Comment 25968

By kevlahan (registered) | Posted July 17, 2008 at 10:18:32

Nice article Ryan!

As Eric Miller (transportation engineer at UofT profiled by Christopher Hume in the Star ) told me: "we teach traffic engineers about these paradoxes, but they tend to forget about them once they start working in the real world".

Regarding Braess's paradox, it is actually more extreme and relevant than you suggest:

  1. Adding a new 'shortcut' with ZERO travel time (the ultimate shortcut) can increase the travel times for EVERYONE. The converse is also true: removing a "fast" shortcut can decrease travel times for EVERYONE. These results assume that the total volume of traffic is unchanged.

  2. On random networks, roughly 50% of new routes will lead to slow downs for everyone. This suggests that Braess's paradox type slowdowns should be quite common!

When combined with the induced demand effect, it is clear that increasing lane capacity is unlikely to improve traffic in the long run.

The exception is where increasing lane capacity and traffic speed drives out local residents and businesses, which then reduces demand. This is likely what happened to Hamilton downtown!

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