Visiting Speaker

Dr Lewis Lehe

Assistant Professor Department of Civil and Environmental Engineering University of Illinois Urbana-Champaign

Date: Mon 1 August 2023, webinar


Dr Lewis Lehe is an assistant professor in the Department of Civil and Environmental Engineering at University of Illinois Urbana-Champaign. His group’s research ( ) focuses on the economics of travel in cities. He holds MS and PhD degrees in civil engineering from UC Berkeley, and an MA in Transport Economics from the University of Leeds.


This talk presents a form of positive feedback that can arise when congestion penalizes transit relative to driving. I illustrate the idea using a simple model of a bus route with elastic demand. When the bus fleet size is fixed,  a rise in congestion makes each bus take longer to finish its route, and so bus frequency falls and wait times rise. This makes some bus riders switch to driving, which in turn causes more congestion. This circular logic can lead to the existence of multiple, stable equilibrium levels of ridership and amplify policy impacts. I also discuss what happens when the bus fleet size depends on its ridership, which intensifies positive feedback.

For more information contact:

Dr Elnaz Irannezhad