Are Wider Streets More Congested?

In a recent Planetizen blog post, Brett Toderian had an interesting insight: "When vehicles are moving, they take up much more space. The faster they move, the more separation distance and space between vehicles is needed."  This makes intuitive sense to me: when I am driving on a 20 mph street, I am willing to drive only a few feet behind other cars, while when driving 60 mph I don't feel comfortable getting so close to the car in front of me. 

This statement illustrates why wide streets doesn't always reduce congestion.  If the road is wider, drivers move faster.  If drivers move faster, they take up more space- which means that making cars move faster is like dumping extra cars on the road.  So even if making traffic move faster doesn't increase the overall number of drivers on the road*, it may actually increase the likelihood of congestion.

*Which it often does by making long-distance commuting more desirable.