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Research Projects

STATUS: Complete YEAR: 2021 TOPIC AREA: Public transit, land use, and urban mobility Transportation planning, policy, and finance CENTER: NCST

Evaluating Accessibility of Los Angeles Metropolitan Area Using Data-Driven Time-Dependent Reachability Analysis

Project Summary

Project number: NCST-21-21
Funding source: U.S. Department of Transportation
Contract number: 69A3551747114
Funding amount: $100,000
Performance period: 8/16/2021 to 8/15/2022

Project description

Accessibility to people, goods, services and places forms the basis of economic development and public health in cities. The better and more efficient this access, the greater the economic benefits and the better the public health. As the population and employment grow in a city and the level of agglomeration gets higher, the city tends to have higher GDP per capita and higher levels of productivity. The way in which cities facilitate accessibility through their urban forms and transport systems also impacts directly on other measures of human development and well-being. Reliable evaluation of accessibility of city blocks for transport systems over a large area is essential for policymakers to achieve effective city planning as well as to improve well-being of citizens.

One major way to evaluate the accessibility of public facilities (e.g., hospitals) is reachability analysis which determines whether certain city blocks can be reached from at least one of these facilities within a certain time span. An isochrone is generally defined as a curve drawn on a map connecting points to which moving objects (e.g., cars) leaving from a specific origin can arrive at the same time and it is one of the most efficient tools used in addressing the reachability analysis problem. Thus, their construction is an important step for many transportation applications. As an example, in urban planning, isochrones are essential when assessing the placement of public services like hospitals and fire departments. However, normal isochrones do not always convey the information one needs. For example, when analyzing the transit system of a city, it is more critical to assess the average commute time in order to get to work and, similarly, in emergency response, we need to understand whether an emergency scene can be reached in time from emergency centers. Approaching this problem with normal isochrones does not yield correct insights because we want to analyze how long it takes for a vehicle to travel towards a single target point instead of away from it, facing, therefore, different traffic dynamics. We refer to this as reverse reachability analysis and we use reverse isochrone maps as a tool to examine it. Note that these are not symmetric as different traffic directions carry different traffic dynamics and therefore normal and reverse isochrone maps with the same parameters will most likely lead to different results.

The objective of this proposal is to investigate how accessibility of city blocks is quantified through the transport systems and real traffic flow data from the Los Angeles Metropolitan Area. We will formally define the isochrone and reverse isochrone problems, describe our approach to solving them and provide a functional system that uses our large traffic data repository of LA County (Archived Data Management System, ADMS) and is capable of visualizing the reachability in various ways. Unlike other studies, our approach is purely data-driven and does not depend on mathematical graph-theory to compute the isochrone which is computationally expensive. Instead, we focus on directly processing the large amount of traffic flow data that our research center, the Integrated Media Systems Center at USC, has acquired from the Los Angeles County in the past decade. We plan to use a data-driven grid-based approach for constructing time-dependent isochrone and reverse isochrone maps considering specific time of day, week, and month with the past, present and future predicted data. The outcome of our work will be 1) addressing the isochrone problem, for both single-source and multi-source scenarios, 2) addressing the reverse isochrone problem, for both single-target and multi-target scenarios, and 3) constructing time- dependent isochrone map display web applications.

To demonstrate the results from the proposed research, we will develop a web application in which users can select a location, travel time, and the time of year and see the evaluated accessibility info in the form of various isochrone and/or reverse isochrone maps. This research will exploit real-world big traffic sensor data collected from the Regional Integration of Intelligent Transportation Systems (RIITS) in the last ten years under our existing Archived Traffic Data Management System (ADMS) project.


Cyrus Shahabi
Helen N. and Emmett H. Jones Professor of Engineering
3737 Watts Way
Charles Lee Powell Hall (PHE) 306ALos Angeles, CA 90089-0781
United States
[email protected]


Seon Kim
Research Scientist
Integrated Media Systems Center
Los Angeles, CA 90089
United States
[email protected]