Often things we think of as pedestrian can offer a wealth of opportunity. This article from today's edition of The Wall Street Journal (free at this writing) is an excellent example.
The story is about a skywalk that was built in Mumbai, India. It's just one of a network of 50 planned for the city. They are necessary because the city's population is growing. Mumbai is also, according to the article, one of the more pedestrian metropolises in the world.
The article offers several different points of departure: discussions about externalities, the role of government, resource constraints (scarcity), and unexpected consequences are all available. Additionally, there is video, a slide show, and an interactive graphic to help illustrate the problem.
I highly recommend the article. And I look forward to hearing how you think it can be used (or if it can be used) in your classroom.