Whenever there are large scale public demonstrations, one of which can turn into a riot at any time - all it takes is a trigger set off by either a small group of agitators, police, or infiltrators.
I got to thinking, what if you were caught in such a situation, or got caught someplace where something even more dangerous came about - like martial law?
How could you high-tail it to the other end of town without falling into the hands of rioters, police, etc? There might be one solution for you: an underground escape route!
If it is possible for you to have access to an urban sewer system through larger culverts or aqueducts, then you could well have an expressway to freedom.
All it takes is a bit of curiosity and some spare time to scope one out and see where it leads. If it is possible, with some imagination, you could even get the plans for the city's sewer system with a creative excuse - say like a urban planning project for university or something along those lines.
An urban, underground escape route might prove to be an interesting thing to research, in fact the notion of finding hidden urban vanishing points is so interesting to some that there are whole networks of people devoted to urban exploration: