‘Liquid’ vs. ‘Gas’ Models for Traffic

Thanks for your article on reversible lanes (Street Smart, May 9). However, you should be aware engineers are shifting their thinking toward traffic. Formerly, the model for traffic was a “liquid,” where adding capacity relieved congestion. Now engineers are starting to see traffic as a “gas,” where traffic volume expands to fill the capacity.

Researchers at the University of California Institute for Transportation Studies recently published the results of their study on capacity expansion projects in many California cities. They found that within one year, 60% of the new capacity was consumed with new, or longer, trips. Within five years, 90% of the capacity was consumed.

Evidence from capacity reductions verifies this thinking--removing capacity reduces traffic. Collapse of the Nimitz Freeway in Oakland caused 45,000 trips a day to disappear. After the Northridge quake, and the closure of the Santa Monica and the Antelope Valley freeways, the air quality was great. And the closure of the Central and Embarcadero freeways in San Francisco resulted in less traffic.

PETER JACOBSEN, PE

Pasadena