Evolution of road infrastructures in large urban areas

TL;DR

This paper analyzes the emergence of urban freeways and ring roads in large US cities through empirical data and a cost-benefit model, revealing critical thresholds for infrastructure development and discussing future urban planning scenarios.

Contribution

It introduces a simple model predicting infrastructure thresholds based on cost-benefit analysis, linking empirical data to urban infrastructure evolution and future planning.

Findings

Urban freeways emerge at around 10,000 commuters.

Ring roads develop at approximately 100,000 commuters.

Removing freeways can be socially beneficial due to pollution and health costs.

Abstract

Most cities in the US and in the world were organized around car traffic. In particular, large structures such as urban freeways or ring roads were built for reducing car traffic congestion. With the evolution of public transportation, working conditions, the future of these structures and the organization of large urban areas is uncertain. Here, we analyze empirical data for US cities and show that they display two transitions at different thresholds. For the first threshold of order $T_c^{FW}\sim 10^4$ commuters, we observe the emergence of a urban freeway. The second threshold is larger and of the order $T_c^{RR}\sim 10^5$ commuters above which a ring road emerges. In order to understand these empirical results, we propose a simple model based on a cost-benefit analysis which relies on the balance between construction and maintenance costs of infrastructures and the trip duration decrease (including the effect of congestion). This model predicts indeed such transitions and allows us to compute explicitly the commuter's thresholds in terms of critical parameters such as the average value of time, average capacity of roads, typical construction cost, etc. Furthermore, this analysis allows us to discuss possible scenarios for the future evolution of these structures. In particular, we show that in many cases it is beneficial to remove urban freeways due to their large social cost (that includes pollution, health cost, etc). This type of information is particularly useful at a time when many cities must confront with the dilemma of renovating these aging structures or converting them into another use.