There's a thought-provoking article (free at this writing) from the weekend edition of The Wall Street Journal. It makes a case that traffic jams actually help the environment.
Now, before you write the article off as the ravings of some anti-environmentalist, I suggest you take a look at it. The main idea is that more congested streets provide the proper incentive for people to move to public transportation, and other environment-friendly alternatives.
Specifically, the author states that ideas like congestion-pricing, where fees are paid to use highways and streets based on time of day and/or volume of traffic may actually cause more environmental problems, while "solving" congestion problems.
In the author's view, reduced (or better managed) traffic volume makes the commute more enjoyable, and may actually encourage people to stay in their cars - just shift times of travel where possible. Further, he does not say to get rid of congestion pricing, rather to find the right price. (For example, he finds it "absurd" that, in New York, "the East River bridges still don't charge tolls and that curbside parking in much of the city is free." On that last point, I'm not sure that much of that free parking is convenient to where people work, but there evidently are spots available.
You could use this as a discussion starter when talking about externalities or demand elasticity (the incentive of a substitute). It might be interesting to see where the discussion goes.
I look forward to your comments.