have alternative means of transportation besides cars. Also, it could be a way to reduce congestion in any of the routes proposed on a routing problem (instead of inducing a fee to use streets that allow people to arrive faster to a certain destination), and you could also add a rationing element to it if necessary. For places like Mexico City that have lots of traffic and an excessive amount of cars, I think a model like the one I´m describing could illustrate interesting ideas. It could also allow relevant analysis related to Pareto efficiency, Nash Equilibria, utilitarian traffic, socialism, among other concepts.
In my opinion it is important to have models that point out the pros and cons of public transportation. Nowadays we face a world with an increasing population. Also, pollution is growing day by day. Both of these problems are good reasons to reduce the number of cars that are used everyday, for instance. Therefore, the need of alternative ways of transportation is a relevant thing to analyze.
Given the name of the chapter, the addition of some sort of public transport to the routing problem could be very enlightening. There are good examples in real life that illustrate how policy interventions can affect public transportation in a negative way. In Mexico City, for example, the malfunctioning of line 12 of the subway is good evidence of how adding an extra public transportation route is not always the best idea. Line 12 interfered with other routes when it presented performance issues, and it slowed people´s arrival to their destinations. Thus, there are efficiency concepts that can be incorporated in a model with public transportation systems. You could also study the equal or unequal treatment of equals (for example, by forcing some people to use the bus in order to reduce congestion, which would probably cause some sort of unintended consequence as well).
It´s very likely that some sort of model that incorporates the things I mentioned already exists. I think it might be a good addition for the book to add something like that, or even just to point out it existance. Thank you for your time.