Look down a surface water drain some time -- you'll not see the surface water sewer full of water unless a torrential downpour is in progress
There is demand for a road because there are drivers who wish to go from Point A to Point B and Road Z provides a segment of that journey. Yes, certainly creation of another route between A and B might displace some traffic from Road Z if the alternative is shorter, less annoying / more pleasant -- But the total traffic on Road Z and the alternative will be no more than the original traffic on Road Z as the same number of people still want to go from A to B -- make all the roads wider -- you get Less Traffic on each -- not more
People are not water molecules, however. They have minds (presumably) and intentions and the ability to choose what they do.
The notion that widening roads leads to more traffic is not original with me. It's a well studied and continually confirmed principle that was first observed back in the 1960s.
Presentation on 'fundamental law of highway congestion' first proposed by Anthony Downs in 1962, later elaborated in the 90s.
His book Stuck in Traffic (1992), which detailed the economic disadvantages of traffic congestion and proposed road pricing as the only effective means of alleviating it, was denounced by traffic engineers for its insistence on the futility of congestion relief measures. However, enough of his gloomy predictions about congestion were proven right that he successfully published a second edition, Still Stuck in Traffic (2004).
Lewis-Mogridge position: traffic expands to meet the available road space
Recent paper studying effects on urban roadways:
We investigate the effect of lane kilometers of roads on vehicle-kilometers traveled (VKT) for different
types of roads in the United States (US). For interstate highways in metropolitan areas we find that VKT
increases one for one with interstate highways, confirming the ‘fundamental law of highway congestion’
suggested by Anthony Downs (1962; 1992). We also uncover suggestive evidence that this law may extend
beyond interstate highways to a broad class of major urban roads, a ‘fundamental law of road congestion’.
These results suggest that increased provision of interstate highways and major urban roads is unlikely to
relieve congestion of these roads.